Christopher K Walker

Interests

Research

Star and Planet Formation;Life Cycle of the Interstellar Medium;Astronomical Instrumentation;Balloon-borne Telescopes;Space-based Telescopes;CubeSats

Teaching

Courses:Life in the Universe: Astr 202;Physical Universe: Astr 170b;Radio Astronomy: Astr 585;Interdisciplinary Studies;Authored Textbook: "TeraHertz Astronomy"

Courses

Scholarly Contributions

Journals/Publications

• Bergner, J. B. (2022). Astrochemistry with the Orbiting Astronomical Satellite for Investigating Stellar Systems (OASIS). Frontiers in Astrophysics and Space Sciences, 8, 246.
• Stanke, T. (2022). The APEX Large CO Heterodyne Orion Legacy Survey (ALCOHOLS). I. Survey overview. Astronomy & Astrophysics, 658, 38.
• Kloosterman, J. L., Hayton, D. J., Ren, Y., Kao, T. Y., Hovenier, J. N., Gao, J. R., Klapwijk, T. M., Hu, Q., Walker, C. K., & Reno, J. L. (2013). Hot electron bolometer heterodyne receiver with a 4.7-THz quantum cascade laser as a local oscillator. Applied Physics Letters, 102(1).
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  Abstract: We report on a heterodyne receiver designed to observe the astrophysically important neutral atomic oxygen [OI] line at 4.7448 THz. The local oscillator is a third-order distributed feedback quantum cascade laser operating in continuous wave mode at 4.741 THz. A quasi-optical, superconducting NbN hot electron bolometer is used as the mixer. We recorded a double sideband receiver noise temperature (T r e c D S B) of 815 K, which is ∼ 7 times the quantum noise limit (h ν 2 k B) and an Allan variance time of 15 s at an effective noise fluctuation bandwidth of 18 MHz. Heterodyne performance was confirmed by measuring a methanol line spectrum. © 2013 American Institute of Physics.
• Walker, C. E., & Pompea, S. M. (2013). Tales from the Dark Side: Teacher Professional Development , Support , Activities, Student Research amp Presentations. AGU Fall Meeting Abstracts, D793.
• Brisset, F., Dugne, O., Robaut, F., 'ar, J. L., & Walker, C. T. (2012). PREFACE: EMAS 2011: 12th European Workshop on Modern Developments in Microbeam Analysis. Materials Science and Engineering Conference Series, 32(1), 011001.
• Groppi, C. E., Wheeler, C. H., Mani, H., McGarey, P., Veach, T., Weinreb, S., Russell, D., Kooi, J. W., Lichtenberger, A. W., Walker, C. K., & Kulesa, C. (2012). The Kilopixel Array Pathfinder Project (KAPPa), a 16 pixel integrated heterodyne focal plane array. Proceedings of SPIE - The International Society for Optical Engineering, 8452.
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  Abstract: KAPPa (the Kilopixel Array Pathfinder Project) is developing key technologies to enable the construction of heterodyne focal plane arrays in the terahertz frequency regime with ∼1000 pixels. The leap to ~1000 pixels requires solutions to several key technological problems before the construction of such a focal plane is possible. The KAPPa project will develop a small (16-pixel) 2D integrated heterodyne focal plane array for the 660 GHz atmospheric window as a technological pathfinder towards future kilopixel heterodyne focal plane arrays. © 2012 SPIE.
• Kloosterman, J., Cottam, T., Swift, B., Lesser, D., Schickling, P., Groppi, C., Borden, M., Towner, A., Schmidt, P., Kulesa, C., D'Aubigny, C., Walker, C., Golish, D., Weinreb, S., Jones, G., Mani, H., Kooi, J., Lichtenberger, A., Puetz, P., & Narayanan, G. (2012). First observations with SuperCam and future plans. Proceedings of SPIE - The International Society for Optical Engineering, 8452.
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  Abstract: Supercam is a 345 GHz, 64-pixel heterodyne imaging array for the Heinrich Hertz Submillimeter Telescope (HHSMT). By integrating SIS mixer devices with Low Noise Amplifiers (LNAs) in 8 - 1x8 pixel modules, the size needed for the cryostat and the complexity of internal wiring is significantly reduced. All subsystems including the optics, cryostat, bias system, IF boxes, and spectrometer have been integrated for all 64 pixels. In the spring of 2012, SuperCam was installed on the HHSMT for an engineering run where it underwent system level tests and performed first light observations. In the fall of 2012 SuperCam will begin a 500 square degree survey of the Galactic Plane in 12CO J=3-2. This large-scale survey will help answer fundamental questions about the formation, physical conditions, and energetics of molecular clouds within the Milky Way. The data set will be available via the web to all interested researchers. © 2012 SPIE.
• Kulesa, C. A., Ashley, M. C., Augarten, Y., Bonner, C. S., Burton, M. G., Bycroft, L., Lawrence, J., Lesser, D. H., Loomis, J., Luong-Van, D. M., Martin, C. L., McLaren, C., Stapleton, S., Storey, J. W., Swift, B. J., Tothill, N. F., Walker, C. K., & Young, A. G. (2012). Opportunities for terahertz facilities on the high plateau. Proceedings of the International Astronomical Union, 8(S288), 256-263.
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  Abstract: While the summit of the Antarctic Plateau has long been expected to harbor the best ground-based sites for terahertz (THz) frequency astronomical investigations, it is only recently that direct observations of exceptional THz atmospheric transmission and stability have been obtained. These observations, in combination with recent technological advancements in astronomical instrumentation and autonomous field platforms, make the recognition and realization of terahertz observatories on the high plateau feasible and timely. Here, we will explore the context of terahertz astronomy in the era of Herschel, and the crucial role that observatories on the Antarctic Plateau can play. We explore the important scientific questions to which observations from this unique environment may be most productively applied. We examine the importance and complementarity of Antarctic THz astronomy in the light of contemporary facilities such as ALMA, CCAT, SOFIA and (U)LDB ballooning. Finally, building from the roots of THz facilities in Antarctica to present efforts, we broadly highlight future facilities that will exploit the unique advantages of the Polar Plateau and provide a meaningful, lasting astrophysical legacy. Copyright © 2013 International Astronomical Union.
• Padovan, S., Cable, M. L., Cumbers, J., Gentry, D., Harrison, T. N., Naidu, S., Parker, C. W., Potter, R., Reimuller, J., Shkolyar, S., Suer, T. K., Szalay, J., Trammell, H. J., Walker, C. C., & Whitten, J. (2012). Flyby of Io with Repeat Encounters (FIRE): Designing a New Frontiers mission to study the most volcanic body in the solar system. AGU Fall Meeting Abstracts, A2014.
• Scheller, M., Young, A. G., Yarborough, J. M., Moloney, J. V., Koch, S. W., Y., C., & Walker, C. K. (2012). Heterodyne detection of intracavity generated terahertz radiation. IEEE Transactions on Terahertz Science and Technology, 2(3), 271-277.
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  Abstract: Heterodyne detection is used to characterize the terahertz (THz) emission of a novel room-temperature continuous wave source based on difference frequency generation within the cavity of a dual-color vertical external cavity surface emitting laser. Employing the high intracavity intensities allows for the generation of mW powers in a wide frequency range within the terahertz spectrum. Experimental results of heterodyne detection are presented for the emission frequencies of 820 GHz and 1.9 THz using Schottky and hot electron bolometer mixers. Simultaneous emission of multiple narrow-line THz frequency components is observed. © 2012 IEEE.
• Scheller, M., Young, A., Yarborough, J. M., Moloney, J. V., Koch, S. W., D'Aubigny, C. D., & Walker, C. (2012). Intracavity generation of continuous wave terahertz radiation. Proceedings of SPIE - The International Society for Optical Engineering, 8240.
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  Abstract: We present a terahertz source based on difference frequency generation within a laser cavity. Combining the high intracavity intensities of a dual-color vertical external cavity surface emitting laser (VECSEL) with the high nonlinear coefficient of a periodically poled lithium niobate crystal enables the generation of milliwatts of continuous wave terahertz radiation. As the frequency spacing between the two simultaneously oscillating laser lines can be adjusted freely, the entire range of the terahertz gap can be covered. We discuss different approaches for the wavelength control of the dual-color laser sources as well as emission characteristics of the nonlinear crystal. Exemplarily, we chose the frequencies 1.9 THz to characterize the source in term of the beam shape, the linewidth and power scalability. To investigate the emitted THz spectrum, heterodyne detection is employed. © 2012 SPIE.
• Walker, C. K., & Groppi, C. E. (2012). Introduction to the special section on 22nd international symposium on space terahertz technology. IEEE Transactions on Terahertz Science and Technology, 2(1), 10-11.
• Zhou, X., Ashley, M. C., Cui, X., Feng, L., Gong, X., Jingyao, H. u., Jiang, Z., Kulesa, C. A., Lawrence, J. S., Liu, G., Luong-Van, D., Jun, M. a., MacRi, L. M., Meng, Z., Moore, A. M., Qin, W., Shang, Z., Storey, J. W., Sun, B., , Travouillon, T., et al. (2012). Progress and results from the chinese small telescope ARray (CSTAR). Proceedings of the International Astronomical Union, 8(S288), 231-238.
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  Abstract: In 2008 January the 24th Chinese expedition team successfully deployed the Chinese Small Telescope ARray (CSTAR) to Dome A, the highest point on the Antarctic plateau. CSTAR consists of four 14.5cm optical telescopes, each with a different filter (g, r, i and open) and has a 4.5°×4.5° field of view (FOV). Based on the CSTAR data, initial statistics of astronomical observational site quality and light curves of variable objects were obtained. To reach higher photometric quality, we are continuing to work to overcome the effects of uneven cirrus cloud cirrus, optical ghosts and intra-pixel sensitivity. The snow surface stability is also tested for further astronomical observational instrument and for glaciology studies. Copyright © 2013 International Astronomical Union.
• Scheller, M., Yarborough, J. M., Young, A., Fallahi, M., Moloney, J. V., Koch, M., Koch, S. W., D'Aubigny, C., & Walker, C. (2011). Continuous wave terahertz source for remote detection and as a local oscillator for THz receivers. IRMMW-THz 2011 - 36th International Conference on Infrared, Millimeter, and Terahertz Waves.
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  Abstract: We present initial results of a room-temperature continuous wave terahertz source as a local oscillator for a terahertz receiver or as an emitter for remote detection. The source is based on intracavity difference frequency generation within a dual color vertical external cavity surface emitting laser and delivers mW powers with a narrow linewidth. We present results at the frequencies of 820 GHz and 1.9 THz. © 2011 IEEE.
• Ashley, M. C., Allen, G., Bonner, C. S., Bradley, S. G., Cui, X., Everett, J. R., Feng, L., Gong, X., Hengst, S., Hu, J., Jiang, Z., Kulesa, C. A., Lawrence, J. S., Li, Y., Luong-Van, D., McCaughrean, M. J., Moore, A. M., Pennypacker, C., Qin, W., , Riddle, R., et al. (2010). PLATO-A robotic observatory for the Antarctic plateau. EAS Publications Series, 40, 79-84.
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  Abstract: PLATO is a fully-robotic observatory designed for operation in Antarctica. It generates its own electricity (about 1 kW), heat (sufficient to keep two 10-foot shipping containers comfortably above 0°C when the outside temperature is at -70°C, and connects to the internet using the Iridium satellite system (providing μ30 MB/day of data transfer). Following a successful first year of operation at Dome A during 2008, PLATO was upgraded with new instruments for 2009. © 2010 EAS, EDP Sciences.
• Bernasconi, P. N., Neufeld, D., Walker, C., Hollenbach, D., Kawamura, J., & Stuzki, J. (2010). The stratospheric terahertz observatory. Johns Hopkins APL Technical Digest (Applied Physics Laboratory), 28(3), 238-239.
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  Abstract: The Stratospheric Terahertz Observatory (STO) is a long-duration balloon (LDB) experiment designed to address a key problem in modern astrophysics. STO is a collaborative effort led by the University of Arizona and APL with a significant contribution from the JHU Department of Physics and Astronomy. It is a balloon-borne observing platform that will carry an 80-cm-aperture telescope in the stratosphere at an altitude of ̃120,000 feet. The telescope will feed a heterodyne receiver consisting of eight phonon-cooled hot-electron bolometer (HEB) mixers, optimized for the dominant interstellar cooling line [CII].
• Gong, X., Wang, L., Cui, X., Feng, L., Yuan, X., Ashley, M. C., Allen, G., Bonner, C. S., Bradley, S. G., Everett, J. R., Hengst, S., Hu, J., Jiang, Z., Kulesa, C. A., Lawrence, J. S., Li, Y., Luong-Van, D., McCaughrean, M. J., Moore, A. M., , Pennypacker, C., et al. (2010). Dome A site testing and future plans. EAS Publications Series, 40, 65-72.
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  Abstract: In January 2005, members of a Chinese expedition team were the first humans to visit Dome A on the Antarctic plateau, a site predicted to be one of the very best astronomical sites on earth. In 2006, the Chinese Center for Antarctic Astronomy (CCAA) was founded to promote the development of astronomy in Antarctica, especially at Dome A. CCAA has since taken part in two traverses to Dome A, organized by the Polar Research Institute of China (PRIC), in the austral summers of 2007/2008 and 2008/2009. These traverses resulted in the installation of many site-testing and science instruments, supported by the PLATO observatory. The Chinese Small Telescope ARray (CSTAR) has produced excellent results from Dome A. Our future plans include further site-testing work, and the following full-scale science instruments: three 0.5-m Antarctic Schmidt Telescopes (AST3), and a proposed 4-m telescope for wide-field infrared high spatial-resolution surveys. The first AST3 telescope is under construction and is scheduled for installation in 2011. © 2010 EAS, EDP Sciences.
• Groppi, C. E., Love, B., Underhill, M., & Walker, C. (2010). Automated CNC micromachining for integrated THz waveguide circuits. 21st International Symposium on Space Terahertz Technology 2010, ISSTT 2010, 291-294.
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  Abstract: Computer Numerically Controlled (CNC) machining of splitblock waveguide circuits has become the primary method of constructing terahertz waveguide circuits. The majority of these circuits have been made on traditional CNC machining centers or on custom-made laboratory machining systems. At both the University of Arizona and Arizona State University, we have developed techniques for machining splitblock waveguide circuits using purpose-built ultra high precision CNC machining centers designed for micromachining. These systems combine the automation of a traditional CNC machining center, including a high capacity toolchanger, workpiece and tool metrology systems and a large work volume, with the precision of custom laboratory systems. The systems at UofA and ASU are built by Kern Micro and deliver typical measured dimensional accuracies of 2-3 microns. Waveguide surface finish has been measured with a Veeco white light interferometric microscope to be Ra~75 nm. Tools of sizes between 25 microns and 10mm are available, with toolchanger capacities of 24-32 tools. The automated toolchanger and metrology systems allow a metal blank to be machined into the final part in one machining cycle, including both micromachining operations and traditional machining operations. This allows for perfect registration between all block features, in addition to very short cycle times. Even the most complicated blocks have machining cycle times of no more than a few hours. Workpiece and tool metrology systems also allow for fast setup times and straightforward part re-work. In addition, other high-throughput techniques such as palletization are enabled for the simultaneous manufacture of large numbers of blocks. Using these machines, we have successfully produced waveguide circuits at frequencies ranging from W-band to 2.7 THz, including highly integrated blocks. The Supercam project relied on these machines to produce integrated 8-pixel SIS mixer array units with integrated low noise amplifiers, bias tees and blind mate connectors. In addition, the 64-way corporate power divider used for LO multiplexing was machined using these techniques. This system consists of 17 split-block circuits containing E-plane power dividers, waveguide twists, diagonal horns and all associated flanges. The final system consists of a single WR-3 UG-387 input and an 8x8 array of 11mm aperture diagonal feedhorn outputs. This is one of the largest submillimeter waveguide circuits ever constructed. Future large focal plane arrays and other applications requiring highly integrated waveguide circuits will critically depend on this type of highly automated micromachining technology. We present the capabilities and machining process used with these machining centers, along with several waveguide circuits that were manufactured with this process including measured results from these circuits. Future directions for improving manufacturing quality and automation for large focal plane arrays will be discussed, including the use of palletization, insitu metrology, and automatic workpiece changers. Using these techniques, construction of the necessary waveguide blocks for even kilopixel class heterodyne array receivers should be realizable in a manageable time with high part yield and relatively low incremental cost.
• Groppi, C., Walker, C., Kulesa, C., Golish, D., Kloosterman, J., Weinreb, S., Jones, G., Barden, J., Mani, H., Kuiper, T., Kooi, J., & Lichtenberger, A. (2010). Testing and integration of supercam, a 64-pixel array receive for the 350 GHz atmospheric window. 21st International Symposium on Space Terahertz Technology 2010, ISSTT 2010, 319-324.
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  Abstract: We report on laboratory testing and telescope integration of SuperCam, a 64 pixel imaging spectrometer designed for operation in the astrophysically important 870 micron atmospheric window. SuperCam will be used to answer fundamental questions about the physics and chemistry of molecular clouds in the Galaxy and their direct relation to star and planet formation. The Supercam key project is a fully sampled Galactic plane survey covering over 500 square degrees of the Galaxy in 12CO(3-2) and 13CO(3-2) with 0.3 km/s velocity resolution. SuperCam will have several times more pixels than any existing spectroscopic imaging array at submillimeter wavelengths. The exceptional mapping speed that will result, combined with the efficiency and angular resolution provided by the HHT will make SuperCam a powerful instrument for probing the history of star formation in our Galaxy and nearby galaxies. SuperCam will be used to answer fundamental questions about the physics and chemistry of molecular clouds in the Galaxy and their direct relation to star and planet formation. Through Galactic surveys, particularly in CO and its isotopomers, the impact of Galactic environment on these phenomena will be realized. These studies will serve as "finder charts" for future focused research (e.g. with ALMA) and markedly improve the interpretation, and enhance the value of numerous contemporary surveys. In the past, all heterodyne focal plane arrays have been constructed using discrete mixers, arrayed in the focal plane. SuperCam reduces cryogenic and mechanical complexity by integrating multiple mixers and amplifiers into a single array module with a single set of DC and IF connectors. These modules are housed in a closed-cycle cryostat with a 1.5W capacity 4K cooler. The Supercam instrument is currently undergoing laboratory testing with four of the eight mixer array modules installed in the cryostat (32 pixels). Work is now underway to perform the necessary modifications at the 10m Heinrich Hertz Telescope to accept the Supercam system. Supercam will be installed in the cassegrain cabin of the HHT, including the optical system, IF processing, spectrometers and control electronics. Supercam will be integrated with the HHT during the 2009-2010 observing season with 32 pixels installed. The system will be upgraded to 64 pixels during the summer of 2010 after assembly of the four additional mixer modules is completed.
• Groppi, C., Walker, C., Kulesa, C., Golish, D., Kloosterman, J., Weinreb, S., Jones, G., Bardin, J., Mani, H., Kuiper, T., Kooi, J., Lichtenberger, A., Cecil, T., Puetz, P., Narayanan, G., & Hedden, A. (2010). Test and integration results from SuperCam: A 64-pixel array receiver for the 350 GHz atmospheric window. Proceedings of SPIE - The International Society for Optical Engineering, 7741.
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  Abstract: We report on both laboratory and telescope integration results from SuperCam, a 64 pixel imaging spectrometer designed for operation in the astrophysically important 870 micron atmospheric window. SuperCam will be used to answer fundamental questions about the physics and chemistry of molecular clouds in the Galaxy and their direct relation to star and planet formation. The SuperCam key project is a fully sampled Galactic plane survey covering over 500 square degrees of the Galaxy in 12CO(3-2) and 13CO(3-2) with 0.3 km/s velocity resolution In the past, all heterodyne focal plane arrays have been constructed using discrete mixers, arrayed in the focal plane. SuperCam reduces cryogenic and mechanical complexity by integrating multiple mixers and amplifiers into a single array module with a single set of DC and IF connectors. These modules are housed in a closed-cycle cryostat with a 1.5W capacity 4K cooler. The SuperCam instrument is currently undergoing laboratory testing with four of the eight mixer array modules installed in the cryostat (32 pixels). Work is now underway to perform the necessary modifications at the 10m Heinrich Hertz Telescope to accept the SuperCam system. SuperCam will be installed in the cassegrain cabin of the HHT, including the optical system, IF processing, spectrometers and control electronics. SuperCam will be integrated with the HHT during the 2009-2010 observing season with 32 pixels installed. The system will be upgraded to 64 pixels during the summer of 2010 after assembly of the four additional mixer modules is completed. © 2010 SPIE.
• Lockwood, N. P., Cartwright, K. L., Gensheimer, P. D., Shiffler, D. A., D'Aubigny, C. Y., Walker, C. K., Young, A., Fairchild, S. B., & Maruyama, B. (2010). 1.5: Development of field emission cathodes, electron gun and a slow wave structure for a terahertz traveling wave tube. 2010 IEEE International Vacuum Electronics Conference, IVEC 2010, 25-26.
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  Abstract: High power terahertz (THz) sources and amplifiers hold the potential to greatly improve remote sensing and high bandwidth communication. To enable these applications, a Traveling Wave Tube (TWT) operating at 0.22 THz and a multi-cathode Field Emission (FE) electron gun are developed and characterized using a Particle-in-Cell simulation. Three candidate high current density cathode materials, Halfnium Carbide (HfC), carbon fibers, and Carbon Nanotubes (CNTs) were tested, characterized and their emission properties compared and used to verify simulations. A current of 3.0 mAmps for a single 100 micron diameter single walled nanotube rope was experimentally achieved and used as the basis of the FE gun design. Simulations of the FE gun and THz TWT were coupled and the effects of multiple and single tip FE gun beam characteristics on the TWT gain, bandwidth, and efficiencies are examined for several beam optic configurations. © 2010 IEEE.
• Narayanan, D., Groppi, C. E., Kulesa, C. A., & Walker, C. K. (2010). Erratum: Warm-dense molecular gas in the ISM of starbursts, LIRGs, and ULIRGs (Astrophysical Journal (2005) 630 (269)). Astrophysical Journal Letters, 721(1), 921-.
• Tothill, N. F., Kulesa, C. A., Walker, C. K., Yang, J., Ashley, M. C., Lawrence, J. S., Luong-Van, D., Storey, J. W., Yang, H., Zhou, X., Zhu, Z., Bardin, J., Cui, X., Everett, J., Feng, L., Golish, D., Gong, X., Hu, J., Jacobs, K., , Jiang, Z., et al. (2010). THz astrophysics from Dome A. EAS Publications Series, 40, 275-280.
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  Abstract: The THz spectral region includes a number of important transitions which allow us to trace the evolution of the interstellar medium. Because of the opacity of the atmosphere in this spectral range, the best sites for ground-based THz observations are on the Antarctic Plateau; of these sites, Dome A is expected to be the best. THz survey science can be carried out with small telescopes, easing logistical constraints. By deploying a submillimetre-wave tipper/telescope to Dome A, we have trialled several technologies for such an instrument, and we are able to test whether the site quality is sufficient for THz surveys. © 2010 EAS, EDP Sciences.
• Walker, C., Kulesa, C., Bernasconi, P., Eaton, H., Rolander, N., Groppi, C., Kloosterman, J., Cottam, T., Lesser, D., Martin, C., Stark, A., Neufeld, D., Lisse, C., Hollenbach, D., Kawamura, J., Goldsmith, P., Langer, W., Yorke, H., Sterne, J., , Skalare, A., et al. (2010). The Stratospheric THz Observatory (STO). Proceedings of SPIE - The International Society for Optical Engineering, 7733(PART 1).
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  Abstract: The Stratospheric TeraHertz Observatory (STO) is a NASA funded, Long Duration Balloon (LDB) experiment designed to address a key problem in modern astrophysics: understanding the Life Cycle of the Interstellar Medium (ISM). STO will survey a section of the Galactic plane in the dominant interstellar cooling line [C II] (1.9 THz) and the important star formation tracer [N II] (1.46 THz) at ∼1 arc minute angular resolution, sufficient to spatially resolve atomic, ionic and molecular clouds at 10 kpc. STO itself has three main components; 1) an 80 cm optical telescope, 2) a THz instrument package, and 3) a gondola [1]. Both the telescope and gondola have flown on previous experiments [2,3]. They have been reoptimized for the current mission. The science flight receiver package will contain four [CII] and four [NII] HEB mixers, coupled to a digital spectrometer. The first engineering test flight of STO was from Ft. Sumner, NM on October 15, 2009. The ∼30 day science flight is scheduled for December 2011. © 2010 SPIE.
• Walker, C., Kulesa, C., Kloosterman, J., Lesser, D., Cottam, T., Groppi, C., Zmuidzinas, J., Edgar, M., Radford, S., Goldsmith, P., Langer, W., Yorke, H., Kawamura, J., Mehdi, I., Hollenbach, D., Stutzki, J., Huebers, H., Gao, J. R., & Martin, C. (2010). Large format heterodyne arrays for observing far-infrared lines with SOFIA. Proceedings of SPIE - The International Society for Optical Engineering, 7741.
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  Abstract: In the wavelength regime between 60 and 300 microns there are a number of atomic and molecular emission lines that are key diagnostic probes of the interstellar medium. These include transitions of [CII], [NII], [OI], HD, H 2D+, OH, CO, and H2O, some of which are among the brightest global and local far-infrared lines in the Galaxy. In Giant Molecular Clouds (GMCs), evolved star envelopes, and planetary nebulae, these emission lines can be extended over many arc minutes and possess complicated, often self absorbed, line profiles. High spectral resolution (R> 105) observations of these lines at sub-arcminute angular resolution are crucial to understanding the complicated interplay between the interstellar medium and the stars that form from it. This feedback is central to all theories of galactic evolution. Large format heterodyne array receivers can provide the spectral resolution and spatial coverage to probe these lines over extended regions. The advent of large format (~100 pixel) spectroscopic imaging cameras in the far-infrared (FIR) will fundamentally change the way astronomy is performed in this important wavelength regime. While the possibility of such instruments has been discussed for more than two decades, only recently have advances in mixer and local oscillator technology, device fabrication, micromachining, and digital signal processing made the construction of such instruments tractable. These technologies can be implemented to construct a sensitive, flexible, heterodyne array facility instrument for SOFIA. The instrument concept for StratoSTAR: Stratospheric Submm/THz Array Receiver includes a common user mounting, control system, IF processor, spectrometer, and cryogenic system. The cryogenic system will be designed to accept a frontend insert. The frontend insert and associated local oscillator system/relay optics would be provided by individual user groups and reflect their scientific interests. Rapid technology development in this field makes SOFIA the ideal platform to operate such a modular, continuously evolving instrument. © 2010 SPIE.
• Walker, C., Kulesa, C., Koostermann, J., Cottam, T., Groppi, C., Bernasconi, P., Eaton, H., Rolander, N., Neufeld, D., Lisse, C., Stark, A., Hollenbach, D., Kawamura, J., Goldsmith, P., Langer, W., Yorke, H., Sterne, J., Skalare, A., Mehdi, I., , Weinreb, S., et al. (2010). The stratospheric THz observatory (STO): 1st test flight. 21st International Symposium on Space Terahertz Technology 2010, ISSTT 2010, 29-32.
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  Abstract: The Stratospheric TeraHertz Observatory (STO) is a NASA funded, Long Duration Balloon (LDB) experiment designed to address a key problem in modern astrophysics: understanding the Life Cycle of the Interstellar Medium (ISM). STO will survey a section of the Galactic plane in the dominant interstellar cooling line [C II] (1.9 THz) and the important star formation tracer [N II] (1.46 THz) at ~1 arc minute angular resolution, sufficient to spatially resolve atomic, ionic and molecular clouds at 10 kpc. STO itself has three main components; 1) an 80 cm optical telescope, 2) a THz instrument package, and 3) a gondola [1]. Both the telescope and gondola have flown on previous experiments [2,3]. They have been reoptimized for the current mission. The science flight receiver package will contain four [CII] and four [NII] HEB mixers, each with its own digital spectrometer. The first engineering test flight of STO was from Ft. Sumner, NM on October 15, 2009. Test flight instrumentation was used to evaluate STO's ability to point and track using gyroscopes/star cameras with a loadvarying, cryogenic system onboard. The science flight receiver electronics and control system were also tested.
• Xu, X., Narayanan, D., & Walker, C. (2010). Molecular disk properties in early-type galaxies. Astrophysical Journal Letters, 721(2 PART 2), L112-L116.
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  Abstract: We study the simulated CO emission from elliptical galaxies formed in the mergers of gas-rich disk galaxies. The cold gas not consumed in the merger-driven starburst quickly resettles into a disk-like configuration. By analyzing a variety of arbitrary merger orbits that produce a range of fast- to slow-rotating remnants, we find that molecular disk formation is a fairly common consequence of gas-rich galaxy mergers. Hence, if a molecular disk is observed in an early-type merger remnant, it is likely the result of a "wet merger" rather than a "dry merger."We compare the physical properties from our simulated disks (e.g., size and mass) and find reasonably good agreement with recent observations. Finally, we discuss the detectability of these disks as an aid to future observations. © 2010 The American Astronomical Society. All rights reserved.
• Y., C., Walker, C. K., Young, A. G., Gensheimer, P., Golish, D. R., & Groppi, C. E. (2010). Terahertz traveling wave tube amplifiers as high-power local oscillators for large heterodyne receiver arrays. Proceedings of SPIE - The International Society for Optical Engineering, 7741.
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  Abstract: The size of existing and projected submillimeter heterodyne receiver arrays is rapidly increasing. As receiver arrays grow ever larger, the local oscillator power they require increases as well. We have developed Terahertz (THz) Traveling Wave Tube Amplifiers (TWTA) that promise to provide more than enough power in the 200 to 700 GHz frequency range to pump the largest arrays being planned for submillimeter telescopes. This technology combines revolutionary carbon nanotube cathodes and electron gun design with unique software modeling and micro-fabrication capabilities. We review key enabling technologies that make this breakthrough possible, present the design, realization, computer models and preliminary results of the THz TWT we have fabricated at 220 and 350 GHz. © 2010 SPIE.
• Yang, H., Kulesa, C. A., Walker, C. K., Tothill, N. F., Yang, J., Ashley, M. C., Cui, X., Feng, L., Lawrence, J. S., Luong-Van, D., Mccaughrean, M. J., Storey, J. W., Wang, L., Zhou, X., & Zhu, Z. (2010). Exceptional terahertz transparency and stability above dome A, Antarctica. Publications of the Astronomical Society of the Pacific, 122(890), 490-494.
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  Abstract: We present the first direct measurements of the terahertz atmospheric transmission above Dome A, the highest point on the Antarctic plateau at an elevation of 4.1 km. The best-quartile atmospheric transmission during the Austral winter is 80% at a frequency of 661 GHz (453 μm), corresponding to a precipitable water vapor column of 0.1 mm. Daily averages as low as 0.025 mm were observed. The Antarctic atmosphere is very stable, and excellent observing conditions generally persist for many days at a time. The exceptional conditions over the high Antarctic plateau open new far-infrared spectral windows to ground-based observation. These windows contain important spectral-line diagnostics of star formation and the interstellar medium which would otherwise only be accessible to airborne or space telescopes. © 2010. The Astronomical Society of the Pacific. All rights reserved.
• Zhou, X., Fan, Z., Jiang, Z., Ashley, M. C., Cui, X., Feng, L., Gong, X., Jingyao, H. u., Kulesa, C. A., Lawrence, J. S., Liu, G., Luong-Van, D., Jun, M. a., Moore, A. M., Qin, W., Shang, Z., Storey, J. W., Sun, B., Travouillon, T., , Walker, C. K., et al. (2010). The first release of the CSTAR point source catalog from dome A, Antarctica. Publications of the Astronomical Society of the Pacific, 122(889), 347-353.
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  Abstract: In 2008 January the twenty-fourth Chinese expedition team successfully deployed the Chinese Small Telescope ARray (CSTAR) to Dome A, the highest point on the Antarctic plateau. CSTAR consists of four 14.5 cm optical telescopes, each with a different filter (g, r, i, and open) and has a 4:5° × 4:5° field of view (FOV). It operates robotically as part of the Plateau Observatory, PLATO, with each telescope taking an image every ∼30 s throughout the year whenever it is dark. During 2008, CSTAR 1 performed almost flawlessly, acquiring more than 0.3 million i-band images for a total integration time of 1728 hr during 158 days of observations. For each image taken under good sky conditions, more than 10,000 sources down to ∼16 th magnitude could be detected. We performed aperture photometry on all the sources in the field to create the catalog described herein. Since CSTAR has a fixed pointing centered on the south celestial pole (decl: = -90°), all the sources within the FOVof CSTAR were monitored continuously for several months. The photometric catalog can be used for studying any variability in these sources, and for the discovery of transient sources such as supernovae, gamma-ray bursts, and minor planets. © 2010. The Astronomical Society of the Pacific.
• Zou, H., Zhou, X., Jiang, Z., AshLey, M. C., Cui, X., Feng, L., Gong, X., Jingyao, H. u., KuLesa, C. A., Lawrence, J. S., Liu, G., Luong-Van, D., Jun, M. a., Moore, A. M., Pennypacker, C. R., Qin, W., Shang, Z., Storey, J. W., Sun, B., , Travouillon, T., et al. (2010). Sky brightness and transparency in the i-band at Dome a, Antarctica. Astronomical Journal, 140(2), 602-611.
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  Abstract: The i-band observing conditions at Dome A on the Antarctic plateau have been investigated using data acquired during 2008 with the Chinese Small Telescope Array. The sky brightness, variations in atmospheric transparency, cloud cover, and the presence of aurorae are obtained from these images. The median sky brightness of moonless clear nights is 20.5 mag arcsec-2 in the SDSS i band at the south celestial pole (which includes a contribution of about 0.06 mag from diffuse Galactic light). The median over all Moon phases in the Antarctic winter is about 19.8 mag arcsec-2. There were no thick clouds in 2008. We model contributions of the Sun and the Moon to the sky background to obtain the relationship between the sky brightness and transparency. Aurorae are identified by comparing the observed sky brightness to the sky brightness expected from this model. About 2% of the images are affected by relatively strong aurorae. © 2010. The American Astronomical Society. All rights reserved.
• Zou, H., Zhou, X., Jiang, Z., Ashley, M. C., Cui, X., Feng, L., Gong, X., Jingyao, H. u., Kulesa, C. A., Lawrence, J. S., Liu, G., Luong-Van, D., Jun, M. a., Moore, A. M., Pennypacker, C. R., Qin, W., Shang, Z., Storey, J. W., Sun, B., , Travouillon, T., et al. (2010). Erratum: The sky brightness and transparency in i-band at dome a, Antarctica (Astronomical Journal (2010) 140 (602)). Astronomical Journal, 140(6), 2146-.
• Golish, D., Swain, M., Walker, C., & D'Aubigny, C. D. (2009). An integrated optics beam combiner realized with magic-tees. Journal of Infrared, Millimeter, and Terahertz Waves, 30(8), 884-898.
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  Abstract: Integrated Optics (IO) beam combiners implemented in waveguide are a compact, low-mass alternative to beam combiners designed using quasi-optical techniques. Here we present modeling (performed in CST Microwave Studio at 30 THz) and experimental results (done at X-Band) for IO beam combiners based on "magic-T" waveguide structures. A magic-T can be configured either for beam combining or nulling. This work was originally done in support of the Terrestrial Planet Finder (TPF) project, and has a wide variety of potential interferometric applications. © 2009 Springer Science+Business Media, LLC.
• Hedden, A., Li, H., Tong, E., Paine, S., Blundell, R., Kawamura, J., Groppi, C., Kulesa, C., Walker, C., Lange, G. D., Mani, H., & Weinreb, S. (2009). A field-deployed 810 GHz receiver incorporating a superconducting mixer developed for Herschel space telescope and a SiGe low noise amplifier. IEEE MTT-S International Microwave Symposium Digest, 949-952.
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  Abstract: We have constructed an 810 GHz receiver system incorporating a HIFI Band-3 superconductor-insulator-superconductor (SIS) mixer developed for Herschel space observatory and a wide-band SiGe low noise amplifier (LNA) designed at Caltech. The instrument is currently installed at the RLT telescope (elevation 5500 m) in northern Chile. Hot/cold (280K/72K) load measurements performed at the telescope yield noise temperatures of 225 K (Y-factor = 1.7) including receiver optics. First-light observations indicate that the receiver is highly sensitive and functions stably. We present details of the receiver system, its performance at the telescope, and first-light observations with a Herschel mixer. © 2009 IEEE.
• Tothill, N. F., Löhr, A., Parshley, S. C., Stark, A. A., Lane, A. P., Harnett, J. I., Wright, G. A., Walker, C. K., Bourke, T. L., & Myers, P. C. (2009). Large-scale CO maps of the Lupus Molecular Cloud Complex. Astrophysical Journal, Supplement Series, 185(1), 98-123.
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  Abstract: Fully sampled degree-scale maps of the 13CO 2-1 and CO 4-3 transitions toward three members of the Lupus Molecular Cloud Complex - Lupus I, III, and IV - trace the column density and temperature of the molecular gas. Comparison with IR extinction maps from the c2d project requires most of the gas to have a temperature of 8-10 K. Estimates of the cloud mass from 13CO emission are roughly consistent with most previous estimates, while the line widths are higher, around 2 km s-1. CO 4-3 emission is found throughout Lupus I, indicating widespread dense gas, and toward Lupus III and IV. Enhanced line widths at the NW end and along the edge of the B 228 ridge in Lupus I, and a coherent velocity gradient across the ridge, are consistent with interaction between the molecular cloud and an expanding H I shell from the Upper-Scorpius subgroup of the Sco-Cen OB Association. Lupus III is dominated by the effects of two HAe/Be stars, and shows no sign of external influence. Slightly warmer gas around the core of Lupus IV and a low line width suggest heating by the Upper-Centaurus-Lupus subgroup of Sco-Cen, without the effects of an H I shell. © 2009. The American Astronomical Society. All rights reserved.
• Yang, H., Allen, G., Ashley, M. C., Bonner, C. S., Bradley, S., Cui, X., Everett, J. R., Feng, L., Gong, X., Hengst, S., Hu, J., Jiang, Z., Kulesa, C. A., Lawrence, J. S., Li, Y., Luong-Van, D., McCaughrean, M. J., Moore, A. M., Pennypacker, C., , Qin, W., et al. (2009). The PLATO dome a site-testing observatory: Instrumentation and first results. Publications of the Astronomical Society of the Pacific, 121(876), 174-184.
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  Abstract: The PLATeau Observatory (PLATO) is an automated self-powered astrophysical observatory that was deployed to Dome A, the highest point on the Antarctic plateau, in 2008 January. PLATO consists of a suite of site-testing instruments designed to quantify the benefits of the Dome A site for astronomy, and science instruments designed to take advantage of the unique observing conditions. Instruments include CSTAR, an array of optical telescopes for transient astronomy; Gattini, an instrument to measure the optical sky brightness and cloud cover statistics; DASLE, an experiment to measure the statistics of the meteorological conditions within the near-surface layer; Pre-HEAT, a submillimeter tipping radiometer measuring the atmospheric transmission and water vapor content and performing spectral line imaging of the Galactic plane; and Snodar, an acoustic radar designed to measure turbulence within the near-surface layer. PLATO has run completely unattended and collected data throughout the winter 2008 season. Here we present a detailed description of the PLATO instrument suite and preliminary results obtained from the first season of operation. © 2009. The Astronomical Society of the Pacific. All rights reserved.
• Bussmann, R. S., Narayanan, D., Shirley, Y. L., Juneau, S., Wu, J., Solomon, P. M., Bout, P. V., Moustakas, J., & Walker, C. K. (2008). The star formation rate-dense gas relation in galaxies as measured by HCN(3-2) emission. Astrophysical Journal, 681(2 PART 2), L73-L76.
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  Abstract: We present observations made with the 10 m Heinrich Hertz Submillimeter Telescope of HCN(3-2) emission from a sample of 30 nearby galaxies ranging in infrared luminosity from 1010 to 1012.5Lȯ and HCN(3-2) luminosity from 106 to 109 K km s -1 pc2. We examine the correlation between the infrared luminosity and HCN(3-2) luminosity and And that the best-fit linear regression has a slope (in log-log space) of 0.74 ± 0.12. Including recently published data from Graciá-Carpio et al. tightens the constraints on the best-fit slope to 0.79 ± 0.09. This slope below unity suggests that the HCN(3-2) molecular line luminosity is not linearly tracing the amount of dense gas. Our results are consistent with predictions from recent theoretical models that find slopes below unity when the line luminosity depends on the average gas density with a power-law index greater than a KennicuttSchmidt index of 1.5. © 2008, The American Astronomical Society. All rights reserved.
• Groppi, C., Walker, C., Kulesa, C., Golish, D., Kloosterman, J., Pütz, P., Weinreb, S., Kuiper, T., Kooi, J., Jones, G., Bardin, J., Mani, H., Lichtenberger, A., Cecil, T., Hedden, A., & Narayanan, G. (2008). SuperCam: A 64 pixel heterodyne imaging spectrometer. Proceedings of SPIE - The International Society for Optical Engineering, 7020.
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  Abstract: We report on the development of SuperCam, a 64 pixel imaging spectrometer designed for operation in the astrophysically important 870 micron atmospheric window. SuperCam will be used to answer fundamental questions about the physics and chemistry of molecular clouds in the Galaxy and their direct relation to star and planet formation. The Supercam key project is a fully sampled Galactic plane survey covering over 500 square degrees of the Galaxy in 12CO(3-2) and 13CO(3-2) with 0.3 km/s velocity resolution.
• Groppi, C., Walker, C., Kulesa, C., Golish, D., Kloosterman, J., Pütz, P., Weinreb, S., Kuiper, T., Kooi, J., Jones, G., Bardin, J., Manidz, H., Lichtenberger, A., Cecil, T., Hedden, A., & Narayanan, G. (2008). Large format heterodyne arrays for terahertz astronomy. Proceedings of the 19th International Symposium on Space Terahertz Technology, ISSTT 2008, 154-161.
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  Abstract: For future ground, airborne and space based single aperture telescopes, multipixel heterodyne imaging arrays are necessary to take full advantage of platform lifetime, and facilitate science requiring wide field spectral line imaging. A first generation of heterodyne arrays with ~10 pixels has already been constructed, i.e. CHAMP, SMART, HERA, DesertStar, PoleStar and HARP. Our group is now constructing SuperCam, a 64 pixel heterodyne array for operation in the 350 GHz atmospheric window. This instrument will realize another order of magnitude increase in array pixel count. Several new techniques were used for SuperCam to maximize integration and modularity. Unlike other SIS array receivers, SuperCam is built around 8 pixel linear mixer modules, rather than independent mixer blocks. These modules house 8 single ended waveguide mixers with SOI substrate SIS devices. Each device is tab bonded to a MMIC based LNA. These modules dissipate only 8 mW of heat, while still maintaining 5 K IF noise temperature and 32 dB gain. Blind mate IF and DC connectors allow each module to be inserted in or removed from the focal plane as a unit. The modules are machined using a state-of-the-art CNC micromilling machine acquired specifically for this project. IF signals are processed by 8 channel IF downconverter boards, which provide gain, baseband downconversion and IF total power monitoring. A real-time FFT spectrometer implemented with high speed ADCs and Xilinx 4 FPGAs produce spectra of the central 250 MHz of each channel at 0.25 km/s spectral resolution. For arrays with an additional order of magnitude increase in pixel count, several additional technical problems must be overcome. Kilopixel arrays will require advances in device fabrication, cryogenics, micromachining, IF processing and spectrometers. In addition, seemingly straightforward receiver systems will require new approaches to realize a kilopixel heterodyne array with manageable complexity and cost. Wire count and 4K heat load must all be reduced significantly compared to SuperCam. IF and DC cabling and interconnects may be replaced with multiconductor microstrip or stripline ribbon. Parallel biasing of LNAs, magnets and even SIS devices is feasible if device uniformity is good enough. IF processing will require further integration, possibly with integrated MMIC chips containing all parts of a IF downconversion chain. Continued advances in FFT spectrometers could allow processing many hundreds of gigahertz of IF bandwidth for a realizable cost. We present results from final SuperCam receiver integration and testing, and concepts for expanding heterodyne arrays to kilopixel scales in the future.
• Krejny, M., Chuss, D., D'Aubigny, C. D., Golish, D., Houde, M., Hui, H., Kulesa, C., Loewenstein, R. F., Moseley, S. H., Novak, G., Voellmer, G., Walker, C., & Wollack, E. (2008). The hertz/VPM polarimeter: Design and first light observations. Applied Optics, 47(24), 4429-4440.
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  PMID: 18716650;Abstract: We present first results of Hertz/VPM, the first submillimeter polarimeter employing the dual Variable-delay Polarization Modulator (dual-VPM). This device differs from previously used polarization modulators in that it operates in translation, rather than mechanical rotation. We discuss the basic theory behind this device and its potential advantages over the commonly used half-wave plate. The dual-VPM was tested both at the Submillimeter Telescope Observatory and in the laboratory. In each case we present a detailed description of the setup. We discovered that properties of the VPM wire grids (diameter and spacing) caused behavior that differs from theoretical predictions for ideal wire grid performance. By modifying the polarimeter settings to compensate for this behavior, we found that the dual-VPM system is robust, operating with high efficiency and low instrumental polarization. This device is well suited for air- and space-borne applications. © 2008 Optical Society of America.
• Kulesa, C. A., Walker, C. K., Schein, M., Golish, D., Tothill, N., Siegel, P., Weinreb, S., Jones, G., Bardin, J., Jacobs, K., Martin, C. L., Storey, J., Ashley, M., Lawrence, J., Luong-Van, D., Everett, J., Wang, L., Feng, L., Zhu, Z., , Yan, J., et al. (2008). Dome A, Antarctica: Prospectives for terahertz astronomy from the ground. Proceedings of the 19th International Symposium on Space Terahertz Technology, ISSTT 2008, 354-360.
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  Abstract: Over a decade of site testing and operation of submillimeter telescopes has shown that the high Antarctic Plateau (South Pole) and Chilean Atacama desert (Chajnantor) are exceptional ground-based sites for submillimeter and terahertz astronomy. The highest sites at both locations (Dome A and the Chajnantor and Sairecabur summits) show great promise in yielding even more favorable conditions. To test the conditions at Dome A, we have deployed Pre-HEAT, a 20 cm aperture submillimeter-wave telescope with a 660 GHz (450 micron) Schottky diode heterodyne receiver and digital FFT spectrometer for the Plateau Observatory (PLATO) developed by the University of New South Wales. In January 2008 it was deployed to Dome A, the summit of the Antarctic plateau, as part of a scientific traverse led by the Polar Research Institute of China. Dome A may be one of the best sites in the world for ground based Terahertz astronomy, based on the exceptionally cold, dry and stable conditions which prevail there. Pre-HEAT is measuring the 450 micron sky opacity at Dome A and mapping the Galactic Plane in the 13CO J=6-5 line, constituting the first submillimeter measurements from Dome A. It is field-testing many of the key technologies for its namesake, a successor mission called HEAT: the High Elevation Antarctic Terahertz telescope. Exciting prospects for submillimeter astronomy from Dome A and the status of Pre-HEAT will be presented.
• Kulesa, C. A., Walker, C. K., Schein, M., Golish, D., Tothill, N., Siegel, P., Weinreb, S., Jones, G., Bardin, J., Jacobs, K., Martin, C. L., Storey, J., Ashley, M., Lawrence, J., Luong-Van, D., Everett, J., Wang, L., Feng, L., Zhu, Z., , Yan, J., et al. (2008). Pre-HEAT: Submillimeter site testing and astronomical spectra from dome a, antarctica. Proceedings of SPIE - The International Society for Optical Engineering, 7012.
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  Abstract: Pre-HEAT is a 20 cm aperture submillimeter-wave telescope with a 660 GHz (450 micron) Schottky diode heterodyne receiver and digital FFT spectrometer for the Plateau Observatory (PLATO) developed by the University of New South Wales. In January 2008 it was deployed to Dome A, the summit of the Antarctic plateau, as part of a scientific traverse led by the Polar Research Institute of China and the Chinese Academy of Sciences. Dome A may be one of the best sites in the world for ground based Terahertz astronomy, based on the exceptionally cold, dry and stable conditions which prevail there. Pre-HEAT is measuring the 450 micron sky opacity at Dome A and mapping the Galactic Plane in the 13CO J=6-5 line, constituting the first submillimeter measurements from Dome A. It is field-testing many of the key technologies for its namesake - a successor mission called HEAT: the High Elevation Antarctic Terahertz telescope. Exciting prospects for submillimeter astronomy from Dome A and the status of Pre-HEAT will be presented.
• Lawrence, J. S., Allen, G. R., Ashley, M. C., Bonner, C., Bradley, S., Cui, X., Everett, J. R., Feng, L., Gong, X., Hengst, S., Hu, J., Jiang, Z., Kulesa, C. A., Li, Y., Luong-Van, D., Moore, A. M., Pennypacker, C., Qin, W., Riddle, R., , Shang, Z., et al. (2008). The PLATO antarctic site testing observatory. Proceedings of SPIE - The International Society for Optical Engineering, 7012.
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  Abstract: Over a decade of site testing in Antarctica has shown that both South Pole and Dome C are xceptional sites for astronomy, with certain atmospheric conditions superior to those at existing mid-latitude sites. However, the highest point on the Antarctic plateau, Dome A, is expected to experience colder atmospheric temperatures, lower wind speeds, and a turbulent boundary layer that is confined closer to the ground. The Polar Research Institute of China, who were the first to visit the Dome A site in January 2005, plan to establish a permanently manned station there within the next decade. As part of this process they conducted a second expedition to Dome A, arriving via overland traverse in January 2008. This traverse involved the delivery and installation of the PLATeau Observatory (PLATO). PLATO is an automated self-powered astrophysical site testing observatory, developed by the University of New South Wales. A number of international institutions have contributed site testing instruments measuring turbulence, optical sky background, and sub-millimetre transparency. In addition, a set of science instruments are providing wide-field high time resolution optical photometry and terahertz imaging of the Galaxy. We present here an overview of the PLATO system design and instrumentation suite.
• Narayanan, D., Cox, T. J., Shirley, Y., Davé, R., Hernquist, L., & Walker, C. K. (2008). Molecular star formation rate indicators in galaxies. Astrophysical Journal, 684(2), 996-1008.
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  Abstract: We derive a physical model for the observed relations between star formation rate (SFR) and molecular line (CO and HCN ) emission in galaxies and show how these observed relations are reflective of the underlying star formation law. We do this by combining 3D non-LTE radiative transfer calculations with hydrodynamic simulations of isolated disk galaxies and galaxy mergers. We demonstrate that the observed SFR-molecular line relations are driven by the relationship between molecular line emission and gas density and anchored by the index of the underlying Schmidt law controlling the SFR in the galaxy. Lines with low critical densities (e.g., CO J = 1-0) are typically thennalized and trace the gas density faithfully. In these cases, the SFR will be related to line luminosity with an index similar to the Schmidt law index. Lines with high critical densities greater than the mean density of most of the emitting clouds in a galaxy (e.g., CO J = 3-2, HCN J = 1-0) will have only a small amount of thermalized gas and consequently a superlinear relationship between molecular line luminosity (Imol) and mean gas density (ñ). This results in an SFR-line luminosity index less than the Schmidt index for high critical density tracers. One observational consequence of this is a significant redistribution of light from the small pockets of dense, thermalized gas to diffuse gas along the line of sight, and prodigious emission from subthermally excited gas. At the highest star formation rates, the SFR-Lmol slope tends to the Schmidt index, regardless of the molecular transition. The fundamental relation is the Kennicutt-Schmidt law, rather than the relation between SFR and molecular line luminosity. Our model for SFR-molecular line relations quantitatively reproduces the slopes of the observed SFR-CO (J = 1-0), CO (J = 3-2), and HCN (J = 1-0) relations when a Schmidt law with index of ∼1.5 describes the SFR. We use these results to make imminently testable predictions for the SFR-molecular line relations of unobserved transitions. © 2008. The American Astronimical Society. All rights reserved.
• Narayanan, D., Yuexing, L. i., Cox, T. J., Hernquist, L., Hopkins, P., Chakrabarti, S., Davé, R., Matteo, T. D., Gao, L., Kulesa, C., Robertson, B., & Walker, C. K. (2008). The nature of co emission from z ∼ 6 quasars. Astrophysical Journal, Supplement Series, 174(1), 13-30.
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  Abstract: We investigate the nature of molecular gas emission from z ∼ 6 quasars via the commonly observed tracer of H2, carbon monoxide (CO). We achieve this by combining non-LTE radiative transfer calculations with merger-driven models of z ∼ 6 quasar formation that arise naturally in A cold dark matter structure formation simulations. Motivated by observational constraints, we consider four representative z ∼ 6 quasars formed in the halo mass range ∼ 1012-1013 M⊙ from different merging histories. Our main results are as follows. We find that, owing to massive starbursts and funneling of dense gas into the nuclear regions of merging galaxies, the CO is highly excited during both the hierarchical buildup of the host galaxy and the quasar phase, and the CO flux density peaks between J = 5 and 8. The CO morphology of z ∼ 6 quasars often exhibits multiple CO emission peaks which arise from molecular gas concentrations which have not yet fully coalesced. Both of these results are found to be consistent with the sole CO detection at z ∼ 6, in quasar J1148+5251. Quasars which form at z ∼ 6 display a large range of sight line-dependent line widths. The sight line dependencies are such that the narrowest line widths are when the rotating molecular gas associated with the quasar is viewed face-on (when the LB is largest) and broadest when the quasar is seen edge-on (and the LB is lowest). Thus, we find that for all models selection effects exist such that quasars selected for optical luminosity are preferentially seen to be face-on which may result in CO detections of optically luminous quasars at z ∼ 6 having line widths narrower than the median. The mean sight line-averaged line width is found to be reflective of the circular velocity of the host halo and thus scales with halo mass. For example, the mean line width for the ∼1012 M⊙ halo is σ ∼ 300 km s-1, while the median for the ∼1013 M⊙ quasar host is σ ∼ 650 km s-1. Depending on the host halo mass, approximately 2%-10% of sight lines in our modeled quasars are found to have narrow line widths compatible with observations of J1148+5251. When considering the aforementioned selection effects, these percentages increase to 10%-25% for quasars selected for optical luminosity. When accounting for both temporal evolution of CO line widths in galaxies, as well as the redshift evolution of halo circular velocities, these models can self-consistently account for the observed line widths of both submillimeter galaxies and quasars at z ∼ 2. Finally, we find that the dynamical mass derived from the mean sight line-averaged line widths provide a good estimate of the total mass and allow for a massive molecular reservoir, supermassive black hole, and stellar bulge, consistent with the local MBH-Mbul relation. © 2008. The American Astronomical Society. All rights reserved.
• Narayanan, E., Cox, T. J., Kelly, B., Davé, R., Hernquist, L., Matteo, T. D., Hopkins, P. F., Kulesa, C., Robertson, B., & Walker, C. K. (2008). The role of galactic winds on molecular gas emission from galaxy mergers. Astrophysical Journal, Supplement Series, 176(2), 331-354.
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  Abstract: Galactic winds from starbursts and active galactic nuclei (AGNs) are thought to play an important role in driving galaxies along the starburst-AGN sequence. Here, we assess the impact of these winds on the CO emission from galaxy mergers and, in particular, search for signatures of starburst and AGN-feedback-driven winds in the simulated CO morphologies and emission-line profiles. We do so by combining a three-dimensional non-LTE molecular line radiative transfer code with smoothed particle hydrodynamic (SPH) simulations of galaxy mergers that include prescriptions for star formation, black hole growth, a multiphase interstellar medium (ISM), and the winds associated with star formation and black hole growth. Our main results are (1) Galactic winds can drive outflows of masses ∼108-109 M ⊙ which may be imaged via CO emission-line mapping. (2) AGN-feedback-driven winds are able to drive detectable CO outflows for longer periods of time than starburst-driven winds owing to the greater amount of energy imparted to the ISM by AGN feedback compared to star formation. (3) Galactic winds can control the spatial extent of the CO emission in postmerger galaxies, and may serve as a physical motivation for the subkiloparsec scale CO emission radii observed in local advanced mergers. (4) Secondary emission peaks at velocities greater than the circular velocity are seen in the CO emission lines in all models, regardless of the associated wind model. In models with winds, however, these high-velocity peaks are seen to preferentially correspond to outfiowing gas entrained in winds, which is not the case in the model without winds. The high-velocity peaks seen in models without winds are typically confined to velocity offsets (from the systemic) ≤ 1.7 times the circular velocity, whereas the models with AGN-feedback-driven winds can drive high-velocity peaks to ∼2.5 times the circular velocity. © 2008. The American Astronomical Society. All rights reserved.
• Tothill, N. F., Kulesa, C. A., Walker, C. K., Ashley, M. C., Bonner, C., Cui, X., Everett, J., Feng, L., Hengst, S., Hu, J., Jiang, Z., Lawrence, J. S., Li, Y., Luong-Van, D., Moore, A. M., Pennypacker, C., Qin, W., Riddle, R., Storey, J. W., , Shang, Z., et al. (2008). Astrophysics from Dome A. EAS Publications Series, 33, 301-306.
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  Abstract: Dome A, the summit of the Antarctic plateau, is expected to have even better atmospheric conditions for ground-based astronomy than Dome C. Instruments to evaluate and exploit Dome A's astronomical potential must operate within logistical constraints, which are currently very stringent. Instrumentation now at Dome A exemplifies the techniques and solutions required by this environment. Future instrumentation and infrastructure will allow the qualities of the site to be exploited much more fully. © EAS, EDP Sciences 2008.
• Walker, C. K., Kulesa, C. A., Groppi, C. E., Young, E., McMahon, T., Bernasconi, P., Lisse, C., Neufeld, D., Hollenbach, D., Kawamura, J., Goldsmith, P., Langer, W., Yorke, H., Sterne, J., Skalare, A., Mehdi, I., Weinreb, S., Kooi, J., Stutzki, J., , Graf, U., et al. (2008). The stratospheric terahertz observatory (STO): An LDB experiment to investigate the life cycle of the interstellar medium. Proceedings of the 19th International Symposium on Space Terahertz Technology, ISSTT 2008, 9-13.
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  Abstract: The Stratospheric Terahertz Observatory (STO) is a balloon-borne, 0.8-meter telescope designed to investigate the structure of the interstellar medium and the life cycle of interstellar clouds. In its first long duration flight, STO will use two, 4-beam HEB receiver arrays to survey part of the Galactic Plane in the [C II] line at 158 microns (the brightest spectral line in the Galaxy) and the [N II] line at 205 microns (a tracer of the star formation rate). At ~1' angular resolution and < 1 km/s velocity resolution, STO will detect every interstellar cloud with AV > 0.3 in the surveyed region, and, through excitation and kinematic diagnostics provided by [C II] and [N II] line emission, will illustrate how atomic and molecular clouds are formed and dispersed in the Galaxy. STO will make 3- dimensional maps of the structure, dynamics, turbulence, energy balance, and pressure of the Milky Way's Interstellar Medium (ISM), as well as the star formation rate. In future flights, STO will observe the important far-infrared lines of [O I], [N II], and HD.
• Walker, C. K., Kulesa, C., Groppi, C., & Golish, D. (2008). Future prospects for THz spectroscopy. Proceedings of SPIE - The International Society for Optical Engineering, 7020.
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  Abstract: Many of the most astrophysically important transitions of atoms and molecules occur at THz frequencies. These transitions allow us to probe the formation and evolution of stars, planets, galaxies, and even the Universe itself. A recent confluence of technologies involving mixers, local oscillators, micromachining, IF amplifiers, and digital signal processing now make the fabrication of large format (∼1000 pixel), heterodyne arrays at THz frequencies possible. Planned observatories at high altitude sites (e.g. the high Atacama and Dome A in Antarctica) and in the stratosphere (e.g. SOFIA and balloon-borne telescopes) can serve as platforms from which THz arrays can provide unprecedented access to a powerful window to the Universe. In this paper the scientific impact and technical roadmap to the realization and implementation of THz arrays will be discussed.
• Bussmann, R. S., Wong, T. W., Hedden, A. S., Kulesa, C. A., & Walker, C. K. (2007). A CO (J = 3-2) outflow survey of the Elias 29 region. Astrophysical Journal, 657(1 II), L33-L36.
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  Abstract: We present a 5′ × 5′ integrated intensity map of 12CO (J = 3-2) emission from the ρ Ophiuchi cloud core that traces low-luminosity outflow emission from two protostars: Elias 29 and, most likely, LFAM 26. The morphology of the outflow from Elias 29 is bipolar and has a curved axis that traces the S-shaped symmetry seen in H2 emission. The outflow from LFAM 26 is a new detection and oriented in the east-west direction near the plane of the sky with most of the blueshifted emission being absorbed by intervening clouds. The outflow axis of this object also appears to intersect a knot of H2 emission previously attributed to Elias 29. LFAM 26 is a low-luminosity source (Lbol = 0.06 L⊙), which, in combination with the observed outflow, makes it a candidate very low luminosity object (VeLLO). We derive lower limits to the gas column densities and energetics for both outflows. The mechanical luminosities for Elias 29 and LFAM 26 are 6.4 and 10.3 × 10-3 L⊙, respectively. © 2007. The American Astronomical Society. All rights reserved.
• Groppi, C., Walker, C., Kulesa, C., Golish, D., Putz, P., Gensheimer, P., Hedden, A., Bussmann, S., Weinreb, S., Jones, G., Barden, J., Mani, H., Kuiper, T., Kooi, J., Lichtenberger, A., Cecil, T., Narayanan, G., & Narayanan, G. (2007). SuperCam : A 64 pixel superheterodyne camera. Proceedings of the Eighteenth International Symposium on Space Terahertz Technology 2007, ISSTT 2007, 264-269.
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  Abstract: We report on the development of SuperCam, a 64 pixel, superheterodyne camera designed for operation in the astrophysically important 870 μm atmospheric window. SuperCam will be used to answer fundamental questions about the physics and chemistry of molecular clouds in the Galaxy and their direct relation to star and planet formation. The advent of such a system will provide an order of magnitude increase in mapping speed over what is now available and revolutionize how observational astronomy is performed in this important wavelength regime. SuperCam is constructed by stacking eight, 1x8 rows of tunerless, SIS mixers. The SIS junctions use SOI (Silicon on Insulator) technology, with beamleads for device positioning and IF and ground electrical connections. The mixer modules are fabricated using a Kern MMP-2522 micromilling machine purchased specifically for this task. The IF output of each SIS device is directly connected to a low-noise, broadband MMIC amplifier module integrated into the mixer block. The instantaneous IF bandwidth of each pixel is 2 GHz, with a center frequency of 5 GHz. An IF processor constructed of eight 8-channel modules provides IF amplification, total power monitoring and baseband downconversion. A spectrum of the central 250 MHz or 500 MHz of each IF band is provided by the Omnisys real-time FFT spectrometer system, based on Xilinx Virtex 4 FPGAs. This spectrometer can operate in either 32 channel mode (500 MHz/channel) or 64 channel mode (250 MHz/channel). Local oscillator power is provided by a Virginia Diodes solid-state multiplier chain whose output is divided between the pixels with a matrix of waveguide power dividers. The mixer array is cooled to 4K by a closed-cycle cryostat with two cryocoolers. SuperCam will reside at the Cassegrain focus of the 10m Heinrich Hertz telescope (HHT) with a dedicated reimaging optics system. All subsystems of SuperCam have completed the development stage, and are undergoing testing. We present test results for the SuperCam LNA modules, integration of LNAs in a test mixer, IF processor performance, spectrometer performance, cryogenic system verification, and end-to-end measurements of the IF chain and backend. Results from the fabrication, construction and testing of prototype SOI mixers, in both single pixel and 8 pixel versions will be shown. We will enter the final fabrication stage in early 2007, with expected completion in late 2007. Science operations are expected to begin in Spring, 2008.
• Groppi, C., Walker, C., Kulesa, C., Golish, D., Pütz, P., Gensheimer, P., Hedden, A., Bussmann, S., Weinreb, S., Wadefalk, N., Jones, G., Barden, J., Mani, H., Kuiper, T., Kooi, J., Lichtenberger, A., & Narayanan, G. (2006). Supercam : A 64 pixel superheterodyne camera. 17th International Symposium on Space Terahertz Technology 2006, ISSTT 2006, 240-243.
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  Abstract: We report on the development of SuperCam, a 64 pixel, superheterodyne camera designed for operation in the astrophysically important 870 μm atmospheric window. SuperCam will be used to answer fundamental questions about the physics and chemistry of molecular clouds in the Galaxy and their direct relation to star and planet formation. The advent of such a system will provide an order of magnitude increase in mapping speed over what is now available and revolutionize how observational astronomy is performed in this important wavelength regime. Unlike bolometric detectors, heterodyne receiver systems are coherent, retaining information about both the amplitude and phase of the incident photon stream. From this information a high resolution spectrum of the incident light can be obtained without multiplexing. SuperCam will be constructed by stacking eight, 1x8 rows of tunerless, SIS mixers. The IF output of each mixer will be connected to a low-noise, broadband MMIC amplifier integrated into the mixer block. The instantaneous IF bandwidth of each pixel will be ~2 GHz, with a center frequency of 5 GHz. A spectrum of the central 500 MHz of each IF band will be provided by the array spectrometer. Local oscillator power is provided by a frequency multiplier whose output is divided between the pixels by using a matrix of waveguide power dividers. The mixer array will be cooled to 4K by a closed-cycle refrigeration system. SuperCam will reside at the Cassegrain focus of the 10m Heinrich Hertz telescope (HHT) with a dedicated reimaging optics system. We report on single pixel integrated LNA testing, cryogenic system testing, performance of the prototype backend spectrometer module, and the fabrication of the first 1x8 array module. This module will be tested on the HHT in 2006, with the first engineering run of the full array in late 2007. The array is designed and constructed so that it may be readily scaled to higher frequencies.
• Groppi, C., Walker, C., Kulesa, C., Pütz, P., Golish, D., Gensheimer, P., Hedden, A., Bussmann, S., Weinreb, S., Kuiper, T., Kooi, J., Jones, G., Bardin, J., Mani, H., Lichtenberger, A., & Narayanan, G. (2006). SuperCam, a 64-pixel heterodyne imaging array for the 870 micron atmospheric window. Proceedings of SPIE - The International Society for Optical Engineering, 6275.
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  Abstract: We report on the development of SuperCam, a 64 pixel, superheterodyne camera designed for operation in the astrophysically important 870 μm atmospheric window. SuperCam will be used to answer fundamental questions about the physics and chemistry of molecular clouds in the Galaxy and their direct relation to star and planet formation. The advent of such a system will provide an order of magnitude increase in mapping speed over what is now available and revolutionize how observational astronomy is performed in this important wavelength regime. Unlike the situation with bolometric detectors, heterodyne receiver systems are coherent, retaining information about both the amplitude and phase of the incident photon stream. From this information a high resolution spectrum of the incident light can be obtained without multiplexing. SuperCam will be constructed by stacking eight, 1×8 rows of fixed tuned, SIS mixers. The IF output of each mixer will be connected to a low-noise, broadband MMIC amplifier integrated into the mixer block. The instantaneous IF bandwidth of each pixel will be ∼2 GHz, with a center frequency of 5 GHz. A spectrum of the central 500 MHz of each IF band will be provided by the array spectrometer. Local oscillator power is provided by a frequency multiplier whose output is divided between the pixels by using a matrix of waveguide power dividers. The mixer array will be cooled to 4K by a closed-cycle refrigeration system. SuperCam will reside at the Cassegrain focus of the 10m Heinrich Hertz telescope (HHT). A prototype single row of the array will be tested on the HHT in 2006, with the first engineering run of the full array in late 2007. The array is designed and constructed so that it may be readily scaled to higher frequencies.
• Hedden, A. S., Walker, C. K., Groppi, C. E., & Butner, H. M. (2006). Star formation in the Northern cloud complex of NGC 2264. Astrophysical Journal Letters, 645(1 I), 345-356.
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  Abstract: We have made continuum and spectral line observations of several outflow sources in the Mon OBI dark cloud (NGC 2264) using the Heinrich Hertz Telescope (HHT) and ARO 12 m millimeter-wave telescope. This study explores the kinematics and outflow energetics of the young stellar systems observed and assesses the impact star formation is having on the surrounding cloud environment. Our data set incorporates 12CO (3-2),13CO (3-2), and 12CO (1-0) observations of outflows associated with the sources IRAS 06382+1017 and IRAS 06381+1039, known as IRAS 25 and 27, respectively, in the northern cloud complex. Complementary 870 μm continuum maps were made with the HHT 19 channel bolometer array. Our results indicate there is a weak ≤0.5% coupling between outflow kinetic energy and turbulent energy of the cloud. An analysis of the energy balance in the IRAS 25 and 27 cores suggests they are maintaining their dynamical integrity except where outflowing material directly interacts with the core, such as along the outflow axes.
• Hedden, A., Pütz, P., D'Aubigny, C., Golish, D., Groppi, C., Walker, C., Williams, B., Qing, H. u., & Reno, J. (2006). Micromachined spatial filters for quantum cascade lasers. 17th International Symposium on Space Terahertz Technology 2006, ISSTT 2006, 181-184.
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  Abstract: Quantum Cascade Lasers (QCL) are the most promising technology for producing compact, high power (> 1 mW), coherent signal sources above 2 THz. Due to their small size (10 μm × 25 μm) and rectangular cross-section, the output beam from a QCL laser cavity is highly divergent and non-Gaussian. A single mode Gaussian beam is desirable for efficient coupling to optical systems. We have designed a waveguide spatial filter for this purpose. The 2.7 THz spatial filter consists of two diagonal feed horns connected by one wavelength of square waveguide (92 μm on a side). The mode filtering efficiency and far field beam pattern of the structure have been modeled in CST Microwave Studio. We have fabricated the filter in tellurium copper using a Kern MMP micromilling machine. We present measurements of the QCL's throughput and emergent power pattern with and without the filter. Our preliminary findings suggest that spatial filtering significantly improves the QCL beam pattern, and further measurements are being made to more rigorously explore these results.
• Krejny, M., Chuss, D. T., Novak, G., Voellmer, G. M., Wollack, E. J., Walker, C. K., Jackson, M., Benford, D. J., Staguhn, J. G., Moseley Jr., S. H., Kulesa, C., Y., C., Golish, D. R., & Loewenstein, R. F. (2006). The variable-delay polarization modulator. Proceedings of SPIE - The International Society for Optical Engineering, 6275.
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  Abstract: Precise astronomical polarization measurements generally require the use of polarization modulation. We have developed a new modulator, the Variable-delay Polarization Modulator (VPM) which uses two modified Martin-Puplett interferometers to induce a physical path length difference between polarization components. This highly durable and efficient device can easily be adapted to a wide range of wavelengths and temperatures, making it well suited for air- and space-borne facilities. This paper discusses the basic modulator design and a comparison to the half-wave plate, as well as details of VPM tests conducted at the Submillimeter Telescope Observatory (SMTO).
• Narayanan, D., Cox, T. J., Robertson, B., Davé, R., Matteo, T. D., Hernquist, L., Hopkins, P., Kulesa, C., & Walker, C. K. (2006). Molecular outflows in galaxy merger simulations with embedded active galactic nuclei. Astrophysical Journal Letters, 642(2 II), L107-L110.
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  Abstract: We study the effects of feedback from active galactic nuclei (AGNs) on emission from molecular gas in galaxy mergers by combining hydrodynamic simulations that include black holes with a three-dimensional, non-local thermodynamic equilibrium (LTE) radiative transfer code. We find that molecular clouds entrained in AGN winds produce an extended CO morphology with significant off-nuclear emission, which may be detectable via contour mapping. Furthermore, kinematic signatures of these molecular outflows are visible in emission-line profiles when the outflow has a large line-of-sight velocity. Our results can help interpret current and upcoming observations of luminous infrared galaxies, as well as provide a detailed test of subresolution prescriptions for supermassive black hole growth in galaxy-scale hydrodynamic simulations. © 2006. The American Astronomical Society. All rights reserved.
• Narayanan, D., Groppi, C. E., Kulesa, C. A., & Walker, C. K. (2006). Erratum: Warm, dense molecular gas in the ISM of starbursts, LIRGs, and ULIRGs (The Astrophysical Journal (2005) 630 (269)). Astrophysical Journal Letters, 642(1 I), 616-.
• Narayanan, D., Kulesa, C. A., Boss, A., & Walker, C. K. (2006). Molecular line emission from gravitationally unstable protoplanetary disks. Astrophysical Journal Letters, 647(2 I), 1426-1436.
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  Abstract: In the era of high-resolution submillimeter interferometers, it will soon be possible to observe the neutral circum-stellar medium directly involved in gas giant planet (GGP) formation at physical scales previously unattainable. In order to explore possible signatures of GGP formation via disk instabilities, we have combined a three-dimensional (3D), nonlocal thermodynamic equilibrium (LTE) radiative transfer code with a 3D, finite differences hydrodynamical code to model molecular emission lines from the vicinity of a 1.4MJ self-gravitating proto-GGP. Here we explore the properties of rotational transitions of the commonly observed dense gas tracer, HCO+ . Our main results are as follows: (1) Very high lying HCO+ transitions (e.g., HCO+ J = 7-6) can trace dense clumps around circumstellar disks. Depending on the molecular abundance, the proto-GGP may be directly imageable by the Atacama Large Millimeter Array (ALMA). (2) HCO+ emission lines are heavily self-absorbed through the proto-GGP's dense molecular core. This signature is nearly ubiquitous and only weakly dependent on assumed HCO+ abundances. The self-absorption features are most pronounced at higher angular resolutions. Dense clumps that are not self-gravitating only show minor self-absorption features. (3) Line temperatures are highest through the proto-GGP at all assumed abundances and inclination angles. Conversely, due to self-absorption in the line, the velocity-integrated intensity may not be. High angular resolution interferometers such as the Submillimeter Array (SMA) and ALMA may be able to differentiate between competing theories of GGP formation. © 2006. The American Astronomical Society. All rights reserved.
• Pütz, P., Hedden, A., Gensheimer, P., Golish, D., Groppi, C. E., Kulesa, C., Narayanan, G., Lichtenberger, A., Kooi, J. W., Wadefalk, N., Weinreb, S., & Walker, C. K. (2006). 345 GHz prototype SIS mixer with integrated MMIC LNA. International Journal of Infrared and Millimeter Waves, 27(10), 1365-1379.
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  Abstract: We report on heterodyne measurements at submillimeter wavelengths using a receiver with a Superconductor-Insulator-Superconductor (SIS) mixer device and a Microwave Monolithic Integrated Circuit (MMIC) cryogenic low noise amplifier (LNA) module integrated into the same block. The mixer characterization presented in this work demonstrates the feasibility of operating a MMIC LNA in close proximity to the SIS device without penalty in mixer performance due to heating effects. Verification of this functionality is crucial for the ongoing development of SuperCam, a 64-pixel focal plane array receiver consisting of eight, 1 × 8 integrated mixer/LNA modules. The test setup included a mixer block modified to accept a MMIC amplifier. Our tests show that the LNA can be operated over a broad range of V drain voltages from 0.40-1.40 V, corresponding to dissipative powers of 2.6-29 mW. We observe no significant effect on the measured uncorrected receiver noise temperatures in the 345 GHz band. © Springer Science+Business Media, LLC 2006.
• Schau, H. C., Jennette, B., & Walker, C. K. (2006). Design of a compact low-cost THz imaging scanner. Proceedings of SPIE - The International Society for Optical Engineering, 6212.
• Datesman, A. M., Schultz, J. C., Lichtenberger, A. W., Golish, D., Walker, C. K., & Kooi, J. (2005). Fabrication and characterization of niobium diffusion-cooled hot-electron bolometers on silicon nitride membranes. IEEE Transactions on Applied Superconductivity, 15(2 PART I), 928-931.
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  Abstract: We have successfully fabricated niobium diffusion-cooled hot-electron bolometer (HEB) mixers on membranes of silicon nitride less than one micron thick. This advance has allowed us to construct a 1 × 5 HEB receiver array intended for operation at 1.45 THz. This article provides an overview of the integration of the HEB array chip with silicon micromachined backshorts and feedhorns, discusses materials issues surrounding the device fabrication, reports resistance and I-V measurements, and compares HEBs fabricated on silicon nitride to similar devices on quartz substrates. © 2005 IEEE.
• Kulesa, C. A., Hungerford, A. L., Walker, C. K., Zhang, X., & Lane, A. P. (2005). Large-scale CO and [C I] emission in the ρ ophiuchi molecular cloud. Astrophysical Journal Letters, 625(1 I), 194-209.
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  Abstract: We present a comprehensive study of the ρ Ophiuchi molecular cloud that addresses aspects of the physical structure and condition of the molecular cloud and its photodissociation region (PDR) by combining far-infrared and submillimeter-wave observations with a wide range of angular scale and resolution. We present 1600 arcmin2 maps (2.3 pc2) with 0.1 pc resolution in submillimeter CO (4 → 3) and [C I] (3P 1 → 3P0) line emission from the Antarctic Submillimeter Telescope and Remote Observatory (AST/RO) and pointed observations in the CO (7 → 6) and [C I] (3P2 → 3P1) lines. Within the large-scale maps, smaller spectral line maps of 3000 AU resolution over ∼90 arcmin2 (0.2 pc 2) of the cloud in CO, CS, HCO+, and their rare isotopomers are made at the Heinrich Hertz Telescope (HHT) in Arizona. Comparison of CO, HCO+, and [C I] maps with far-infrared observations of atomic and ionic species from the Infrared Space Observatory (ISO) far-infrared and submillimeter continuum emission and near-infrared H 2 emission allows clearer determination of the physical and chemical structure of the ρ Oph PDR, since each species probes a different physical region of the cloud structure. Although a homogeneous plane-parallel PDR model can reproduce many of the observations described here, the excitation conditions needed to produce the observed HCO+ and [O I] emission imply inhomogeneous structure. Strong chemical gradients are observed in HCO + and CS; the former is ascribed to a local enhancement in the H 2 ionization rate, and the latter is principally due to shocks. Under the assumption of a simple two-component gas model for the cloud, we find that [C II] and [C I] emission predominantly arises from the lower density envelopes (103-104 cm-3) that surround denser cloud condensations, or "clumps." The distribution of [C I] is very similar to that of C18O and is generally consistent with illumination from the "far" side of the cloud. A notable exception is found at the western edge of the cloud, where UV photons create a PDR viewed "edge-on." The abundance of atomic carbon is accurately modeled using a radiation field that decreases with increasing projected distance from the exciting star HD 147889 and a total gas column density that follows that of C18O, decreasing toward the edges of the cloud. In contrast to the conclusions of other studies, we find that no nonequilibrium chemistry is needed to enhance the [C I] abundance. Each spectral line is traced to a particular physical component of the cloud and PDR. Although CO rotational line emission originates from both dense condensations and diffuse envelopes, the millimeter-wave transitions mostly find their origins in envelope material, whereas the high-J submillimeter lines stem more from the dense clumps. Submillimeter HCO+ and infrared [O I] and [CII] emission indicate clump surface temperatures of 50-200 K, an ultraviolet radiation field with IUV ≈ 10-90, densities of 105-106 cm -3, and interior temperatures of ≤20 K. This study highlights the value of large-scale infrared and submillimeter mapping for the interpretation of molecular cloud physical and chemical structure, and important future observations are highlighted. © 2005. The American Astronomical Society. All rights reserved.
• Narayanan, D., Groppi, C. E., Kulesa, C. A., & Walker, C. K. (2005). Warm, dense molecular gas in the ISM of starbursts, LIRGs, and ULIRGs. Astrophysical Journal Letters, 630(1 I), 269-279.
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  Abstract: The role of star formation in luminous and ultraluminous infrared galaxies (LIRGs, LIR ≥ 1011 L⊙; ULIRGs, L IR ≥ 1012 L⊙) is a hotly debated issue: while it is clear that starbursts play a large role in powering the IR luminosity in these galaxies, the relative importance of possible enshrouded AGNs is unknown. It is therefore important to better understand the role of star-forming gas in contributing to the infrared luminosity in IR-bright galaxies. The J = 3 level of 12CO lies 33 K above ground and has a critical density of ∼ 1.5 × 104 cm-3. The 12CO J = 3-2 line serves as an effective tracer for warm, dense molecular gas heated by active star formation. Here we report on 12CO J = 3-2 observations of 17 starburst spiral galaxies, LIRGs, and ULIRGs, which we obtained with the Heinrich Hertz Submillimeter Telescope on Mount Graham, Arizona. Our main results are as follows. (1) We find a nearly linear relation between the infrared luminosity and warm, dense molecular gas such that the infrared luminosity increases as the warm, dense molecular gas to the power 0.92; we interpret this to be roughly consistent with the recent results of Gao & Solomon. (2) We find LIR/MH2 warm, dense ratios ranging from ∼38 to ∼482 L⊙/M⊙ using a modified CO-H2 conversion factor of 8.3 × 1019 cm-2 (K km s-1)-1 derived in this paper. © 2005. The American Astronomical Society. All rights reserved.
• Schultz, J., Lichtenberger, A., Weikle, R., Lyons, C., Bass, R., Bryerton, E., Pan, S. K., Groppi, C., Kooi, J., & Walker, C. (2005). Application of ultra-thin silicon technology to submillimeter detection and mixing. WMSCI 2005 - The 9th World Multi-Conference on Systemics, Cybernetics and Informatics, Proceedings, 9, 171-175.
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  Abstract: Superconducting based SIS and HEB detectors continue to yield improved noise temperatures at submillimeter wavelengths. These higher frequencies present new challenges, particularly for waveguide based designs where the tolerances for mounting small mixer chips become quite narrow. Also, conventional millimeter wavelength techniques for making the IF and ground connections are more prone to error. As the device technology for these SIS and HEB-based detectors matures, there is also an increased interest in integrated receiver arrays. These challenges call for simpler mounting designs and more repeatable assembly techniques. Our research group, at the University of Virginia, is meeting these challenges with a new ultra-thin mixer chip technology, with integrated gold beam leads, first reported in [1]. We have since further developed and improved on this technology. We have several ongoing SIS, HEB and OMT projects which utilize these capabilities. Most important to this technology is the transition from the conventional use of quartz as a circuit substrate material to that of ultra-thin (
• Walker, C. K., & Kulesa, C. A. (2005). Terahertz astronomy from the coldest place on Earth. The Joint 30th International Conference on Infrared and Millimeter Waves and 13th International Conference on Terahertz Electronics, 2005. IRMMW-THz 2005, 1, 3-4.
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  Abstract: Many atoms and molecules have emission lines that occur at THz frequencies. These emission lines can be used to probe the conditions in Giant Molecular Clouds (GMCs) throughout our galaxy. From studying the star formation process in GMCs we will gain a better understanding of the origin of planetary systems like our own. The high Antarctic plateau offers unique opportunities for observatories optimized for this important wavelength regime. © 2005 IEEE.
• Chuss, D. T., Benford, D. J., Walker, C., Moseley, S. H., Novak, G., Staguhn, J. G., & Wollack, E. J. (2004). A 350 micron array polarimeter using translational modulators. Proceedings of SPIE - The International Society for Optical Engineering, 5492(PART 3), 1450-1460.
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  Abstract: We describe a polarimeter that will be a testbed for a novel polarization modulator. This modulator is composed of two modified Martin-Puplett interferometers in which the input polarization state is transformed by adjusting the phase delays between linear orthogonal polarizations in each of the two stages. This type of modulator represents a potential improvement over existing technology in the following three ways. First, these modulators can fully characterize the polarization state by measuring Stokes Q, U, and V. The characterization of V is especially useful as a diagnostic for systematic errors. Second, the Martin-Puplett modulators can be easily retuned to observe at multiple frequencies. Finally, the small translations required for modulation can be accomplished with fewer moving parts than the rotational motions in wave plate modulators, thus potentially leading to modulators with longer lifetimes than those currently employed in polarimeters. The next generation of polarimeters, designed for both the study of Galactic magnetic fields in the far-infrared and submillimeter and the measurement of the polarized component of the Cosmic Microwave Background, will need to incorporate modulators with these attributes. This prototype polarimeter will be a modified version of the Hertz polarimeter that had until recently been operating on the Caltech Submillimeter Observatory. After modification, Hertz will be moved to the Heinrich Hertz Telescope in Arizona. We will utilize dynamic scheduling to efficiently observe during the best fraction of weather at this site.
• Groppi, C. E., Kulesa, C., Walker, C., & Martin, C. L. (2004). Millimeter and submillimeter survey of the R coronae australis region. Astrophysical Journal Letters, 612(2 I), 946-955.
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  Abstract: Using a combination of data from the Antarctic Submillimeter Telescope and Remote Observatory (AST/RO), the Arizona Radio Observatory Kitt Peak 12 m telescope, and the Arizona Radio Observatory 10 m Heinrich Hertz Telescope, we have studied the most active part of the R CrA molecular cloud in multiple transitions of carbon monoxide, HCO+, and 870 μm continuum emission. Since R CrA is nearby (130 pc), we are able to obtain physical spatial resolution as high as 0.01 pc over an area of 0.16 pc2, with velocity resolution finer than 1 km s-1. Mass estimates of the protostar driving the millimeter-wave emission derived from HCO+, dust continuum emission, and kinematic techniques point to a young, deeply embedded protostar of ∼0.5-0.75 M⊙, with a gaseous envelope of similar mass. A molecular outflow is driven by this source that also contains at least 0.8 M⊙ of molecular gas with ∼0.5 L ⊙ of mechanical luminosity. HCO+ lines show the kinematic signature of infall motions, as well as bulk rotation. The source is most likely a Class 0 protostellar object not yet visible at near-IR wavelengths. With the combination of spatial and spectral resolution in our data set, we are able to disentangle the effects of infall, rotation, and outflow toward this young object.
• Groppi, C. E., Walker, C. K., Kulesa, C., Golish, D., Hedden, A., Narayanan, G., Lichtenberger, A. W., Kooi, J., Graf, U. U., & Heyminck, S. (2004). First results from desert star: A 7 pixel 345 GHz heterodyne array receiver for the heinrich hertz telescope. Proceedings of SPIE - The International Society for Optical Engineering, 5498, 290-299.
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  Abstract: We present the first astronomical results from DesertSTAR, a 7 pixel heterodyne array receiver designed for operation in the astrophysically rich 345 GHz atmospheric window. DesertSTAR was constructed for the 10m Heinrich Hertz Telescope located at 3150m elevation on Mt. Graham, Arizona. This receiver promises to increase mapping speed at the HHT by a factor of ∼15 over the facility's existing single beam, dual polarization receiver. DesertSTAR uses tunerless, single-ended waveguide SIS mixers to achieve uncorrected receiver noise temperatures of ∼60K. The instantaneous bandwidth is 2 GHz, with a 5 GHz Intermediate Frequency, offering 1600 km/s of velocity coverage. Cryogenic isolators are employed between the mixers and low noise amplifiers to assure a flat IF passband. The system uses a Joule-Thompson closed-cycle refrigerator with 180W capacity at 70K and 1.8W capacity at 4K. A novel reflective phase grating is used for Local Oscillator multiplexing, while a simple Mylar beamsplitter is used as an LO diplexer. Optics include only polyethelene mixer lenses and a single, cold, flat mirror, maximizing simplicity for high efficiency and easy optical alignment. The computer controlled bias system provides low noise bias for the SIS junctions, magnets and LNAs through a modular and hardware independent GUI interface, and allows remote operation and monitoring. We present measurements of receiver noise, beam quality, efficiency and stability in addition to astronomical observations obtained during engineering runs at the HHT.
• Martin, C. L., Walsh, W. M., Xiao, K., Lane, A. P., Walker, C. K., & Stark, A. A. (2004). Erratum: The astro survey of the galactic center region. I. The inner 3 degrees (The Astrophysical Journal Supplement Series (2004) 150 (239)). Astrophysical Journal, Supplement Series, 153(1), 395-.
• Martin, C. L., Walsh, W. M., Xiao, K., Lane, A. P., Walker, C. K., & Stark, A. A. (2004). The AST/RO survey of the galactic center region. I. The inner 3 degrees. Astrophysical Journal, Supplement Series, 150(1), 239-262.
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  Abstract: We present fully sampled maps of 461 GHz CO J = 4 → 3, 807 GHz CO J = 7 → 6, and 492 GHz [C I] 3P 1 → 3P 0 emission from the inner 3 degrees of the Galactic center region taken with the Antarctic Submillimeter Telescope and Remote Observatory (AST/RO) in 2001-2002. The data cover -1°.3 < l < 2°, -0°.3 < b < 0°.2 with 0.′5 spacing, resulting in spectra in three transitions at over 24,000 positions on the sky. The CO J = 4 → 3 emission is found to be essentially coextensive with lower J transitions of CO. The CO J = 7 → 6 emission is spatially confined to a far smaller region than the lower J CO lines. The [C I] 3P 1 → 3P 0 emission has a spatial extent similar to the low-J CO emission but is more diffuse. Bright CO J = 7 → 6 emission is detected in the well-known Galactic center clouds Sgr A and Sgr B. We also detect CO J = 4 → 3 and CO J = 7 → 6 absorption from spiral arms in the Galactic disk at velocities near 0 km s -1 along the line of sight to the Galactic center. Analyzing our CO J = 7 → 6 and CO J = 4 → 3 data in conjunction with J = 1 → 0 12CO and 13CO data previously observed with the Bell Laboratories 7 m antenna, we apply a large velocity gradient (LVG) model to estimate the kinetic temperature and density of molecular gas in the inner 200 pc of the Galactic center region. We show maps of the derived distribution of gas density and kinetic temperature as a function of position and velocity for the entire region. Kinetic temperature was found to decrease from relatively high values (>70 K) at cloud edges to low values (
• Pütz, P., Glenz, S., Teipen, R., Tils, T., Honingh, N., Jacobs, K., Hedden, A., Kulesa, C., Groppi, C. E., & Walker, C. K. (2004). High sensitivity 810 GHz SIS receivers at AST/RO. Proceedings of SPIE - The International Society for Optical Engineering, 5498, 509-516.
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  Abstract: In November 2003 the heterodyne receivers WANDA (polarization diplexed 492/810 GHz) and PoleSTAR (2 × 2 810 GHz array) of AST/RO (Antarctic Submillimeter Telescope and Remote Observatory, located at the South Pole) were upgraded with new 810 GHz SIS (Superconductor-Insulator-Superconductor) waveguide mixers from KOSMA. Profiting from device development for the HIFI (Heterodyne Instrument for the Far-Infrared) Band 2 SIS mixers of the Herschel Space Observatory, a factor of ≈ 2 improvement in receiver noise temperature (from 1100 K to 550 K DSB) was achieved with WANDA. The SIS mixer devices employ low-loss NbTiN-Al tuning circuits and are fabricated using electron beam lithographic junction area definition and CMP (Chemical Mechanical Polishing) of the tuning circuit dielectric. With the South Pole being one of the best possible sites for ground-based submillimeter astronomy, the 1.7m telescope currently makes AST/RO well suited for sensitive, large scale spectral line mapping at 810 GHz. Low atmospheric opacity (τ < 1) and, consequently, very low system noise temperatures (< 3000 K) are regularly achieved at 810 GHz, making AST/RO an extremely sensitive observatory at these frequencies. "First light" astronomical measurements made with the upgraded 810 GHz channel of WANDA towards the galactic HII region NGC 3576 in CO J = 7 → 6 (806.65 GHz) and the neutral carbon [CI] 3P2 →3 P1 (809.3 GHz) lines are presented.
• Stark, A. A., Martin, C. L., Walsh, W. M., Xiao, K., Lane, A. P., & Walker, C. K. (2004). Gas density, stability, and starbursts near the inner lindblad resonance of the milky way. Astrophysical Journal Letters, 614(1 II), L41-L44.
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  Abstract: A key project of the Antarctic Submillimeter Telescope and Remote Observatory reported by Martin et al. is the mapping of CO J = 4 → 3 and J = 7 → 6 emission from the inner Milky Way, allowing determination of gas density and temperature. Galactic center gas that Binney et al. identify as being on x2 orbits has a density near 103.5 cm 3, which renders it only marginally stable against gravitational coagulation into a few giant molecular clouds, as discussed by Elmegreen. This suggests a relaxation oscillator mechanism for starbursts in the Milky Way, whereby inflowing gas accumulates in a ring at 150 pc radius until the critical density is reached and the resulting instability leads to the sudden formation of giant clouds and the deposition of 4 × 107 M ⊙ of gas onto the Galactic center. Depending on the accretion rate near the inner Lindblad resonance, this cycle will repeat with a timescale of order 20 Myr.
• Swain, M. R., Walker, C. K., Traub, W. A., Storey, J. W., Coudé, V., Fossat, E., Vakili, F., Stark, A. A., Lloyd, J. P., Lawson, P. R., Burrows, A. S., Ireland, M., Millan-Gabet, R., T., G., Lane, B., Vasisht, G., & Travouillon, T. (2004). The Antarctic planet interferometer. Proceedings of SPIE - The International Society for Optical Engineering, 5491(PART 1), 176-185.
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  Abstract: The Antarctic Planet Interferometer is a concept for an instrument designed to detect and characterize extrasolar planets by exploiting the unique potential of the best accessible site on earth for thermal infrared interferometry. High-precision interferometric techniques under development for extrasolar planet detection and characterization (differential phase, nulling and astrometry) all benefit substantially from the slow, low-altitude turbulence, low water vapor content, and low temperature found on the Antarctic plateau. At the best of these locations, such as the Concordia base being developed at Dome C, an interferometer with two-meter diameter class apertures has the potential to deliver unique science for a variety of topics, including extrasolar planets, active galactic nuclei, young stellar objects, and protoplanetary disks.
• Walker, C. K., Kulesa, C. A., Golish, D., Hedden, A., Jacobs, K., Stutzki, J., Gao, J. R., Kooi, J., Glaister, D., Gully, W., Mehdi, I., Swain, M., & Siegel, P. (2004). Forecast for heat on dome A, Antarctica: The high elevation antarctic terahertz telescope. Proceedings of SPIE - The International Society for Optical Engineering, 5489(PART 1), 470-480.
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  Abstract: We have proposed to develop a prototype 0.5-meter far-infrared telescope and heterodyne receiver/spectrometer system for fully-automated remote operation at the summit of Dome A, the highest point on the Antarctic plateau. The unparalleled stability, exceptional dryness, low wind and extreme cold make Dome A a ground-based site without equal for astronomy at infrared and submillimeter wavelengths. HEAT, the High Elevation Antarctic Terahertz Telescope, will operate in the atmospheric windows between 150 and 400 μm, in which the most crucial astrophysical spectral diagnostics of the formation of galaxies, stars, planets, and life are found. At these wavelengths, HEAT will have high aperture efficiency and excellent atmospheric transmission most of the year. The proposed superheterodyne receiver system will be comprised of 0.8 THz, 1.4 THz and 1.9 THz channels which will observe the pivotal J=7→6 line of CO, the J=2→1 line of atomic carbon, and the far-infrared fine structure lines of N+ and C+, the brightest emission lines in the entire Milky Way Galaxy. When combined with the HEAT telescope, the receiver system represents a uniquely powerful instrument for reconstructing the history of star formation in our Galaxy, with application to the distant Universe. The receiver system itself serves as a valuable testbed for heterodyne Terahertz components, using leading-edge mixer, local oscillator, low-noise amplifier, cryogenic, and digital signal processing technologies that will play essential roles in future Terahertz observatories. The proposed study will pave the way for future astronomical investigations from Dome A.
• Wehmeier, U. J., Swain, M. R., Y., C., Golish, D., & Walker, C. K. (2004). The potential of conductive waveguides for nulling interferometry. Proceedings of SPIE - The International Society for Optical Engineering, 5491(PART 3), 1435-1444.
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  Abstract: We report on a novel approach for implementing a dual Bracewell nulling interferometric beam combiner using miniature conductive waveguides contained in a single monolithic structure. We present modeling results for these devices at mid-infrared wavelengths. Potential applications for these devices in the Terrestrial Planet Finder mission are discussed.
• Y., C., Walker, C. K., & Golish, D. (2004). Mid-infrared spatial filters fabrication using laser chemical etching. Proceedings of SPIE - The International Society for Optical Engineering, 5491(PART 2), 655-666.
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  Abstract: Feedhorns like those commonly used in radio-telescope and radio communication equipment couple very efficiently (>98%) to the fundamental Gaussian mode (TEM00). High order modes are not propagated through a single-mode hollow metallic waveguides. It follows that a back to back feedhorn design joined with a small length of single-mode waveguide can be used as a very high throughput spatial filter. Laser micro machining provides a mean of scaling successful waveguide and quasi-optical components to far and mid infrared wavelengths. A laser micro machining system optimized for THz and far IR applications has been in operation at Steward Observatory for several years and produced devices designed to operate at λ=60μm. A new laser micromachining system capable of producing mid-infrared devices will soon be operational. These proceedings review metallic hollow waveguide spatial filtering theory, feedhorn designs as well as laser chemical etching and the design of a new high-NA UV laser etcher capable of sub-micron resolution to fabricate spatial filters for use in the mid-infrared.
• Kooi, J. W., Chattopadhyay, G., Withington, S., Rice, F., Zmuidzinas, J., Walker, C., & Yassin, G. (2003). A full-height waveguide to thin-film microstrip transition with exceptional RF bandwidth and coupling efficiency. International Journal of Infrared and Millimeter Waves, 24(3), 261-284.
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  Abstract: We describe a waveguide to thin-film microstrip transition for high-performance submillimetre wave and teraherz applications. The proposed constant-radius probe couples thin-film microstrip line, to full-height rectangular waveguide with better than 99% efficiency (VSWR ≤ 1.20) and 45% fractional bandwidth. Extensive HFSS simulations, backed by scale-model measurements, are presented in the paper. By selecting the substrate material and probe radius, any real impedance between ≈ 15-60 Ω can be achieved. The radial probe gives significantly improved performance over other designs discussed in the literature. Although our primary application is submillimetre wave superconducting mixers, we show that membrane techniques should allow broad-band waveguide components to be constructed for the THz frequency range.
• Lloyd, J. P., Lane, B. F., Swain, M. R., Storey, J. W., Travouillon, T., Traub, W. A., & Walker, C. K. (2003). Extrasolar Planet Science with the Antarctic Planet Interferometer. Proceedings of SPIE - The International Society for Optical Engineering, 5170, 193-199.
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  Abstract: The primary limitation to ground based astronomy is the Earth's atmosphere. The atmosphere above the Antarctic plateau is fundamentally different in many regards compared to the atmosphere at temperate sites. The extreme altitude, cold and low humidity offer a uniquely transparent atmosphere at many wavelengths. Studies at the South Pole have shown additionally that the turbulence properties of the night time polar atmosphere are unlike any mid latitude sites. Despite relatively strong ground layer turbulence, the lack of high altitude turbulence combined with low wind speeds presents favorable conditions for interferometry. The unique properties of the polar atmosphere can be exploited for Extrasolar Planet studies with differential astrometry, differential phase and nulling interferometers. This paper combines the available data on the properties of the atmosphere at the South Pole and other Antarctic plateau sites for Extrasolar Planet science with interferometry.
• Martin, C. L., Walsh, W. M., Xiao, K., Lane, A. P., Walker, C. K., & Stark, A. A. (2003). The Inner 200pc: Hot Dense Gas. Astronomische Nachrichten, 324(SUPPL.1), 93-99.
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  Abstract: We present fully-sampled maps of 461GHz CO J = 4 → 3, 807GHz CO J = 7 → 6, and 492GHz [CI]3P1 → 3P0 emission from the inner 3 degrees of the Galactic Center region taken with the Antarctic Submillimeter Telescope and Remote Observatory (AST/RO) in 2001-2002. The data cover -1°.3 < ℐ < 2°, -0°.3 < b < 0°.2 with 0.5′ spacing, resulting in spectra in 3 transitions at over 24,000 positions on the sky. The CO J = 4 → 3 emission is found to be essentially coextensive with lower-J transitions of CO. The CO J = 7 → 6 emission is spatially confined to a far smaller region than the lower-J CO lines. The [CI] 3P1 → 3P0 emission has a spatial extent similar to the low-J CO emission, but is more diffuse. Bright CO J = 7 → 6 emission is detected in the well-known Galactic Center clouds SgrA and SgrB. Analyzing our CO J = 7 → 6 and CO J = 4 → 3 data in conjunction with J = 1 → 0 12CO and 13CO data previously observed with the Bell Laboratories 7-m antenna, we apply a Large Velocity Gradient (LVG) model to estimate the kinetic temperature and density of molecular gas in the inner 200 pc of the Galactic Center region. Typical pressures in the Galactic Center gas are n(H2) · Tkin ∼ 105.2 Kcm-3. We present an (ℐ, b) map of molecular hydrogen column density derived from our LVG results. © 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
• Gerecht, E., Yngvesson, S., Nicholson, J., Zhuang, Y., Rodriguez-Morales, F., Zhao, X., Dazhen, G. u., Zannoni, R., Coulombe, M., Dickinson, J., Goyette, T., Waldman, J., Groppi, C., Hedden, A., Golish, D., Walker, C., Stark, A., Martin, C., & Lane, A. (2002). TREND - A low noise terahertz receiver user instrument for AST/RO at the South Pole. Proceedings of SPIE - The International Society for Optical Engineering, 4855, 574-582.
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  Abstract: Based on the excellent performance of NbN HEB mixer receivers at THz frequencies which we have established in the laboratory, we are building a Terahertz REceiver with NbN HEB Device (TREND) to be installed on the 1.7 meter diameter AST/RO submillimeter wave telescope at the Amundsen/Scott South Pole Station. TREND is scheduled for deployment during the austral summer season of 2002/2003. The frequency range of 1.25 THz to 1.5 THz was chosen in order to match the good windows for atmospheric transmission and interstellar spectral lines of special interest. The South Pole Station is the best available site for THz observations due to the very cold and dry atmosphere over this site. In this paper, we report on the design of this receiver. In particular, we report on HEB mixer device performance, the quasi-optical coupling design using an elliptical silicon lens and a twin-slot antenna, the laser local oscillator (LO), as well as the mixer block design and the plans for coupling the TREND receiver to the sky beam and to the laser LO at the AST/RO telescope site.
• Groppi, C. E., Walker, C. K., Kulesa, C., Golish, D., Hedden, A., Gensheimer, P., Narayanan, G., Lichtenberger, A. W., Graf, U. U., & Heyminck, S. (2002). Desert STAR: A 7 pixel 345 GHz heterodyne array receiver for the Heinrich Hertz Telescope. Proceedings of SPIE - The International Society for Optical Engineering, 4855, 330-337.
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  Abstract: DesertSTAR is a 7 beam, 345 GHz heterodyne array receiver for the Heinrich Hertz Telescope (HHT) on Mt. Graham, AZ. The instrument uses fixed-backshort Superconductor-Insulator-Superconductor (SIS) mixers with a broadband waveguide probe. Instantaneous bandwidths greater than 2 GHz can be achieved over the entire 345 GHz atmospheric window. A cryostat with Joule-Thompson (JT) mechanical refrigerator allows continuous operation and 1.8W of cooling capacity at 4K, and provides the needed temperature stability for low-noise operation. Local Oscillator (LO) distribution is accomplished with a novel phase grating that yields high efficiency and power uniformity in a hexagonally symmetric geometry. The computer controlled bias system is an evolution of a proven design that is simple, portable to any computer platform, and easily extensible to over 100 channels. It provides control and monitoring of bias, temperature and vacuum from any X-windows capable machine, and writes an instrument status web page visible with any web browser. The 2 GHz Intermediate Frequency (IF) bandwidth allows the future addition of a wideband backend optimized for extragalactic observations, with ∼1700 km/s of velocity coverage. We present measurements of receiver performance and plans for integration on the HHT.
• Narayanan, G., Moriarty-Schieven, G., Walker, C. K., & Butner, H. M. (2002). Detection of infall signatures toward serpens SMM4. Astrophysical Journal Letters, 565(1 I), 319-330.
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  Abstract: We present the detection of kinematic infall signatures toward the Class 0 protostellar system SMM4 in the Serpens cloud core. We have observed the dense molecular gas toward the embedded source using millimeter and submillimeter line transitions of density sensitive molecular tracers, High signal-to-noise ratio maps obtained in HCO+ J = 1 → 0, J = 3 → 2, and J = 4 → 3, and CS J = 2 → 1 show the blue-bulge infall signature. The blue-bulge infall signature can be observed in the centroid velocity maps of protostellar objects when infall dominates over rotation. The line profiles of HCO+ and CS exhibit the characteristic blue asymmetric line profile signature consistent with infall. In addition, HCO+ and CS optical depth profiles obtained using isotopic observations show a red asymmetry also consistent with an infall interpretation. Using three-dimensional radiative transfer models based on the rotating, collapse model of Terebey, Shu, & Cassen, we derive infall parameters of the source. To determine the direction and orientation of molecular outflows in the larger Serpens cluster, wide-field mapping of CO J = 1 → 0 emission was also performed.
• Swain, M. R., Dumont, P. J., Lawson, P. R., Moore, J. D., Smythe, R. F., Walker, C. K., & D'Aubigny, C. D. (2002). Far infrared interferometer technology development: A progress report. Proceedings of SPIE - The International Society for Optical Engineering, 4852(2), 645-651.
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  Abstract: We report on the progress in developing cryogenic delay lines and integrated optics components. These are some of the critical components needed to enable far-infrared direct-detection interferometers. To achieve background-limited performance in the 40 to 400 μm region, the interferometer optics and delay lines must be cooled to near liquid Helium temperatures. Our cryogenic delay line designs incorporate a number of novel features and has been operated at liquid nitrogen temperatures. Our integrated optics effort has focused on producing single-mode spatial filters and beam combiners.
• Walker, C. K., Groppi, C. E., D'Aubigny, C. D., Kulesa, C., Hedden, A., Prober, D., Siddiqi, I., Kooi, J., Chen, G., & Lichtenberger, A. W. (2002). Integrated heterodyne array receivers for submillimeter astronomy. Proceedings of SPIE - The International Society for Optical Engineering, 4855, 349-354.
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  Abstract: The advent of large format (∼100 pixels) spectroscopic imaging cameras at submillimeter wavelength would fundamentally change the way in which astronomy is performed in this important wavelength regime. While the possibility of such instruments has been discussed for more than two decades, only recently have advances in mixer technology, device fabrication, micromachining, digital signal processing, and telescope design made the construction of such an instrument possible and economical. In our paper, we will present the design concept for a 10×10 heterodyne camera.
• Y., C., & Walker, C. K. (2002). Laser induced chemical etching of THZ waveguides and quasi optical devices in silicon. ICALEO 2002 - 21st International Congress on Applications of Laser and Electro-Optics, Congress Proceedings.
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  Abstract: Laser induced, micro-chemical etching is a promising new technology that can be used to fabricate three dimensional structures many millimeters across with micrometer accuracy. Laser micromachining possesses a significant edge over more conventional techniques. It does not require the use of masks and is not confined to crystal planes. A non-contact process, it eliminates tool wear and vibration problems associated with classical milling machines. At the University of Arizona we have constructed the first such laser micromachining system optimized for the fabrication ofTHz and far IR waveguide and quasi-optical components. Our system can machine structures up to 150 mm. in diameter down to a few microns accuracy in a short time, with a remarkable surface finish. This paper presents the design, operation and performance of our system, as well as examples of laser micromachined waveguide and quasi-optical structures. Finally, we compare this technology to existing processes and discuss possible future directions.
• Y., C., Walker, C. K., Golish, D., Swain, M. R., Dumont, P. J., & Lawson, P. R. (2002). Laser micro-machining of waveguide devices for sub-mm and far IR interferometry and detector arrays. Proceedings of SPIE - The International Society for Optical Engineering, 4852(2), 568-580.
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  Abstract: Laser induced, micro-chemical etching is a promising new technology that can be used to fabricate three dimensional structures many millimeters across with micrometer accuracy. Laser micromachining possesses a significant edge over more conventional techniques. It does not require the use of masks and is not confined to crystal planes. A non-contact process, it eliminates tool wear and vibration problems associated with classical milling machines. At the University of Arizona we have constructed the first such laser micromachining system optimized for the fabrication of THz and far IR waveguide and quasi-optical components. Our system can machine many millimeters across down to a few microns accuracy in a short time, with a remarkable surface finish. This paper presents the design, operation and performance of our system, and its applications to waveguide devices for sub millimeter and far IR interferometry.
• Datesman, A. M., Zhang, J. Z., Lichtenberger, A. W., & Walker, C. (2001). Fabrication of a superconducting hot-electron bolometer receiver with micromachined waveguide components. Biennial University/Government/Industry Microelectronics Symposium - Proceedings, 186-189.
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  Abstract: The use of superconducting hot-electron bolometers (HEBs) as mixing elements in terahertz heterodyne receivers has become increasingly attractive in the last few years. Such receivers find application in the fields of radio astronomy, molecular spectroscopy, and atmospheric remote sensing. We have developed a method of fabricating these devices, consisting of a niobium film less than one-quarter micron long and 150 Angstroms thick, using a gallium focused-ion beam (FIB). We intend to build a mixer which incorporates these devices, fabricated on membranes of silicon nitride, with terahertz waveguide components fabricated by laser-micromachining of silicon under CAD control. This paper discusses the three fabrication methods (focused-ion beam, the bulk micromachining of silicon necessary to obtain nitride membranes, and laser micromachining) used in our work.
• Stark, A. A., Bally, J., Balm, S. P., Bania, T. M., Bolatto, A. D., Chamberlin, R. A., Engargiola, G., Huang, M., Ingalls, J. G., Jacobs, K., Jackson, J. M., Kooi, J. W., Lane, A. P., Lo, K. -., Marks, R. D., Martin, C. L., Mumma, D., Ojha, R., Schieder, R., , Staguhn, J., et al. (2001). The antarctic submillimeter telescope and remote observatory (AST/RO). Publications of the Astronomical Society of the Pacific, 113(783), 567-585.
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  Abstract: The Antarctic Submillimeter Telescope and Remote Observatory, a 1.7 m diameter telescope for astronomy and aeronomy studies at wavelengths between 200 and 2000 μm, was installed at the South Pole during the 1994-1995 austral summer. The telescope operates continuously through the austral winter and is being used primarily for spectroscopic studies of neutral atomic carbon and carbon monoxide in the interstellar medium of the Milky Way and the Magellanic Clouds. The South Pole environment is unique among observatory sites for unusually low wind speeds, low absolute humidity, and the consistent clarity of the submillimeter sky. Especially significant are the exceptionally low values of sky noise found at this site, a result of the small water vapor content of the atmosphere. Four heterodyne receivers, an array receiver, three acousto-optical spectrometers, and an array spectrometer are currently installed. A Fabry-Perot spectrometer using a bolometric array and a terahertz receiver are in development. Telescope pointing, focus, and calibration methods as well as the unique working environment and logistical requirements of the South Pole are described.
• Tieftrunk, A. R., Jacobs, K., Martin, C. L., Siebertz, O., Stark, A. A., Stutzki, J., Walker, C. K., & Wright, G. A. (2001). ¹³Ci in high-mass star-forming clouds. Astronomy and Astrophysics, 375(2), L23-L26.
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  Abstract: We report measurements of the 12C/13C abundance ratio in the three galactic regions G 333.0-0.4, NGC 6334 A and G 351.6-1.3 from observations of the 12CI 3P2 → 3P1 transition and the hyperfine components of the corresponding 13CI transition near 809 GHz. These transitions were observed simultaneously with the CO 7-6 line emission at 806 GHz with the AST/RO telescope located at the South Pole. From a simultaneous fit to the 12CI 3P2 → 3P1 transition and the HF components of the corresponding 13CI transition and an independent estimate of an upper limit to the optical depth of the 12CI emission we determine intrinsic 12CI/13CI column density ratios of 23 ± 1 for G 333.0-0.4, 56 ± 14 for NGC 6334 A and 69 ± 12 for G 351.6-1.3. As the regions observed are photon dominated, we argue that the apparent enhancement in the abundance of 13C towards G 333.0-0.4 may be due to strong isotope-selective photodissociation of 13CO, outweighing the effects of chemical isotopic fractionation as suggested by models of PDRs. Towards NGC 6334 A and G 351.6-1.3 these effects appear to be balanced, similar to the situation for the Orion Bar region observed by Keene et al. (1998).
• Y., C., Walker, C. K., & Jones, B. D. (2001). Laser microchemical etching of waveguides and quasi-optical components. Proceedings of SPIE - The International Society for Optical Engineering, 4557, 101-110.
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  Abstract: Laser induced micro chemical etching of silicon can be used to quickly and cheaply machine high-quality three-dimensional structures that would otherwise be nearly impossible to fabricate, in particular THz waveguide structures and quasi-optical components. At the University of Arizona, the construction and characterization of the first laser micro-machining system designed for waveguide components fabrication has been completed. Our system can be used to fabricate focal plane heterodyne mixer arrays, coherent beam combiners, AR grooved silicon lenses, phase gratings, single mode filters and more. Laser micro machining enables the fabrication of three-dimensional structures down to a few microns accuracy and up to 6 inches across in a short time. This presentation discusses the design and performance of our micro-machining system, and illustrates the type, range and performance of quasi-optical components this exciting new technology will make accessible.
• D'Aubigny, C. Y., Walker, C. K., & Jones, B. D. (2000). Laser micromachining of THz systems. Proceedings of SPIE - The International Society for Optical Engineering, 4015, 584-588.
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  Abstract: Laser micromachining is a powerful alternative to the conventional fabrication of waveguide structures, feedhorns and backshorts designed to operate at frequencies greater than 800 GHz. Computer controlled laser etching permits the direct scaling and fabrication of successful waveguide designs to THz frequencies with micrometer tolerances in just a few hours. Laser micromachining can also be used to produce quasi optical components such as anti-reflection(AR) grooved silicon lenses, and local oscillator(LO) phase gratings. In these proceedings we describe the specifics of the laser micromachining facility being completed at the Steward Observatory Radio Astronomy Laboratory and its potential for the fabrication of THz imaging array receivers.
• Groppi, C. E., Walker, C. K., Hungerford, A. L., Narayanan, G., & Lichtenberger, A. (2000). 345 GHz array receiver for the Heinrich Hertz Telescope. Proceedings of SPIE - The International Society for Optical Engineering, 4015, 253-259.
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  Abstract: In this paper, we present the design and construction of DesertSTAR, a seven pixel heterodyne receiver for the 345 GHz atmospheric window for operation on the Heinrich Hertz Telescope located at Mt. Graham, Arizona. The seven beams are arranged in a hexagonal close-packed pattern with one beam spacing. The instrument uses fixed tuned split-block, half-height waveguide mixers with Nb SIS junctions. The mixers have an instantaneous bandwidth of 2 GHz, centered on 5 GHz. The cryostat uses a NRAO Joule-Thompson refrigerator to cool the mixers, isolators, amplifiers and optics to 4 K. The computer controlled bias system allows automated bias optimization and monitoring both locally and remotely. The instrument will take full advantage of the good 345 GHz weather at the HHT and dramatically increase scientific throughput. We expect to have first operations with seven pixels in November, 2000.
• Walker, C. K., Groppi, C. E., d'Aubigny, C., Chen, G., Schieder, R., Narayanan, G., Lichtenberger, A., & Siegel, P. (2000). Far-infrared Array Receiver (FAR) for SOFIA. Proceedings of SPIE - The International Society for Optical Engineering, 4014, 125-136.
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  Abstract: In this paper, we present the design for a 16-channel heterodyne array receiver for use on SOFIA. The array will be capable of using either hot-electron bolometers or membrane mounted Schottky diodes in efficient, low-cost waveguide mounts. Focal plane arrays will be constructed to target astrophysically important lines between approximately 1.9 and 3 THz. Due to the prevailing physical conditions in the interstellar medium, this frequency range is one of the richest in the FIR portion of the spectrum. An array receiver designed for this wavelength range will make excellent use of the telescope and the available atmospheric transmission, and will provide a new perspective on stellar, chemical, and galaxy evolution in the present as well as past epochs. The proposed system uses the most sensitive detectors available in an efficient optical system. The local oscillator will be a compact CO2 pumped far-infrared laser specifically developed for SOFIA. The backend spectrometer will be a 4×4 array of acousto-optic spectrometers (AOS). The spectrometer utilizes proven hardware and technologies to provide broadband performance (≥1 GHz per AOS channel) and high spectral resolution (1 MHz). The proposed instrument will be the fastest and most sensitive heterodyne receiver ever to operate in its wavelength regime.
• Glenn, J., Walker, C. K., & Young, E. T. (1999). Magnetic fields in star formation regions: 1.3 millimeter continuum polarimetry. Astrophysical Journal Letters, 511(2 PART 1), 812-821.
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  Abstract: We present a nine-point λ = 1.3 mm continuum polarization map of the inner arcminute of the DR 21 cloud core. The polarization and position angles are very uniform, and the inferred magnetic field (P.A. ∼ 75°) is nearly orthogonal to the cloud elongation (P.A. ∼ 7°). Applying the virial theorem and comparing the continuum polarimetry, we find that the magnetic field strength must be greater than a few mG to have a significant impact. Turbulent gas motions are probably a more significant source of support against self-gravity in the cloud core than in the magnetic field. We also report a survey of the λ = 1.3 mm polarization of 14 star-forming cloud cores (〈P〉 = 1.6%). The λ = 1.3 mm distribution is similar to the λ = 100 μm and λ = 800 μm polarization distributions in the literature except that the 1.3 mm distribution peaks at P < 1%. We compared our polarimetry of nine of the cloud cores to physical parameters derived from far-infrared photometry in a homogeneous fashion. Consistent with theoretical expectations, the polarizations of these cloud cores do not depend on the λ = 1.3 mm dust optical depth, emission temperature, or emissivity spectral index. Although the sample is very small, it appears that the polarization is larger on average for the cloud cores with mean densities of nH2 > 1.5 × 107 cm-3 than for those with nH2 > 1.5 × 107 cm-3. The sky-plane projection of the magnetic field lines in the seven elongated cloud cores with 800 μm or 1.3 mm polarization detections greater than 3 σ appear randomly distributed with respect to the position angles of cloud core elongations. This implies that magnetic fields do not provide substantial anisotropic support against self-gravity in this sample of star-forming cloud cores. The magnetic fields in the cloud cores also appear randomly oriented with respect to the Galactic plane.
• Narayanan, G., & Walker, C. K. (1998). A parameterized study of the detection of infall in protostellar systems. Astrophysical Journal Letters, 508(2 PART II), 780-790.
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  Abstract: We report new radiative transfer calculations of submillimeter and millimeter molecular line emission from fully three-dimensional models of protostars. The radiative transfer calculations are performed using the parameterized, semianalytic, rotating collapse solutions of Terebey, Shu, & Cassen. These results are compared with the class of "self-consistent," nonspherical, hydrodynamic, rotating collapse solutions (Boss; Walker, Narayanan, & Boss). We explore the sensitivity of the "blue-bulge" infall signature to rotational rate, infall time, sound speed, and abundance gradients in the molecular tracer of choice. At high angular resolutions, models with moderate to high rotational rates exhibit the "polar blue bulge" - a centroid velocity signature of underlying Keplerian rotation in an embedded cloud core. Submillimeter transitions of HCO+ and CS are found to be better than millimeter transitions in detecting infall, especially at early collapse times. © 1998. The American Astronomical Society. All rights reserved.
• Narayanan, G., Walker, C. K., & Buckley, H. D. (1998). The "blue-bulge" infall signature toward IRAS 16293-2422. Astrophysical Journal Letters, 496(1 PART I), 292-310.
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  Abstract: We present the first detection of the "blue-bulge" infall signature toward a protostellar source. The blue-bulge infall signature can be observed in the centroid velocity maps of protostellar objects when infall dominates over rotation. This infall signature can be detected under a wide variety of source conditions. The detection of the blue-bulge infall signature toward a protobinary system such as IRAS 16293-2422 suggests that detailed studies of gravitational collapse toward a large number of protostellar sources may now be possible. Our CS J = 1 → 6 data appear to be tracing gaseous material in the inner circumbinary core, while the CS J = 5 → 4 and HCO+ J = 4 → 3 appear to trace the outer envelope and static core in addition to the inner circumbinary core. The mass accretion rate through the infall region appears to be consistent with an inside-out collapse model for the source. Using three-dimensional radiative transfer models based on the rotating, collapse solution of Terebey, Shu, & Cassen, we derive the infall parameters of the IRAS 16293-2422 cloud core. Our best-fit model suggests that the infall radius of the IRAS 16293-2422 cloud core is ∼39″ (0.03 pc). © 1998. The American Astronomical Society. All rights reserved.
• Stark, A. A., Carlstrom, J. E., Israel, F. P., Menten, K. M., Peterson, J. B., Phillips, T. G., Sironi, G., & Walker, C. K. (1998). Plans for a 10-m submillimeter-wave telescope at the South Pole. Proceedings of SPIE - The International Society for Optical Engineering, 3357, 495-506.
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  Abstract: A 10 meter diameter submillimeter-wave telescope has been proposed for installation and scientific use at the NSF Amundsen-Scott South Pole Station. Current evidence indicates that the South Pole is the best submillimeter-wave telescope site among all existing or proposed ground-based observatories. Proposed scientific programs place stringent requirements on the optical quality of the telescope design. In particular, reduction of the thermal background and offsets requires an off-axis, unblocked aperture, and the large field of view needed for survey observations requires shaped optics. This mix of design elements is well-suited for large-scale (square degree) mapping of line and continuum radiation from submillimeter-wave sources at moderate spatial resolutions (4 to 60 arcsecond beam size) and high sensitivity (milliJansky flux density levels). The telescope will make arcminute angular scale, high frequency Cosmic Microwave Background measurements from the best possible ground-based site, using an aperture which is larger than is currently possible on orbital or airborne platforms. The telescope design is homologous. Gravitational changes in pointing and focal length will be accommodated by active repositioning of the secondary mirror. The secondary support, consisting of a large, enclosed beam, permits mounting of either a standard set of Gregorian optics, or prime focus instrumentation packages for CMBR studies. A tertiary chopper is located at the exit pupil of the instrument. An optical design with a hyperboloidal primary mirror and a concave secondary mirror provides a flat focal surface. The relatively large classical aberrations present in such an optical arrangement can be small compared to diffraction at submillimeter wavelengths. Effective use of this telescope will require development of large (1000 element) arrays of submillimeter detectors which are background-limited when illuminated by antenna temperatures near 50 K. ©2003 Copyright SPIE - The International Society for Optical Engineering.
• Walker, C. K., Hungerford, A. L., Narayanan, G., Groppi, C. E., Bloomstein, T. M., Palmacci, S. T., Stern, M. B., & Curtin, J. E. (1998). Laser micromachining of silicon: A new technique for fabricating terahertz imaging arrays. Proceedings of SPIE - The International Society for Optical Engineering, 3357, 45-52.
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  Abstract: One of the main obstacles encountered in designing low noise, high efficiency, heterodyne receivers and local oscillator sources at submillimeter wavelengths is the quality and cost of waveguide structures. At wavelengths shorter than 400 micrometers, rectangular waveguide structures, feed-horns, and backshorts become extremely difficult to fabricate using standard machining techniques. We have used a new laser milling technique to fabricate high quality, THz waveguide components and feedhorns. Once metallized, the structures have the properties of standard waveguide components. Unlike waveguide components made using silicon wet-etching techniques, laser-etched components can have almost any cross section, from rectangular to circular. Under computer control, the entire waveguide structure (including the corrugated feedhorn a submillimeter-wave mixer or multiplier can be fabricated to micrometer tolerances in a few hours. Laser etching permits the direct scaling of successful waveguide multiplier and mixer designs to THz frequencies. Since the entire process is computer controlled, the cost of fabricating submillimeter waveguide components is significantly reduced. With this new laser etching process, the construction of high performance waveguide array receivers at THz frequencies becomes tractable. In this paper we will describe the laser etching technique and discuss how it can be used to construct THz imaging arrays. We will also describe the construction of a prototype 810 GHz mixer which utilizes these new construction techniques. ©2003 Copyright SPIE - The International Society for Optical Engineering.
• Glenn, J., Walker, C. K., & Jewell, P. R. (1997). HCO⁺ spectropolarimetry and millimeter continuum polarimetry of the DR 21 star-forming region. Astrophysical Journal Letters, 479(1 PART I), 325-331.
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  Abstract: We report the results of a search for linear polarization in emission lines toward the outflow lobes of DR 21 (HCO+ J = 1-0), Mon R2 (HCO+ J = 1-0), and IRAS 16293-2422 (CS J = 2-1). Theoretical models suggest linear polarization in optically thin emission lines may be present at the level of a few percent in these regions. However, no polarization was detected. An upper limit of 0.4% can be placed on the polarization toward DR 21 and Mon R2, and 1.2% toward IRAS 16293-2422. Using published HCO+ interferometry of DR 21, we argue that the lack of polarization is probably due to the presence of clumpy, turbulent gas, which is unresolved by our beam (70″ FWHM). Diminished polarization by multiple scattering cannot be ruled out. We also present new λ = 1.3 mm and λ = 2 mm continuum polarimetry of the DR 21 dust core. Polarizations of P = 1.7% ± 0.2% and P = 0.77% ± 0.13%, respectively, were detected. The λ = 2 mm polarization is lower than the ∼2.0%-2.5% previously reported at millimeter and submillimeter wave-lengths. However, with the unpolarized free-free component of the flux density from H II regions in the star-forming core removed, the level of polarization is consistent with the previous observations. Based on our observations and those previously reported, there is no clear dependence of level of polarization on beam size in the inner ′30″ of the dust core. The λ = 2 mm polarization position angle, 2° ± 5°, is rotated ∼20° with respect to the previous measurements. If the rotation is real, it implies the magnetic field on this expanded scale (42″ FWHM beam) is more perpendicular to the general elongation of the dust cloud than it is within the inner 20″ of the dust core. © 1997. The American Astronomical Society. All rights reserved.
• Glenn, J., Walker, C. K., & Young, E. T. (1997). Cyclops: A single beam 1.3 millimeter polarimeter. International Journal of Infrared and Millimeter Waves, 18(2), 285-300.
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  Abstract: We present the design of a broad band, 1.3 mm polarimeter for astronomical observations and the results of first light observations at the Heinrich Hertz Telescope (HHT). The systematic polarization of the instrument plus telescope is 1.1%. The observed linear polarization of DR 21, P = 1.65%±0.14% and θ = 16° ± 2°, is consistent with previous measurements. Our 0.9% upper limit on the linear polarization of Cepheus A is the first 1.3 mm measurement reported. With Cyclops it is possible to map the magnetic fields in several of the brightest molecular cloud dust cores in the Milky Way. We comment on the expected performance of an achromatic Rexolite half-waveplate for λ = 800 μm to 1.3 mm.
• Narayanan, G., & Walker, C. K. (1996). Evidence for multiple outbursts from the cepheus a molecular outflow. Astrophysical Journal Letters, 466(2 PART I), 844-865.
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  Abstract: We report evidence for multiple episodes of outflow activity in the Cepheus A star-forming region. We present new, high signal-to-noise CSO observations of 12CO J = 3 → 2, 13CO J = 3 → 2, and CS J = 7 → 6 emission. We also present new, interferometric and single-dish observations of 12CO J = 1 → 0 emission toward the Cepheus A molecular outflow. Using line core velocity centroid maps, we argue that the multiple self-absorption features in the CO J = 3 → 2 line profiles are tracing cool shells of material swept-up by an episodic outflow. We present the results of a flexible three-dimensional LTE outflow model and radiative transfer code that best explain the observations as tracing multiple generations of outflow. The difference in the dynamical timescales between the "old" and "new" swept-up shells gives an estimate of ∼1.6 × 105 yr between the two generations of outbursts. The high-resolution 12CO J = 1 → 0 maps obtained by combining single-dish observations with interferometric data clearly show a shell-like morphology at low velocities. This cool shell appears to encompass the hot, extremely high velocity (EHV) winds seen in the J = 3 → 2 transition. The interferometric observations show that the current generation of outflow is being powered by the object Cepheus A-HW 2. There is also evidence for redirection of the blueshifted lobe of the current generation of outflow, possibly by the extended NH3 structure Cep A-3. We present a model of the outflow geometry that can explain most of the observed structures in Cepheus A. The rotating, dense core traced by the CS observations is ∼0.32 pc in diameter and has an estimated dynamical mass of 330 M⊙. The velocity structure of the core suggests that it is being disrupted by the high-velocity winds driving the molecular outflow. This new technique of extracting information from self-absorbed line profiles could be used to study other deeply embedded protostellar systems. Since outflows are believed to be intimately tied to accretion, such studies could lead to constraints on mass accretion models for young stellar objects. © 1996. The American Astronomical Society. All rights reserved.
• Kooi, J. W., Chan, M. S., Bin, M., Bumble, B., LeDuc, H. G., Walker, C. K., & Phillips, T. G. (1995). Development of an 850 GHz waveguide receiver using tuned SIS junctions on 1 μm Si₃N₄ membranes. International Journal of Infrared and Millimeter Waves, 16(2), 349-362.
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  Abstract: We report preliminary development work on a 850 GHz SIS heterodyne receiver employing a tuned niobium tunnel junction on a 1 μm Si3N4 supporting membrane. Since the mixer is meant to be operated well above the superconducting gap frequency of niobium (2Δ/h≈690 GHz) special care has been taken to minimize transmission line loss. We have therefore used junctions with an integrated radial stub RF matching network to tune out the large shunt susceptance of the junction and minimize the niobium film absorption loss. Scale model measurements of the waveguide embedding impedance have been made to aid in the design of the choke structure and RF matching network. Detailed Fourier Transform Spectrometer measurements of tuned junctions or both SiO2 and silicon nitride membranes show response up to 1100 GHz and indicate that the absorption loss in the niobium film is in the order of 4-7 dB at 850 GHz, in fairly good agreement with the theoretical loss calculated from the Mattis-Bardeen theory. The junctions have a center frequency of 800 GHz which presents a 6% downshift from the designed value.
• Wolf-Chase, G. A., & Walker, C. K. (1995). The dense gas component of outflows in the monoceros OB1 dark cloud. Astrophysical Journal Letters, 447(1), 244-256.
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  Abstract: Wolf-Chase, Walker, & Lada (1995, hereafter WWL) found that most of the CS J = 2 → 1 emission in the Mon OB1 dark cloud is concentrated in two regions they refer to as the South Cloud (SCL) and North Cloud (NCL). These regions encompass five of the 10 outflows previously identified in CO by Margulis, Lada, & Snell (1988). The spatial morphologies of the CS J = 5 → 4 and CS J = 7 → 6 emission regions in the SCL (WWL) are significantly different from those of the CS J = 2 → 1 emission. Velocity centroid plots of the CS J = 2 → 1 and CS J = 5 → 4 emission in the line cores indicate that the upper transition primarily traces dense core material, but the lower transition primarily traces dense outflow material. We have found that the CS J = 2 → 1 transition traces a large portion of the dense, low-velocity component of outflows in the Mon OB1 dark cloud. The outflows are identifiable through non-Gaussian, asymmetric wings in the CS J = 2 → 1 lines. This outflow component is not identifiable in CO because the gas in the asymmetric CS line wings is moving at velocities which lie within the core of the much broader CO lines. Two of the outflows which were previously classified as monopolar in CO (Margulis et al. 1988) appear bipolar in CS. We find the mass of this component to be about an order of magnitude greater than previous estimates of the low-velocity outflow component. Comparison of the masses derived for the CS outflows to the masses derived by WWL for the total CS emission in the SCL and NCL indicate that at least 20% of the material in the SCL has been entrained in outflows, and approximately 25%-50% of the material in the NCL is associated with a rotating cloud swept up by an outflow. The addition of the low-velocity CS outflow component to previous estimates of outflow energetics implies that multiple generations of outflows need not be required to support this cloud against collapse. Our results neither support nor rule out the existence of fossil outflows in this cloud. A fully sampled, unbiased survey of the cloud is required to search for such outflows.
• Wolf-Chase, G. A., Walker, C. K., & Lada, C. J. (1995). The relationship of dense gas to star formation in the Monoceros OB1 dark cloud. Astrophysical Journal Letters, 442(1), 197-207.
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  Abstract: We have conducted a CS survey of the 10 outflows and 30 IRAS sources identified by Margulis, Lada, & Snell (1988) and Margulis, Lada, & Young (1989), in the Mon OB1 dark cloud to study the relationship between outflows, young stellar objects, and dense cores in this cloud. We detected the CS J = 2 → 1 transition in the vicinity of 20 of the 30 IRAS sources, six of which are associated with the six most massive outflows in this cloud. We did not detect CS emission toward the four remaining outflows; three of these outflows are not associated with IRAS sources. The brightest CS J = 2 → 1 emission is concentrated in two spatially extended regions encompassing five of the 10 outflows and eight of the IRAS sources. Within the extended regions, CS line widths are ∼30% broader near outflow lobes than on the line of sight toward IRAS sources. There is a strong correlation of CS antenna temperature with IRAS source luminosity and a weaker correlation of CS line width with IRAS source luminosity for IRAS sources associated with outflows in Mon OB1. There is no apparent correlation of either CS antenna temperature or line width with IRAS source luminosity for IRAS sources not associated with outflows (quiescent sources). We have detected the CS J = 5 → 4 transition in the vicinity of four outflow sources in this cloud and around none of the quiescent IRAS sources. The CS J = 5 → 4 emission is extended around the two most massive outflows in the cloud and has been mapped in these regions. CS J = 7 → 6 emission has been detected and mapped about the brightest outflow source in this cloud (Allen's star; Allen 1972). The J = 7 → 6 emission peaks at the position of the IRAS source. The morphology of the J = 7 → 6 emission differs from that of the J = 5 → 4 emission.
• Choi, M., J., N., Jaffe, D. T., & Walker, C. K. (1994). Observation of [C I] toward the GL 2591 and W28 A2 molecular outflows. Astrophysical Journal Letters, 435(2), 734-737.
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  Abstract: Molecular outflows associated with GL 2591 and W28 A2 were observed in the 492 GHz [C I] line. Upper limits are set on the [C I] emission in the extremely high velocity line wings. These limits are discussed in terms of wind-driven and jet-driven models of molecular outflows.
• Kooi, J. W., Walker, C. K., LeDuc, H. G., Schaffer, P. L., Hunter, T. R., Benford, D. J., & Phillips, T. G. (1994). A lownoise 665 GHz SIS quasi-particle waveguide receiver. International Journal of Infrared and Millimeter Waves, 15(3), 477-492.
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  Abstract: We report recent results on a 565-690 GHz SIS heterodyne receiver employing a 0.36 μm2 Nb/AlOx/Nb SIS tunnel junction with high quality circular non-contacting backshort and E-plane tuners in a full height waveguide mount. No resonant tuning structures have been incorporated in the junction design at this time, even though such structures are expected to help the performance of the receiver. The receiver operates to at least the gap frequency of Niobium, ≈ 680 GHz. Typical receiver noise temperatures from 565-690 GHz range from 160K to 230K with a best value of 185K DSB at 648 GHz. With the mixer cooled from 4.3K to 2K the measured receiver noise temperatures decreased by approximately 15%, giving roughly 180K DSB from 660 to 680 GHz. The receiver has a full 1 GHz IF passband and has been successfully installed at the Caltech Submillimeter Observatory in Hawaii. © 1994 Plenum Publishing Corporation.
• Walker, C. K., Maloney, P. R., & Serabyn, E. (1994). Vibrationally excited CS: A new probe of conditions in young protostellar systems. Astrophysical Journal Letters, 437(2 PART 2), L127-L130.
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  Abstract: We present the first detection of vibrationally excited C32S J = 10 → 9 and J = 7 → 6 emission toward a young stellar object. Toward IRAS 16293-2422, the vibrationally excited C32S emission is redshifted ∼3.9 km s-1 from the systemic velocity of the core. The emission must arise in warm (T ≳ 1000 K), dense (n ≳ 1011-1012 cm-3) gas. The most plausible origin for the emission appears to be self-gravitating instabilities in a protostellar accretion disk, which produce waves and shocks.
• Walker, C. K., Narayanan, G., & Boss, A. P. (1994). Spectroscopic signatures of infall in young protostellar systems. Astrophysical Journal Letters, 431(2), 767-782.
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  Abstract: We present the first radiative transfer calculations of millimeter and submillimeter molecular line emission from "self-consistent" models of nonspherical, collapsing, rotating protostellar systems. We find that: (1) the morphology of the gas and dust emission is a strong function of collapse time and angular resolution, (2) centroid velocity maps can provide a distinct infall signature, and (3) the unambiguous detection of infall toward binary systems becomes more difficult with increasing binary separation.
• Latter, W. B., Walker, C. K., & Maloney, P. R. (1993). Detection of the carbon monoxide ion (CO⁺) in the interstellar medium and a planetary nebula. Astrophysical Journal Letters, 419(2 PART 2), L97-L100.
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  Abstract: We report detection of the carbon monoxide ion (CO+) in the interstellar medium (M17SW) and a planetary nebula (NGC 7027). These detections are based on observations of three millimeter and submillimeter transitions in M17SW and one in NGC 7027. Chemical models suggest that CO+ should be most abundant where complex molecules are least likely to be present. In our search for CO+ we therefore minimized the chance of confusion while maximizing the probability of detection by observing regions whose chemistry is dominated by the effects of ultraviolet radiation.
• McGrath, W. R., Walker, C., Yap, M., & Tai, Y. (1993). Silicon micromachined waveguides for millimeter-wave and submillimeter-wave frequencies. IEEE Microwave and Guided Wave Letters, 3(3), 61-63.
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  Abstract: Rectangular waveguide is commonly used up to high millimeter-wave frequencies. However, conventional machining techniques for waveguides operating above a few hundred GHz are complicated and costly. The development of silicon micromachining techniques to create silicon-based waveguide circuits, which can operate up to high submillimeter-wave frequencies, is reported. As a first step, WR-10 waveguide has been fabricated from (110) silicon wafers. Insertion loss measurements of gold plated silicon waveguide show performance comparable to standard metal waveguides. It is suggested that active devices and planar circuits can be integrated with the waveguides, solving the traditional mounting problems.
• Walker, C. K., Narayanan, G., Büttgenbach, T. H., Carlstrom, J. E., Keene, J., & Phillips, T. G. (1993). The detection of [C I] in molecular outflows associated with young stellar objects. Astrophysical Journal Letters, 415(2), 672-679.
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  Abstract: The first observations of atomic carbon in molecular outflows are presented. Most of the outflow regions show similar [C I] and 13CO line profiles suggesting the [C I] emission from outflow sources traces the same volume of gas as the CO emission, as previous studies have suggested for molecular clouds in general. The [C I] and CO column densities for the 11 sources surveyed are computed over wing and line center velocities. If the [C I] column densities derived from line center velocities are probing conditions in the ambient cloud in the vicinity of the infrared source, then a comparison of these values indicates the carbon abundance in the low-velocity component of the outflows is essentially the same as in the ambient cloud; there is no evidence for shock enhancement of [C I] in the swept-up material. A map of the [C I] emission from the central arcminute of the luminous DR 21 outflow is presented. The [C I] emission is detected from the two CO outflow lobes; [C I] emission from the southwest lobe appears as a limb-brightened, conical shell. Outflow parameters derived from [C I] are consistent with those derived from CO, suggesting the [C I] emission arises from ambient cloud material swept-up by the outflow. The presence of carbon in the swept-up component of the outflows indicates that gas phase carbon is present deep within molecular clouds and is not confined solely to surface layers.
• Büttgenbach, T. H., Keene, J., Phillips, T. G., & Walker, C. K. (1992). Submillimeter detection of extragalactic C I emission: IC 342. Astrophysical Journal Letters, 397(1 PART 2), L15-L17.
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  Abstract: We report observations of the 3P1 → 3P0 transition of atomic carbon at 492 GHz in IC 342, made at the Caltech Submillimeter Observatory, which represent the first extragalactic submillimeter detection of C I. The C I emission from IC 342 was found to have a main beam temperature of about 1 K, which is a factor of 2 greater than that for 13CO(2 → 1) in the same size beam. Comparing our data with COBE observations of the Milky Way, we find that the ratio of C I to CO(2 → 1) is approximately a factor of 2 lower in IC 342 than in our Galaxy. The result is interpreted in terms of photodissociation region models which require higher gas densities for IC 342 than for the Milky Way.
• Falgarone, E., Phillips, T. G., & Walker, C. K. (1991). The edges of molecular clouds: Fractal boundaries and density structure. Astrophysical Journal Letters, 378(1), 186-201.
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  Abstract: This paper continues a discussion of the manifestations of the highly nonlinear physical equations underlying the dynamics of the dense interstellar medium. Previously, Falgarone and Phillips confirmed that the velocity field in non-star-forming regions could be explained as a turbulent phenomenon, showing the Kolmogorov scaling of velocity dispersion with spatial extent, and proposed that the excess of large velocity deviations (line wings stronger than predicted by a Gaussian distribution) corresponds to the fundamental property of turbulent flows called intermittency. In the present work we inspect the spatial structure of the dense medium. The observations of clouds at two different distances were carried out at high angular resolution using several transitions of the carbon monoxide molecule. Cloud edge regions were selected for the study to avoid the spatial crowding of emitting components, which obscures the structure of cloud cores. The selected regions include components from the line core and line wings of the parent complexes. We find that spatial structure exists on all scales down to our best angular resolution (0.02 pc) and that the emission arises from structures small compared to the telescope beam. Maps over a large range of scale sizes are self-similar suggesting a fractal structure. This is made quantitative by the use of the area-perimeter relation. A fractional value is found for the dimension parameter, D = 1.36 (with a formal error of ±0.02), in agreement with that found for atmospheric clouds and other interstellar clouds. An unexpected result is that this dimension is the same for the largest entities (∼100 pc) which are self-gravitating and the smallest (∼0.1 pc) which are not. It is also independent of the rotational transition of CO used to map the molecular gas. In addition, the intercept of the logarithmic perimeter-area relation scales with the resolution of the observations in accord with Mandelbrot's prediction for fractals. From the study of the 12CO (J = 3-2) and 12CO (J = 2-1) lines we find that the 12CO (J = 3-2)/12CO (J = 2-1) ratio remains constant at ∼0.55 over the whole range (about a factor of 10) of line intensities available in the maps. We interpret this as a signature of uniform excitation conditions for the CO rotational transitions, throughout the entire range of column densities. It is remarkable that the gas in the line wings cannot be distinguished from that in the line cores on the basis of this ratio. The 13CO/12CO ratios show that the 12CO (J = 2-1) optical depth has a typical value of ∼4. We argue that the 12CO (J = 3-2) emission is almost thermalized and the gas seen in this transition is quite dense (104 cm-3 or more for nH2) and cold (∼10 K). An estimate of the heating and cooling rates for these regions shows that the result is plausible. This fits with the overall framework of dense gas organized in a self-similar distribution of sizes continuing down to a threshold estimated to be smaller than 2000 AU. The dense molecular material is therefore very much non-space filling.
• Walker, C. K., Adams, F. C., & Lada, C. J. (1990). 1.3 millimeter continuum observations of cold molecular cloud cores. Astrophysical Journal, 349(2), 515-528.
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  Abstract: We present the results of a 1.3 mm continuum survey of cold IRAS sources known to be associated with star formation regions. A dozen sources were detected, eleven of which we mapped. By combining our observations with those of other observers and IRAS, we are able to study the properties of the extended dust envelopes in which these IR sources are embedded.
• Walker, C. K., Kooi, J. W., Chant, M., LeDuc, H. G., Schaffer, P. L., Carlstrom, J. E., & Phillips, T. G. (1990). A low-noise 492 GHz SIS waveguide receiver. International Journal of Infrared and Millimeter Waves, 13(6), 785-798.
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  Abstract: In this paper we discuss the design and performance of an SIS waveguide receiver which provides low noise performance from 375 to 510 GHz. At its design frequency of 492 GHz the receiver has a double sideband noise temperature of ∼172 K. By using embedded magnetic field concentrators, we are able to effectively suppress Josephson pair tunneling. Techniques for improving receiver performance are discussed. © 1992 Plenum Publishing Corporation.

Proceedings Publications

• Arenberg, J. (2021). OASIS architecture: key features. In SPIE, 11820, 20.
• Choi, H. (2021). Mid-to-high frequency characterization of inflatable membrane optics. In SPIE, 11820, 10.
• Esparza, M. (2021). Stressed deformable reflector design and pneumatic membrane antenna for cryogenic thermal vacuum chamber testing. In SPIE, 11820, 7.
• Palisoc, A. (2021). Analytical and finite element analysis tool for nonlinear membrane antenna modeling for astronomical applications. In SPIE, 11820, 19.
• Quach, H. (2021). Deflectometry-based thermal vacuum testing for a pneumatic terahertz antenna. In SPIE, 11820, 13.
• Sirsi, S. (2021). Parametric design study of the Orbiting Astronomical Satellite for Investigating Stellar Systems (OASIS) space telescope. In SPIE, 11820, 15.
• Takashima, Y. (2021). All reflective THz telescope design with an inflatable primary antenna for Orbiting Astronomical Satellite for Investigating Stellar Systems (OASIS) mission. In SPIE, 11820, 9.
• Walker, C. K. (2021, August). Orbiting Astronomical Satellite for Investigating Stellar Systems: OASIS. In SPIE, 1180, 52 pp.
• Green, R. F., Allen, L., Alvarez Del Castillo, E. M., Brocious, D. K., Corbally, C. J., Davis, D. R., Falco, E. E., Gabor, P., Hall, J. C., Jannuzi, B., Larson, S. M., Mighell, K. J., Nance, C., Shankland, P. D., Walker, C. E., Williams, G., & Zaritsky, D. F. (2014, jan). Progress in Dark Sky Protection in Southern Arizona. In American Astronomical Society Meeting Abstracts, 223, #413.05.
• Pompea, S. M., Walker, C. E., Dugan, C., Roddy, W. T., & Newhouse, M. (2014, jan). Dark Skies Yuma: An NOAO and APS Program on Light Pollution Education. In American Astronomical Society Meeting Abstracts, 223, #413.04.
• Towner, A. P., Walker, C. K., Tolls, V., & Martin, C. D. (2014, jan). A Possible Protostar in IGGC 22. In American Astronomical Society Meeting Abstracts, 223, #454.36.
• Walker, C. E., & Green, R. F. (2014, jan). IAU Commission 50 on Astronomical Site Protection. In American Astronomical Society Meeting Abstracts, 223, #413.07.
• Jensen, L. J., & Walker, C. E. (2013, jan). Characterizing Sky Brightness Throughout the Night in and around Tucson Arizona. In American Astronomical Society Meeting Abstracts, 221, #156.04.
• Newhouse, M. A., Walker, C. E., Boss, S. K., & Hennig, A. J. (2013, apr). Mobilizing the GLOBE at Night Citizen-Scientist. In Communicating Science: A National Conference on Science Education and Public Outreach, 473, 295.
• Pompea, S. M., Sparks, R. T., Dugan, C., & Walker, C. E. (2013, jan). Lessons Learned from the Arizona Galileoscope Star Party Program. In American Astronomical Society Meeting Abstracts, 221, #201.02.
• Sparks, R. T., Garmany, C. D., Pompea, S. M., & Walker, C. E. (2013, jan). An Extended Day Program on the Tohono O'odham Nation. In American Astronomical Society Meeting Abstracts, 221, #201.03.
• Sparks, R. T., Garmany, K., Siquieros, J. M., Austin, C. L., Pompea, S. M., & Walker, C. E. (2013, apr). An After School Education Program on the Tohono O'odham Nation. In Communicating Science: A National Conference on Science Education and Public Outreach, 473, 267.
• Walker, C. E., & Buxner, S. (2013, apr). The Impact of Light Pollution Education Through a Global Star-Hunting Campaign and Classroom Curricula. In Communicating Science: A National Conference on Science Education and Public Outreach, 473, 321.
• Walker, C. E., Low, R., Zepeda, O., & Valdez, S. (2013, apr). The Science of Storytelling: Indigenous Perspective in Environmental Change. In Communicating Science: A National Conference on Science Education and Public Outreach, 473, 85.
• Walker, C. E., Tafreshi, B., & Simmons, M. (2013, apr). Touch the Cosmos: The 2012 International Earth and Sky Photo Contest. In Communicating Science: A National Conference on Science Education and Public Outreach, 473, 165.
• Fersch, A., & Walker, C. (2012, jan). Light Pollution Around Tucson, AZ And Its Effect On The Spatial Distribution Of Lesser Long-nosed Bats. In American Astronomical Society Meeting Abstracts #219, 219, #141.04.
• Pompea, S. M., Sparks, R. T., Walker, C. E., & Dokter, E. F. (2012, oct). Optics education through the Arizona Galileoscope program. In Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, 8481.
• Walker, C. E., Pompea, S. M., Sparks, R. T., & Newhouse, M. (2012, jan). Seeing Stars: A GLOBE at Night Campaign Update. In American Astronomical Society Meeting Abstracts #219, 219, #141.03.