Shuo Kong

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• Bialy, S., Burkhart, B., Seifried, D., Sternberg, A., Godard, B., Krumholz, M. R., Walch, S., Hamden, E., Haworth, T. J., Turner, N. J., Lee, M. Y., & Kong, S. (2025). The Molecular Cloud Life Cycle. I. Constraining H2 Formation and Dissociation Rates with Observations. Astrophysical Journal, 982(Issue 1). doi:10.3847/1538-4357/adb3a6
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  Molecular clouds (MCs) are the birthplaces of new stars in galaxies. A key component of MCs are photodissociation regions (PDRs), where far-ultraviolet radiation plays a crucial role in determining the gas’s physical and chemical state. Traditional PDR models assume a chemical steady state (CSS), where the rates of H2 formation and photodissociation are balanced. However, real MCs are dynamic and can be out of CSS. In this study, we demonstrate that combining H2 emission lines observed in the far-ultraviolet or infrared with column density observations can be used to derive the rates of H2 formation and photodissociation. We derive analytical formulae that relate these rates to observable quantities, which we validate using synthetic H2 line emission maps derived from the SILCC-Zoom hydrodynamical simulation. Our method estimates integrated H2 formation and dissociation rates with an accuracy ≈30% (on top of the uncertainties in the observed H2 emission maps and column densities). Our simulations, valid for column densities N ≤ 2 × 1022 cm−2, cover a wide dynamic range of H2 formation and photodissociation rates, showing significant deviations from CSS, with 74% of the MC’s mass deviating from CSS by a factor greater than 2. Our analytical formulae can effectively distinguish between regions in and out of CSS. When applied to actual H2 line observations, our method can assess the chemical states of MCs, providing insights into their evolutionary stages and lifetimes. A NASA Small Explorer mission concept, Eos, will be proposed in 2025 and is specifically designed to conduct the types of observations outlined in this study.
• Li, S., Beuther, H., Oliva, A., Elbakyan, V. G., Offner, S. S., Kuiper, R., Qiu, K., Lu, X., Sanhueza, P., Chen, H. R., Zhang, Q., Olguin, F. A., Lee, C. W., Pudritz, R. E., Kong, S., Kuruwita, R. L., Luo, Q., & Liu, J. (2025). Detection of a septuple stellar system in formation via disk fragmentation. Nature Astronomy. doi:10.1038/s41550-025-02682-9
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  Stellar multiple systems play a pivotal role in cluster dynamics and stellar evolution, producing intense astronomical phenomena like X-ray binaries, gamma-ray bursts, type Ia supernova and stellar mergers, which are prime sources of gravitational waves. However, their origin remains poorly understood. Here we report the discovery of a septuple protostellar system embedded in a Keplerian disk within the high-mass star-forming region NGC 6334IN, with close separations of 181–461 au. A stability analysis reveals that the disk surrounding the septuple system is dynamically unstable, indicating that the septuple system formed through disk fragmentation. Previous studies have typically found only two or three members forming through disk fragmentation in both low- and high-mass star-forming regions. Our findings provide compelling observational evidence that the fragmentation of a gravitationally unstable disk is a viable mechanism for the formation of extreme high-order multiplicity, confirming what was previously only a theoretical concept. The results shed new light on the formation of extreme high-order multiplicity in cluster environments.
• Baade, J., Kong, S., Bieging, J., & Folkers, T. (2024). CO Mapping of Cygnus-X—Volume Density Distribution. Astrophysical Journal, 960(1). doi:10.3847/1538-4357/ad0a8f
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  We present CO(2-1) and 13CO(2-1) maps of the Cygnus-X molecular cloud complex using the 10 m Heinrich Hertz Submillimeter Telescope. The maps cover the southern portion of the complex, which is strongly impacted by the feedback from the Cygnus OB2 association. Combining CO(1-0) and 13CO(1-0) maps from the Nobeyama 45 m Cygnus-X CO Survey, we carry out a multitransition molecular line analysis with RADEX and derive the volume density of velocity-coherent gas components. We select those components with a column density in the power-law tail part of the column density probability distribution function (N-PDF) and assemble their volume density into a volume density PDF (ρ-PDF). The ρ-PDF exhibits a power-law shape in the range of 104.5 cm−3 ≲ n H 2 ≲ 105.5 cm−3 with a fitted slope of α = −1.12 ± 0.05. The slope is shallower than what is predicted by simulations of rotationally supported structures or those undergoing gravitational collapse. Applying the same analysis to synthetic observations with feedback may help identify the cause of the shallow slope. The ρ-PDF provides another useful benchmark for testing models of molecular cloud formation and evolution.
• Bieging, J., & Kong, S. (2024). CO in the Draco nebula: the atomic-molecular transition. Monthly Notices of the Royal Astronomical Society, 531(4). doi:10.1093/mnras/stae1419
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  This paper presents maps of the J=2-1 transition of CO towards the Draco nebula intermediate-velocity cloud (IVC). The maps cover 8500 square arcmin with a velocity resolution of 0.33 km s and angular resolution of 38, or 0.11 pc at the cloud distance of 600 pc. The mapped area includes all the emission detected by the Herschel satellite with 250 m intensity >5 MJy sr-1. Previously published observations of the far-infrared (far-IR) emission and the 21 cm line of HI are used to derive the column density distribution of H and the abundance ratio CO/H, as well as the distribution of the molecular fraction of hydrogen, which approaches 90 per cent over much of the brighter parts of the nebula. The CO emission is highly clumpy and closely resembles the structures seen in far-IR images. The kinematics of the CO show supersonic motions between clumps but near-thermal to trans-sonic motions within clumps, consistent with model predictions that the scale length for dissipation of supersonic turbulence should be pc, mediated by kinematic viscosity and/or ambipolar diffusion. Different parts of the nebula show evidence for a spread of molecular formation time-scales of a few 10 yr, comparable with the dynamical time-scale of the infalling gas. The IVC will likely merge with the Galactic interstellar medium in yr, and the densest clumps may form an unbound cluster of low-mass stars.
• Burkhart, B., Bialy, S., Seifried, D., Walch, S., Hamden, E., Haworth, T., Hoadley, K., Kong, S., Johnson, M., Jeffreson, S., Krumholz, M., Lee, M., Sternberg, A., & Turner, N. (2024). The Molecular Cloud Life Cycle. II. Formation and Destruction of Molecular Clouds Diagnosed via H2Fluorescent Emission. Astrophysical Journal, 975(2). doi:10.3847/1538-4357/ad75f8
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  Molecular hydrogen (H2) formation and dissociation are key processes that drive the gas life cycle in galaxies. Using the SImulating the LifeCycle of Molecular Clouds zoom-in simulation suite, we explore the utility of future observations of H2 dissociation and formation for tracking the life cycle of molecular clouds. The simulations used in this work include nonequilibrium H2 formation, stellar radiation, sink particles, and turbulence. We find that at early times in the cloud evolution H2 formation rapidly outpaces dissociation and molecular clouds build their mass from the atomic reservoir in their environment. Rapid H2 formation is also associated with a higher early star formation rate. For the clouds studied here, H2 is strongly out of chemical equilibrium during the early stages of cloud formation but settles into a bursty chemical steady state about 2 Myr after the first stars form. At the latest stage of cloud evolution, dissociation outweighs formation and the clouds enter a dispersal phase. We discuss how theories of the molecular cloud life cycle and star formation efficiency may be distinguished with observational measurements of H2 fluorescence with a space-based high-resolution far-UV spectrometer, such as the proposed Hyperion and Eos NASA Explorer missions. Such missions would enable measurements of the H2 dissociation and formation rates, which we demonstrate can be connected to different phases in a molecular cloud's star-forming life, including cloud building, rapidly star forming, H2 chemical equilibrium, and cloud destruction.
• Kong, S., Smith, R., Whitworth, D., & Hamden, E. (2024). Filamentary Molecular Cloud Formation via Collision-induced Magnetic Reconnection in a Cold Neutral Medium. Astrophysical Journal, 975(1). doi:10.3847/1538-4357/ad73a5
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  We have investigated the possibility of molecular cloud formation via the collision-induced magnetic reconnection (CMR) mechanism of the cold neutral medium (CNM). Two atomic gas clouds with conditions typical of the CNM were set to collide at the interface of reverse magnetic fields. The cloud-cloud collision triggered magnetic reconnection and produced a giant 20 pc filamentary structure that was not seen in the control models without CMR. The cloud, with rich fiber-like substructures, developed a fully molecular spine at 5 Myr. Radiative transfer modeling of dust emission at far-infrared wavelengths showed that the middle part of the filament contained dense cores over a span of 5 pc. Some of the cores were actively forming stars and typically exhibited both connecting fibers in dust emission and high-velocity gas in CO line emission, indicative of active accretion through streamers. Supersonic turbulence was present in and around the CMR filament due to inflowing gas moving at supersonic velocities in the collision midplane. The shocked gas was condensed and transported to the main filament piece by piece by reconnected fields, making the filament and star formation a bottom-up process. Instead of forming a gravitationally bounded cloud that then fragments hierarchically (top-down) and forms stars, the CMR process creates dense gas pieces and magnetically transports them to the central axis to constitute the filament. Since no turbulence is manually driven, our results suggest that CMR is capable of self-generating turbulence. Finally, the resulting helical field should show field reversal on both sides of the filament from most viewing angles.
• Kreckel, K., Egorov, O., Egorova, E., Blanc, G., Drory, N., Kounkel, M., Stringfellow, G., Stutz, A., Zari, E., Barrera-Ballesteros, J., Bizyaev, D., Brownstein, J., Congiu, E., Hillenbrand, L., Ibarra-Medel, H., Jin, Y., Johnston, E., Jones, A., Kim, J., , Kollmeier, J., et al. (2024). SDSS-V Local Volume Mapper (LVM): A glimpse into Orion. Astronomy and Astrophysics, 689. doi:10.1051/0004-6361/202449943
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  Context. The Orion Molecular Cloud complex, one of the nearest (D = 406 pc) and most extensively studied massive star-forming regions, is ideal for constraining the physics of stellar feedback, but its ∼12 deg diameter on the sky requires a dedicated approach to mapping ionized gas structures within and around the nebula. Aims. The Sloan Digital Sky Survey (SDSS-V) Local Volume Mapper (LVM) is a new optical integral field unit (IFU) that will map the ionized gas within the Milky Way and Local Group galaxies, covering 4300 deg2 of the sky with the new LVM Instrument (LMV-I). Methods. We showcase optical emission line maps from LVM covering 12 deg2 inside of the Orion belt region, with 195 000 individual spectra combined to produce images at 0.07 pc (35.3″) resolution. This is the largest IFU map made (to date) of the Milky Way, and contains well-known nebulae (the Horsehead Nebula, Flame Nebula, IC 434, and IC 432), as well as ionized interfaces with the neighboring dense Orion B molecular cloud. Results. We resolve the ionization structure of each nebula, and map the increase in both the [S II]/Hα and [N II]/Hα line ratios at the outskirts of nebulae and along the ionization front with Orion B. [O III] line emission is only spatially resolved within the center of the Flame Nebula and IC 434, and our ∼0.1 pc scale line ratio diagrams show how variations in these diagnostics are lost as we move from the resolved to the integrated view of each nebula. We detect ionized gas emission associated with the dusty bow wave driven ahead of the star σ Orionis, where the stellar wind interacts with the ambient interstellar medium. The Horsehead Nebula is seen as a dark occlusion of the bright surrounding photo-disassociation region. This small glimpse into Orion only hints at the rich science that will be enabled by the LVM.
• Baade, J. C., Kong, S., Bieging, J. H., & Folkers, T. (2023). CO Mapping of Cygnus-X???Volume Density Distribution. The Astrophysical Journal, 960(1), 33.
• Hsieh, C., Arce, H. G., Li, Z., Dunham, M., Offner, S., Stephens, I. W., Stutz, A., Megeath, T., Kong, S., Plunkett, A., Tobin, J. J., Zhang, Y., Mardones, D., Pineda, J. E., Stanke, T., & Carpenter, J. (2023). "The Evolution of Protostellar Outflow Cavities, Kinematics, and Angular Distribution of Momentum and Energy in Orion A: Evidence for Dynamical Cores". apj, 947(1), 25.
• Kong, S., Arce, H. G., Tobin, J. J., Zhang, Y., Maureira, M. J., Kratter, K. M., & Pillai, T. G. (2023). "Binary Formation in a 100 {ensuremath{mu}m Dark Massive Core}". apj, 950(2), 187.
• Kong, S., Ossenkopf-Okada, V., Arce, H. G., Klessen, R. S., & Xu, D. (2023). "CMR Exploration. I. Filament Structure with Synthetic Observations". apjs, 265(2), 58.
• Takemura, H., Nakamura, F., Arce, H. G., Schneider, N., Ossenkopf-Okada, V., Kong, S., Ishii, S., Dobashi, K., Shimoikura, T., Sanhueza, P., Tsukagoshi, T., Padoan, P., Klessen, R. S., Goldsmith, P. F., Burkhart, B., Lis, D. C., S{'anchez-Monge}, '., Shimajiri, Y., & Kawabe, R. (2023). "CARMA-NRO Orion Survey: Unbiased Survey of Dense Cores and Core Mass Functions in Orion A". apjs, 264(2), 35.
• Xu, D., Kong, S., Kaul, A., Arce, H. G., & Ossenkopf-Okada, V. (2023). "CMR Exploration. II. Filament Identification with Machine Learning". apj, 955(2), 113.
• Bialy, S., Bryden, G., Burkhart, B., Chung, H., Godard, B., Hamden, E. T., Hasegawa, Y., Haworth, T. J., Hoadley, K., Hoenk, M., Imara, N., Kennicutt, R., Kiessling, A., Kim, D., Kong, S., Krumholz, M., Lee, M., Luthman, E., McGuire, J. P., , Nikzad, S., et al. (2022).

  Hyperion: the origin of the stars. A far UV space telescope for high-resolution spectroscopy over wide fields

  . Journal of Astronomical Telescopes, Instruments, and Systems, 8(04). doi:10.1117/1.jatis.8.4.044008
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  We present Hyperion, a mission concept recently proposed to the December 2021 NASA Medium Explorer announcement of opportunity. Hyperion explores the formation and destruction of molecular clouds and planet-forming disks in nearby star-forming regions of the Milky Way. It does this using long-slit high-resolution spectroscopy of emission from fluorescing molecular hydrogen, which is a powerful far-ultraviolet (FUV) diagnostic. Molecular hydrogen (H2) is the most abundant molecule in the universe and a key ingredient for star and planet formation but is typically not observed directly because its symmetric atomic structure and lack of a dipole moment mean there are no spectral lines at visible wavelengths and few in the infrared. Hyperion uses molecular hydrogen’s wealth of FUV emission lines to achieve three science objectives: (1) determining how star formation is related to molecular hydrogen formation and destruction at the boundaries of molecular clouds, (2) determining how quickly and by what process massive star feedback disperses molecular clouds, and (3) determining the mechanism driving the evolution of planet-forming disks around young solar-analog stars. Hyperion conducts this science using a straightforward, highly efficient, single-channel instrument design. Hyperion’s instrument consists of a 48-cm primary mirror with an f/5 focal ratio. The spectrometer has two modes, both covering 138.5- to 161.5-nm bandpasses. A low resolution mode has a spectral resolution of R ≥ 10,000 with a slit length of 65 arcmin, whereas the high-resolution mode has a spectral resolution of R ≥ 50,000 over a slit length of 5 armin. Hyperion occupies a 2-week-long high-earth lunar resonance TESS-like orbit and conducts 2 weeks of planned observations per orbit, with time for downlinks and calibrations. Hyperion was reviewed as category I, which is the highest rating possible but was not selected.
• Bieging, J., & Kong, S. (2022). The Star Formation-Gas Density Relation in Four Galactic GMCs: Effects of Stellar Feedback. Astrophysical Journal, 938(2). doi:10.3847/1538-4357/ac9054
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  We present maps of four Galactic giant molecular clouds (GMCs) in the J = 2-1 emission of both CO and 13CO. We use an LTE analysis to derive maps of the CO excitation temperature and column density and the distribution of total molecular gas column density, ςgas. The depletion of CO by freeze-out onto cold dust grains is accounted for by an approximation to the results of Lewis et al., which were derived from far-IR observations with Herschel. The surface density of young stellar objects (YSOs) is obtained from published catalogs. The mean YSO surface density exhibits a power-law dependence on ςgas, with exponents in the range 0.9-1.9. Gas column density probability distribution functions show power-law tails extending to high column densities. The distributions of sonic Mach number, M S , are sharply peaked at MS ∼5-8 for 3 GMCs; a fourth has a broad distribution up to M S = 30, possibly a result of feedback effects from multiple OB stars. An analysis following the methodology of Pokhrel et al. finds that our sample of GMCs shows power-law relations that are somewhat shallower than those found by Pokhrel et al. for the star formation rate versus «ςgas» and versus «ςgas»/t ff in a different sample of clouds. We discuss possible differences in the two samples of star-forming clouds and the effects of stellar feedback on the relation between gas density and star formation rate.
• Hamden, E. T., Kong, S., Smith, R. J., & Whitworth, D. J. (2022).

  Filament formation via collision-induced magnetic reconnection – formation of a star cluster

  . Monthly Notices of the Royal Astronomical Society, 517(4), 4679-4695. doi:10.1093/mnras/stac2932
• Kong, S. (2022). Dense Gas Formation via Collision-induced Magnetic Reconnection in a Disk Galaxy with a Bisymmetric Spiral Magnetic Field. Astrophysical Journal, 933(1). doi:10.3847/1538-4357/ac70cd
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  Recently, a collision-induced magnetic reconnection (CMR) mechanism was proposed to explain a dense filament formation in the Orion A giant molecular cloud. A natural question is whether CMR works elsewhere in the Galaxy. As an initial attempt to answer the question, this paper investigates the triggering of CMR and the production of dense gas in a flat-rotating disk with a modified Bisymmetric spiral (BSS) magnetic field. Cloud-cloud collisions at field reversals in the disk are modeled with the Athena++ code. Under the condition that is representative of the warm neutral medium, the cloud-cloud collision successfully triggers CMR at different disk radii. However, dense gas formation is hindered by the dominating thermal pressure, unless a moderately stronger initial field ≃5 μG is present. The strong-field model, having a larger Lundquist number S L and lower plasma β, activates the plasmoid instability in the collision midplane, which is otherwise suppressed by the disk rotation. We speculate that CMR can be common if more clouds collide along field reversals. However, to witness the CMR process in numerical simulations, we need to significantly resolve the collision midplane with a spatial dynamic range ≃106. If Milky Way spiral arms indeed coincide with field reversals in BSS, it is possible that CMR creates or maintains dense gas in the arms. High-resolution, high-sensitivity Zeeman/Faraday rotation observations are crucial for finding CMR candidates that have helical fields.
• Xu, D., Offner, S., Gutermuth, R., Kong, S., & Arce, H. (2022). A Census of Protostellar Outflows in Nearby Molecular Clouds. Astrophysical Journal, 926(1). doi:10.3847/1538-4357/ac39a0
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  We adopt the deep learning method casi-3d (Convolutional Approach to Structure Identification-3D) to systemically identify protostellar outflows in 12CO and 13CO observations of the nearby molecular clouds, Ophiuchus, Taurus, Perseus, and Orion. The total outflow masses are 267 M Ȯ, 795 M Ȯ, 1305 M Ȯ, and 6332 M Ȯ for Ophiuchus, Taurus, Perseus, and Orion, respectively. We show the outflow mass in each cloud is linearly proportional to the total number of young stellar objects. The estimated total 3D deprojected outflow energies are 9 × 1045 erg, 6 × 1046 erg, 1.2 × 1047 erg, and 6 × 1047 erg for Ophiuchus, Taurus, Perseus, and Orion, respectively. The energy associated with outflows is sufficient to offset turbulent dissipation at the current epoch for all four clouds. All clouds also exhibit a break point in the spatial power spectrum of the outflow prediction map, which likely corresponds to the typical outflow mass and energy injection scale.
• Avison, A., Barnes, A. T., Bigiel, F., Caselli, P., Cosentino, G., Feng, S., Fontani, F., Henshaw, J. D., Jiménez-Serra, I., Kong, S., Law, C. Y., Longmore, S. N., Moser, L., Parker, R. J., Pineda, J. E., Sánchez-Monge, Á., Tan, J. C., & Wang, K. (2021).

  ALMA–IRDC: dense gas mass distribution from cloud to core scales

  . Monthly Notices of the Royal Astronomical Society, 503(3), 4601-4626. doi:10.1093/mnras/stab803
• Hsieh, C., Arce, H., Mardones, D., Kong, S., & Plunkett, A. (2021). Rotating Filament in Orion B: Do Cores Inherit Their Angular Momentum from Their Parent Filament?. Astrophysical Journal, 908(1). doi:10.3847/1538-4357/abd034
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  Angular momentum is one of the most important physical quantities that governs star formation. The initial angular momentum of a core may be responsible for its fragmentation, and can have an influence on the size of the protoplanetary disk. To understand how cores obtain their initial angular momentum, it is important to study the angular momentum of filaments where they form. While theoretical studies on filament rotation have been explored, there exist very few observational measurements of the specific angular momentum in star-forming filaments. We present high-resolution N2D+ ALMA observations of the LBS 23 (HH24-HH26) region in Orion B, which provide one of the most reliable measurements of the specific angular momentum in a star-forming filament. We find the total specific angular momentum (4 × 1020 cm2 s-1), the dependence of the specific angular momentum with radius (j(r) ∝ r 1.83), and the ratio of rotational energy to gravitational energy (β rot ∼ 0.04) comparable to those observed in rotating cores with sizes similar to our filament width (∼0.04 pc) in other star-forming regions. Our filament angular momentum profile is consistent with rotation acquired from ambient turbulence and with simulations that show cores and their host filaments develop simultaneously due to multi-scale growth of nonlinear perturbation generated by turbulence.
• Kong, S., Arce, H., Carpenter, J., Bally, J., Ossenkopf-Okada, V., Sargent, A., Suri, S., McGehee, P., Lis, D., Klessen, R., Mairs, S., Zucker, C., Smith, R., Nakamura, F., Pillai, T., Kauffmann, J., Zhang, S., & Sánchez-Monge, Á. (2021). High-resolution CARMA Observation of Molecular Gas in the North America and Pelican Nebulae. Astronomical Journal, 161(5). doi:10.3847/1538-3881/abec7d
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  In this paper, we present the first results from a CARMA high-resolution 12CO(1-0), 13CO(1-0), and C18O(1-0) molecular line survey of the North America and Pelican (NAP) Nebulae. CARMA observations have been combined with single-dish data from the Purple Mountain 13.7 m telescope, to add short spacings and to produce high-dynamic-range images. We find that the molecular gas is predominantly shaped by the W80 H ii bubble, driven by an O star. Several bright rims noted in the observation are probably remnant molecular clouds, heated and stripped by the massive star. Matching these rims in molecular lines and optical images, we construct a model of the three-dimensional structure of the NAP complex. Two groups of molecular clumps/filaments are on the near side of the bubble: one is being pushed toward us, whereas the other is moving toward the bubble. Another group is on the far side of the bubble, and moving away. The young stellar objects in the Gulf region reside in three different clusters, each hosted by a cloud from one of the three molecular clump groups. Although all gas content in the NAP is impacted by feedback from the central O star, some regions show no signs of star formation, while other areas clearly exhibit star formation activity. Additional molecular gas being carved by feedback includes cometary structures in the Pelican Head region, and the boomerang features at the boundary of the Gulf region. The results show that the NAP complex is an ideal place for the study of feedback effects on star formation.
• Kong, S., Arce, H., Shirley, Y., & Glasgow, C. (2021). Evidence of Core Growth in the Dragon Infrared Dark Cloud: A Path for Massive Star Formation. Astrophysical Journal, 912(2). doi:10.3847/1538-4357/abefe7
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  A sample of 1.3 mm continuum cores in the Dragon infrared dark cloud (also known as G28.37+0.07 or G28.34+0.06) is analyzed statistically. Based on their association with molecular outflows, the sample is divided into protostellar and starless cores. Statistical tests suggest that the protostellar cores are more massive than the starless cores, even after temperature and opacity biases are accounted for. We suggest that the mass difference indicates core mass growth since their formation. The mass growth implies that massive star formation may not have to start with massive prestellar cores, depending on the core mass growth rate. Its impact on the relation between core mass function and stellar initial mass function is to be further explored.
• Kong, S., Ossenkopf-Okada, V., Arce, H., Bally, J., McGehee, P., Suri, S., Klessen, R., Carpenter, J., Lis, D., Nakamura, F., Schilke, P., Smith, R., Mairs, S., Goodman, A., Maureira, M., & Sánchez-Monge, Á. (2021). The CARMA-NRO orion survey: Filament formation via collision-induced magnetic reconnection-the stick in orion A. Astrophysical Journal, 906(2). doi:10.3847/1538-4357/abc687
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  A unique filament is identified in the Herschel maps of the Orion A giant molecular cloud. The filament, which we name the Stick, is ruler-straight and at an early evolutionary stage. Transverse position-velocity diagrams show two velocity components closing in on the Stick. The filament shows consecutive rings/forks in C18O (1−0) channel maps, which is reminiscent of structures generated by magnetic reconnection. We propose that the Stick formed via collision-induced magnetic reconnection (CMR). We use the magnetohydrodynamics code Athena++ to simulate the collision between two diffuse molecular clumps, each carrying an antiparallel magnetic field. The clump collision produces a narrow, straight, dense filament with a factor of >200 increase in density. The production of the dense gas is seven times faster than freefall collapse. The dense filament shows ring/fork-like structures in radiative transfer maps. Cores in the filament are confined by surface magnetic pressure. CMR can be an important dense-gas-producing mechanism in the Galaxy and beyond.
• Liu, M., Tan, J., Marvil, J., Kong, S., Rosero, V., Caselli, P., & Cosentino, G. (2021). Sio outflows as tracers of massive star formation in infrared dark clouds. Astrophysical Journal, 921(1). doi:10.3847/1538-4357/ac0829
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  To study the early phases of massive star formation, we present ALMA observations of SiO(5-4) emission and VLA observations of 6 cm continuum emission toward 32 Infrared Dark Cloud clumps, spatially resolved down to 0.05 pc. Out of the 32 clumps, we detect SiO emission in 20 clumps, and in 11 of them the SiO emission is relatively strong and likely tracing protostellar outflows. Some SiO outflows are collimated, while others are less ordered. For the six strongest SiO outflows, we estimate basic outflow properties. In our entire sample, where there is SiO emission, we find 1.3 mm continuum and infrared emission nearby, but not vice versa. We build the spectral energy distributions (SEDs) of cores with 1.3 mm continuum emission and fit them with radiative transfer models. The low luminosities and stellar masses returned by SED fitting suggest these are early-stage protostars. We see a slight trend of increasing SiO line luminosity with bolometric luminosity, which suggests more powerful shocks in the vicinity of more massive YSOs. We do not see a clear relation between the SiO luminosity and the evolutionary stage indicated by L/M. We conclude that, as a protostar approaches a bolometric luminosity of ∼102 Le, the shocks in the outflow are generally strong enough to form SiO emission. The VLA 6 cm observations toward the 15 clumps with the strongest SiO emission detect emission in four clumps, which is likely from shock-ionized jets associated with the more massive ones of these protostellar cores.
• Takemura, H., Nakamura, F., Kong, S., Arce, H. G., Carpenter, J. M., Ossenkopf-Okada, V., Klessen, R., Sanhueza, P., Shimajiri, Y., Tsukagoshi, T., Kawabe, R., Ishii, S., Dobashi, K., Shimoikura, T., Goldsmith, P. F., Sánchez-Monge, l., Kauffmann, J., Pillai, T. G., Padoan, P., , Ginsberg, A., et al. (2021). The Core Mass Function in the Orion Nebula Cluster Region: What Determines the Final Stellar Masses?. Astrophysical Journal Letters, 910(Issue 1). doi:10.3847/2041-8213/abe7dd
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  Applying dendrogram analysis to the CARMA-NRO C18O (J = 1-0) data having an angular resolution of ∼8″, we identified 692 dense cores in the Orion Nebula Cluster region. Using this core sample, we compare the core and initial stellar mass functions in the same area to quantify the step from cores to stars. About 22% of the identified cores are gravitationally bound. The derived core mass function (CMF) for starless cores has a slope similar to Salpeter's stellar initial mass function (IMF) for the mass range above 1 M o˙, consistent with previous studies. Our CMF has a peak at a subsolar mass of ∼0.1 M o˙, which is comparable to the peak mass of the IMF derived in the same area. We also find that the current star formation rate is consistent with the picture in which stars are born only from self-gravitating starless cores. However, the cores must gain additional gas from the surroundings to reproduce the current IMF (e.g., its slope and peak mass), because the core mass cannot be accreted onto the star with 100% efficiency. Thus, the mass accretion from the surroundings may play a crucial role in determining the final stellar masses of stars.
• Bally, J., Bisbas, T. G., Chambers, E., Kong, S., Lim, W., Lis, D. C., McGehee, P., Nakamura, F., Ossenkopf-Okada, V., Sánchez-Monge, Á., Tan, J. C., & Wu, B. (2020).

  Star cluster formation in Orion A

  . Publications of the Astronomical Society of Japan, 73(Supplement_1), S239-S255. doi:10.1093/pasj/psaa035
• Beaklini, P., Mendoza, E., Canelo, C., Aleman, I., Merello, M., Kong, S., Navarete, F., Janot-Pacheco, E., Abraham, Z., de Almeida, A., Friaça, A., & Lépine, J. (2020). Sulphur-bearing and complex organic molecules in an infrared cold core. Monthly Notices of the Royal Astronomical Society, 491(1). doi:10.1093/mnras/stz3024
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  Since the start of ALMA (Atacama Large Millimeter Array) observatory operation, new and important chemistry of infrared cold core was revealed. Molecular transitions at millimetre range are being used to identify and to characterize these sources. We have investigated the 231 GHz ALMA archive observations of the infrared dark cloud region C9, focusing on the brighter source that we called as IRDC-C9 Main. We report the existence of two substructures on the continuum map of this source: a compact bright spot with high chemistry diversity that we labelled as core, and a weaker and extended one, that we labelled as tail. In the core, we have identified lines of the molecules OCS(19−18), 13CS(5−4), and CH3CH2CN, several lines of CH3CHO and the k-ladder emission of 13CH3CN. We report two different temperature regions, while the rotation diagram of CH3CHO indicates a temperature of 25 K, the rotation diagram of 13CH3CN indicates a warmer phase at temperature of ∼450 K. In the tail, only the OCS(19−18) and 13CS(5−4) lines were detected. We used the NAUTILUS and the RADEX codes to estimate the column densities and the abundances. The existence of hot gas in the core of IRDC-C9 Main suggests the presence of a protostar, which is not present in the tail.
• Feddersen, J., Arce, H., Kong, S., Suri, S., Ossenkopf-Okada, V., Dunham, M., Nakamura, F., Shimajiri, Y., Bally, J., & Sánchez-Monge, L. (2020). The CARMA-NRO Orion Survey: Protostellar Outflows, Energetics, and Filamentary Alignment. Astrophysical Journal, 896(1). doi:10.3847/1538-4357/ab86a9
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  We identify 45 protostellar outflows in CO maps of the Orion A giant molecular cloud from the Combined Array for Research in Millimeter-wave Astronomy-Nobeyama Radio Observatory Orion survey. Our sample includes 11 newly detected outflows. We measure the mass and energetics of the outflows, including material at low velocities, by correcting for cloud contributions. The total momentum and kinetic energy injection rates of outflows are comparable to the turbulent dissipation rate of the cloud. We also compare the outflow position angles to the orientation of C18O filaments. We find that the full sample of outflows is consistent with being randomly oriented with respect to the filaments. A subsample of the most reliable measurements shows a moderately perpendicular outflow-filament alignment that may reflect accretion of mass across filaments and onto the protostellar cores.
• Feddersen, J., Arce, H., Kong, S., Ossenkopf-Okada, V., & Carpenter, J. (2019). The CARMA-NRO Orion Survey: Statistical Signatures of Feedback in the Orion A Molecular Cloud. Astrophysical Journal, 875(2). doi:10.3847/1538-4357/ab0e7d
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  We investigate the relationship between turbulence and feedback in the Orion A molecular cloud using maps of 12CO(1?0), 13CO(1?0), and C18O(1?0) from the CARMA-NRO Orion survey. We compare gas statistics with the impact of feedback in different parts of the cloud to test whether feedback changes the structure and kinematics of molecular gas. We use principal component analysis, the spectral correlation function, and the spatial power spectrum to characterize the cloud. We quantify the impact of feedback with momentum injection rates of protostellar outflows and wind-blown shells as well as the surface density of young stars. We find no correlation between shells or outflows and any of the gas statistics. However, we find a significant anticorrelation between young star surface density and the slope of the 12CO spectral correlation function, suggesting that feedback may influence this statistic. While calculating the principal components, we find peaks in the covariance matrix of our molecular line maps offset by 1-3 km s-1toward several regions of the cloud that may be produced by feedback. We compare these results to predictions from molecular cloud simulations.
• Kong, S. (2019). The Core Mass Function in the Infrared Dark Cloud G28.37+0.07. Astrophysical Journal, 873(1). doi:10.3847/1538-4357/aaffd5
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  In this paper, we analyze the 1.3 mm continuum ALMA data that cover the majority of the infrared dark cloud (IRDC) G28.37+0.07. With a spatial resolution of 0.″5 (2500 au at 5 kpc), the continuum image reveals five groups of dense cores. Each core group has a projected physical scale of about 1 pc, with core masses spanning a dynamic range of about 100. We use the dendrogram method (astrodendro) and a newly developed graph method (astrograph) to identify individual cores. The core masses are estimated through the millimeter continuum flux, assuming constant temperature and using an NH 3 -based gas temperature. We construct core mass functions (CMFs) based on the two methods and fit a power-law relation dN/d log M ∝ M -α to the CMFs for M > 0.79 M o . In the constant-temperature scenario, astrograph gives α = 0.80 ± 0.10, while astrodendro gives α = 0.71 ± 0.11, both significantly shallower than the Salpeter-type initial mass function with α = 1.35. In the scenario where the NH 3 gas temperature is applied to cores, astrograph gives α = 1.37 ± 0.06, while astrodendro gives α = 0.87 ± 0.07. Regional CMF slope variation is seen between the core groups. We also compare CMFs in three different environments, including IRDC G28.37+0.07, IRDC clumps, and G286.21+0.17, using the identical dendrogram method. Results show that IRDCs have smaller α than the cluster-forming cloud G286.21+0.17.
• Kong, S., Arce, H., Maureira, M., Caselli, P., Tan, J., & Fontani, F. (2019). Widespread Molecular Outflows in the Infrared Dark Cloud G28.37+0.07: Indications of Orthogonal Outflow-filament Alignment. Astrophysical Journal, 874(1). doi:10.3847/1538-4357/ab07b9
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  We present ALMA CO(2-1) observations toward a massive infrared dark cloud G28.37+0.07. The ALMA data reveal numerous molecular (CO) outflows with a wide range of sizes throughout the cloud. Sixty-two 1.3 mm continuum cores were identified to be driving molecular outflows. We have determined the position angle in the plane-of-sky of 120 CO outflow lobes and studied their distribution. We find that the distribution of the plane-of-sky outflow position angles peaks at about 100°, corresponding to a concentration of outflows with an approximately east-west direction. For most outflows, we have been able to estimate the plane-of-sky angle between the outflow axis and the filament that harbors the protostar that powers the outflow. Statistical tests strongly indicate that the distribution of outflow-filament orientations is consistent with most outflow axes being mostly orthogonal to their parent filament in three dimensions. Such alignment may result from filament fragmentation or continuous mass transportation from the filament to the embedded protostellar core. The latter is suggested by recent numerical studies with moderately strong magnetic fields.
• Kong, S., Arce, H., Sargent, A., Mairs, S., Klessen, R., Bally, J., Padoan, P., Smith, R., Maureira, M., Carpenter, J., Ginsburg, A., Stutz, A., Goldsmith, P., Meingast, S., McGehee, P., Suri, S., Pineda, J., Alves, J., Feddersen, J., , Kauffmann, J., et al. (2019). The CARMA-NRO Orion Survey: Core Emergence and Kinematics in the Orion A Cloud. Astrophysical Journal, 882(1). doi:10.3847/1538-4357/ab311e
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  We have investigated the formation and kinematics of submillimeter (submm) continuum cores in the Orion A molecular cloud. A comparison between submm continuum and near-infrared extinction shows a continuum core detection threshold of A V ∼ 5-10 mag. The threshold is similar to the star formation extinction threshold of A V ∼ 7 mag proposed by recent work, suggesting a universal star formation extinction threshold among clouds within 500 pc to the Sun. A comparison between the Orion A cloud and a massive infrared dark cloud G28.37+0.07 indicates that Orion A produces more dense gas within the extinction range 15 mag ≲ A V ≲ 60 mag. Using data from the CARMA-NRO Orion Survey, we find that dense cores in the integral-shaped filament (ISF) show subsonic core-to-envelope velocity dispersion that is significantly less than the local envelope line dispersion, similar to what has been found in nearby clouds. Dynamical analysis indicates that the cores are bound to the ISF. An oscillatory core-to-envelope motion is detected along the ISF. Its origin is to be further explored.
• Tanabe, Y., Nakamura, F., Tsukagoshi, T., Shimajiri, Y., Ishii, S., Kawabe, R., Feddersen, J., Kong, S., Arce, H., Bally, J., Carpenter, J., & Momose, M. (2019). Nobeyama 45 m mapping observations toward Orion A. I. Molecular outflows. Publications of the Astronomical Society of Japan, 71(Issue). doi:10.1093/pasj/psz100
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  We conducted an exploration of 12CO molecular outflows in the Orion A giant molecular cloud to investigate outflow feedback using 12CO ($J = 1\!-\!0$) and ${}^{13}$CO ($J = 1\!-\!0$) data obtained by the Nobeyama 45 m telescope. In the region excluding the center of OMC 1, we identified 44 12CO (including 17 newly detected) outflows based on the unbiased and systematic procedure of automatically determining the velocity range of the outflows and separating the cloud and outflow components. The optical depth of the 12CO emission in the detected outflows is estimated to be approximately 5. The total momentum and energy of the outflows, corrected for optical depth, are estimated to be $1.6 \times 10^{2}\, M_{\odot }\:$km$\:$s$^{-1}$ and $1.5\times 10^{46}\:$erg, respectively. The momentum and energy ejection rate of the outflows are estimated to be 36% and 235% of the momentum and energy dissipation rates of the cloud turbulence, respectively. Furthermore, the ejection rates of the outflows are comparable to those of the expanding molecular shells estimated by Feddersen et al. (2018, ApJ, 862, 121). Cloud turbulence cannot be sustained by the outflows and shells unless the energy conversion efficiency is as high as 20%.
• Cheng, Y., Tan, J., Liu, M., Kong, S., Lim, W., Andersen, M., & Da Rio, N. (2018). The Core Mass Function in the Massive Protocluster G286.21+0.17 Revealed by ALMA. Astrophysical Journal, 853(2). doi:10.3847/1538-4357/aaa3f1
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  We study the core mass function (CMF) of the massive protocluster G286.21+0.17 with the Atacama Large Millimeter/submillimeter Array via 1.3 mm continuum emission at a resolution of 1.″0 (2500 au). We have mapped a field of 5.′3 × 5.′3 centered on the protocluster clump. We measure the CMF in the central region, exploring various core detection algorithms, which give source numbers ranging from 60 to 125, depending on parameter selection. We estimate completeness corrections due to imperfect flux recovery and core identification via artificial core insertion experiments. For masses M 1 M o, the fiducial dendrogram-identified CMF can be fit with a power law of the form dN/dlog M ∝ M -α with α ≃ 1.24 ± 0.17, slightly shallower than, but still consistent with, the index of the Salpeter stellar initial mass function of 1.35. Clumpfind-identified CMFs are significantly shallower with α ≃ 0.64 ± 0.13. While raw CMFs show a peak near 1 M o, completeness-corrected CMFs are consistent with a single power law extending down to ∼0.5 M o, with only a tentative indication of a shallowing of the slope around ∼1 M o. We discuss the implications of these results for star and star cluster formation theories.
• Feddersen, J., Arce, H., Kong, S., Shimajiri, Y., Nakamura, F., Hara, C., Ishii, S., Sasaki, K., & Kawabe, R. (2018). Expanding CO Shells in the Orion A Molecular Cloud. Astrophysical Journal, 862(2). doi:10.3847/1538-4357/aacaf2
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  We present the discovery of expanding spherical shells around low- to intermediate-mass young stars in the Orion A giant molecular cloud using observations of 12CO(10) and 13CO(1 0) from the Nobeyama Radio Observatory 45 m telescope. The shells have radii from 0.05 to 0.85 pc and expand outward at 0.8 5 km s-1. The total energy in the expanding shells is comparable to protostellar outflows in the region. Together, shells and outflows inject enough energy and momentum to maintain the cloud turbulence. The mass-loss rates required to power the observed shells are two to three orders of magnitude higher than predicted for line-driven stellar winds from intermediate-mass stars. This discrepancy may be resolved by invoking accretion-driven wind variability. We describe in detail several shells in this paper and present the full sample in the online journal.
• Kong, S., Tan, J., Arce, H., Caselli, P., Fontani, F., & Butler, M. (2018). Core Emergence in a Massive Infrared Dark Cloud: A Comparison between Mid-IR Extinction and 1.3 mm Emission. Astrophysical Journal Letters, 855(2). doi:10.3847/2041-8213/aab151
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  Stars are born from dense cores in molecular clouds. Observationally, it is crucial to capture the formation of cores in order to understand the necessary conditions and rate of the star formation process. The Atacama Large Millimeter/submillimeter Array (ALMA) is extremely powerful for identifying dense gas structures, including cores, at millimeter wavelengths via their dust continuum emission. Here, we use ALMA to carry out a survey of dense gas and cores in the central region of the massive (∼105 M o) infrared dark cloud (IRDC) G28.37+0.07. The observation consists of a mosaic of 86 pointings of the 12 m array and produces an unprecedented view of the densest structures of this IRDC. In this first Letter about this data set, we focus on a comparison between the 1.3 mm continuum emission and a mid-infrared (MIR) extinction map of the IRDC. This allows estimation of the "dense gas" detection probability function (DPF), i.e., as a function of the local mass surface density, Σ, for various choices of thresholds of millimeter continuum emission to define "dense gas." We then estimate the dense gas mass fraction, f dg, in the central region of the IRDC and, via extrapolation with the DPF and the known Σ probability distribution function, to the larger-scale surrounding regions, finding values of about 5% to 15% for the fiducial choice of threshold. We argue that this observed dense gas is a good tracer of the protostellar core population and, in this context, estimate a star formation efficiency per free-fall time in the central IRDC region of ff ∼ 10%, with approximately a factor of two systematic uncertainties.
• Kong, S., Tan, J., Caselli, P., Fontani, F., Wang, K., & Butler, M. (2018). Zooming in to Massive Star Birth. Astrophysical Journal, 867(2). doi:10.3847/1538-4357/aae1b2
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  We present high-resolution (0 2, 1000 au) 1.3 mm ALMA observations of the massive infrared dark cloud clump, G028.37+00.07-C1, thought to harbor the early stages of massive star formation. Using N2D +(3-2), we resolve the previously identified C1-S core, separating the bulk of its emission from two nearby protostellar sources. C1-S is thus identified as a massive (∼50M⊙), compact (∼0.1 pc diameter) starless core, e.g., with no signs of outflow activity. Being highly deuterated, this is a promising candidate for a pre-stellar core on the verge of collapse. An analysis of its dynamical state indicates a sub-virial velocity dispersion compared to a trans-Alfvenic turbulent core model. However, virial equilibrium could be achieved with sub-Alfvenic conditions involving magnetic field strengths of ∼2 mG.
• Liu, M., Tan, J., Cheng, Y., & Kong, S. (2018). The Core Mass Function across Galactic Environments. II. Infrared Dark Cloud Clumps. Astrophysical Journal, 862(2). doi:10.3847/1538-4357/aacb7c
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  We study the core mass function (CMF) within 32 dense clumps in seven infrared dark clouds (IRDCs) with the Atacama Large Millimeter/submillimeter Array via 1.3 mm continuum emission at a resolution of ∼1″. We have identified 107 cores with the dendrogram algorithm, with a median radius of about 0.02 pc. Their masses range from 0.261 to 178 M o. After applying completeness corrections, we fit the combined IRDC CMF with a power law of the form and derive an index of α ≃ 0.86 ± 0.11 for M ≥ 0.79 M o and α ≃ 0.70 ± 0.13 for M ≥ 1.26 M o, which is a significantly more top-heavy distribution than the Salpeter stellar initial mass function index of 1.35. We also make a direct comparison of these IRDC clump CMF results to those measured in the more evolved protocluster G286 derived with similar methods, which have α ≃ 1.29 ± 0.19 and 1.08 ± 0.27 in these mass ranges, respectively. These results provide a hint that, especially for the M ≥ 1.26 M o range where completeness corrections are modest, the CMF in high pressure, early-stage environments of IRDC clumps may be top-heavy compared to that in the more evolved, global environment of the G286 protoclusters. However, larger samples of cores probing these different environments are needed to better establish the robustness of this potential CMF variation.
• Imara, N., Lada, C., Lewis, J., Bieging, J., Kong, S., Lombardi, M., & Alves, J. (2017). X Marks the Spot: Nexus of Filaments, Cores, and Outflows in a Young Star-forming Region. Astrophysical Journal, 840(2). doi:10.3847/1538-4357/aa6d74
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  We present a multiwavelength investigation of a region of a nearby giant molecular cloud that is distinguished by a minimal level of star formation activity.With our new 12CO(J=2-1) and 13CO(J=2-1) observations of a remote region within the middle of the California molecular cloud, we aim to investigate the relationship between filaments, cores, and a molecular outflow in a relatively pristine environment. An extinction map of the region from Herschel Space Observatory observations reveals the presence of two 2 pc long filaments radiating from a highextinction clump. Using the 13CO observations, we show that the filaments have coherent velocity gradients and that their mass-per-unit-lengths may exceed the critical value above which filaments are gravitationally unstable. The region exhibits structure with eight cores, at least one of which is a starless, prestellar core. We identify a lowvelocity, low-mass molecular outflow that may be driven by a flat spectrum protostar. The outflow does not appear to be responsible for driving the turbulence in the core with which it is associated, nor does it provide significant support against gravitational collapse.
• Kong, S., Tan, J., Caselli, P., Fontani, F., Liu, M., & Butler, M. (2017). A HUNT for MASSIVE STARLESS CORES. Astrophysical Journal, 834(2). doi:10.3847/1538-4357/834/2/193
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  We carry out an ALMA N2D+(3-2) and 1.3 mm continuum survey of 32 high-mass surface density regions of even infrared dark clouds, with the aim of finding massive starless cores that may form the initial conditions for the formation of massive stars. Cores showing strong N2D+(3-2) emission are expected to be highly deuterated and indicative of early, potentially pre-stellar stages of star formation. We also present maps of these regions in ancillary line tracers, including C18O(2-1), DCN(3-2), and DCO+(3-2). Over 100 N2D+ cores are identified with our newly developed core-finding algorithm, based on connected structures in position-velocity space. The most massive core has ∼70 M⊙ (potentially ∼170 M⊙) and so may be representative of the initial conditions or early stages of massive star formation. The existence and dynamical properties of such cores constrain massive star formation theories. We measure the line widths and thus velocity dispersion of six of the cores with strongest N2D+ (3-2) line emission, finding results that are generally consistent with virial equilibrium of pressure confinedcores.
• Barnes, A., Kong, S., Tan, J., Henshaw, J., Caselli, P., Fontani, F., & Jiménez-Serra, I. (2016). Widespread deuteration across the IRDC G035.39-00.33. Monthly Notices of the Royal Astronomical Society, 458(2). doi:10.1093/mnras/stw403
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  Infrared Dark Clouds (IRDCs) are cold, dense regions that are usually found within Giant Molecular Clouds. Ongoing star formation within IRDCs is typically still deeply embedded within the surrounding molecular gas. Characterizing the properties of relatively quiescent IRDCs may therefore help us to understand the earliest phases of the star formation process. Studies of local molecular clouds have revealed that deuterated species are enhanced in the earliest phases of star formation. In this paper, we test this towards IRDC G035.39-00.33. We present an 80 arcsec by 140 arcsec map of the J = 2 → 1 transition of N2D+, obtained with the Institut de Radioastronomie Millimétrique 30 m telescope telescope. We find that N2D+ is widespread throughout G035.39-00.33. Complementary observations of N2H+ (1 - 0) are used to estimate the deuterium fraction, DN2H+frac ≡ N(N2D+)/N(N2H+). We report a mean DN2H+frac = 0.04 ± 0.01, with a maximum of DN2H+frac = 0.09 ± 0.02. The mean deuterium fraction is ~3 orders of magnitude greater than the interstellar [D]/[H] ratio. High angular resolution observations are required to exclude beam dilution effects of compact deuterated cores. Using chemical modelling, we find that the average observed values of DN2H+frac are in agreement with an equilibrium deuterium fraction, given the general properties of the cloud. This implies that the IRDC is at least ~3 Myr old, which is ~8 times longer than the mean free-fall time of the observed deuterated region.
• Goodson, M., Kong, S., Tan, J., Heitsch, F., & Caselli, P. (2016). STRUCTURE, DYNAMICS, and DEUTERIUM FRACTIONATION of MASSIVE PRE-STELLAR CORES. Astrophysical Journal, 833(2). doi:10.3847/1538-4357/833/2/274
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  High levels of deuterium fraction in N2H+ are observed in some pre-stellar cores. Single-zone chemical models find that the timescale required to reach observed values (DN2H+frac ≡ N2D+/ N2H+ ≳ 0.1) is longer than the free-fall time, possibly 10 times longer. Here, we explore the deuteration of turbulent, magnetized cores with 3D magnetohydrodynamics simulations. We use an approximate chemical model to follow the growth in abundances of N2H+ and N2D+. We then examine the dynamics of the core using each tracer for comparison to observations. We find that the velocity dispersion of the core as traced by N2D+ appears slightly sub-virial compared to predictions of the Turbulent Core Model of McKee & Tan, except at late times just before the onset of protostar formation. By varying the initial mass surface density, the magnetic energy, the chemical age, and the ortho-to-para ratio of H2, we also determine the physical and temporal properties required for high deuteration. We find that low initial ortho-to-para ratios (≲ 0.1) and/or multiple free-fall times (≳ 3) of prior chemical evolution are necessary to reach the observed values of deuterium fraction in pre-stellar cores.
• Kong, S., Tan, J., Caselli, P., Fontani, F., Pillai, T., Butler, M., Shimajiri, Y., Nakamura, F., & Sakai, T. (2016). THE DEUTERIUM FRACTION in MASSIVE STARLESS CORES and DYNAMICAL IMPLICATIONS. Astrophysical Journal, 821(2). doi:10.3847/0004-637x/821/2/94
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  We study deuterium fractionation in two massive starless/early-stage cores, C1-N and C1-S, in Infrared Dark Cloud G028.37+00.07, which was first identified by Tan et al. with ALMA. Line emission from multiple transitions of N2H+ and N2D+ were observed with the ALMA, CARMA, SMA, JCMT, NRO 45 m, and IRAM 30 m telescopes. By simultaneously fitting the spectra, we estimate the excitation conditions and deuterium fraction, Dfrac N2H+ ≡ [N2D+]/[N2H+], with values of Dfrac N2H+ ≃ 0.2-0.7, several orders of magnitude above the cosmic [D]/[H] ratio. Additional observations of o-H2D+ are also presented that help constrain the ortho-to-para ratio of H2, which is a key quantity affecting the degree of deuteration. We then present chemodynamical modeling of the two cores, especially exploring the implications for the collapse rate relative to free-fall, αff. In order to reach the high level of observed deuteration of N2H+, we find that the most likely evolutionary history of the cores involves collapse at a relatively slow rate, ≲ one-tenth of free-fall.
• Tan, J., Kong, S., Zhang, Y., Fontani, F., Caselli, P., & Butler, M. (2016). AN ORDERED BIPOLAR OUTFLOW FROM A MASSIVE EARLY-STAGE CORE. Astrophysical Journal Letters, 821(1). doi:10.3847/2041-8205/821/1/l3
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  We present ALMA follow-up observations of two massive, early-stage core candidates, C1-N and C1-S, in IRDC G028.37+00.07, that were previously identified by their N2D+(3-2) emission, and show high levels of deuteration of this species. The cores are also dark at far-infrared wavelengths up to ∼100 μm. We detect 12CO(2-1) from a narrow, highly collimated bipolar outflow that is being launched from near the center of the C1-S core, which is also the location of the peak 1.3 mm dust continuum emission. This protostar, C1-Sa, has associated dense gas traced by C18O(2-1) and DCN(3-2), from which we estimate that it has a radial velocity that is near the center of the range exhibited by the C1-S massive core. A second outflow-driving source is also detected within the projected boundary of C1-S, but it appears to be at a different radial velocity. After considering the properties of the outflows, we conclude that C1-Sa is a promising candidate for an early-stage massive protostar and as such it shows that these early phases of massive star formation can involve highly ordered outflow, and thus accretion, processes, similar to models developed to explain low-mass protostars.
• Kong, S., Caselli, P., Tan, J., Wakelam, V., & Sipilä, O. (2015). The deuterium fractionation timescale in dense cloud cores: A parameter space exploration. Astrophysical Journal, 804(2). doi:10.1088/0004-637x/804/2/98
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  The deuterium fraction, [N2D+]/[N2H+], may provide information about the ages of dense, cold gas structures, which are important for comparing dynamical models of cloud core formation and evolution. Here we introduce a complete chemical network with species containing up to three atoms, with the exception of the oxygen chemistry, where reactions involving H3O+ and its deuterated forms have been added, significantly improving the consistency with comprehensive chemical networks. Deuterium chemistry and spin states of H2 and H3+ isotopologues are included in this primarily gas-phase chemical model. We investigate the dependence of deuterium chemistry on these model parameters: density (), temperature, cosmic ray ionization rate, and gas-phase depletion factor of heavy elements (). We also explore the effects of time-dependent freeze-out of gas-phase species and the dynamical evolution of density at various rates relative to free-fall collapse. For a broad range of model parameters, the timescales to reach large values of , observed in some low- and high-mass starless cores, are relatively long compared to the local free-fall timescale. These conclusions are unaffected by introducing time-dependent freeze-out and considering models with evolving density, unless the initial 10. For fiducial model parameters, achieving requires collapse to be proceeding at rates at least several times slower than that of free-fall collapse, perhaps indicating a dynamically important role for magnetic fields in supporting starless cores and thus the regulation of star formation.
• Kong, S., Lada, C., Lada, E., Bieging, J., Lombardi, M., Forbrich, J., Alves, J., & Román-Zúñiga, C. (2015). The relationship between the dust and gas-phase co across the california molecular cloud. Astrophysical Journal, 805(1). doi:10.1088/0004-637x/805/1/58
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  We present results of an extinction-CO line survey of the southeastern part of the California molecular cloud (CMC). Deep, wide-field, near-infrared images were used to construct a sensitive, relatively high resolution (∼0.5 arcmin) (NICEST) extinction map of the region. The same region was also surveyed in the 12CO(2-1), 13CO(2-1), and C18O(2-1) emission lines at the same angular resolution. These data were used to investigate the relation between the molecular gas, traced by CO emission lines, and the dust column density, traced by extinction, on spatial scales of 0.04 pc across the cloud. We found strong spatial variations in the abundances of 13CO and C18O that were correlated with variations in gas temperature, consistent with temperature-dependent CO depletion/desorption on dust grains. The 13CO-to-C18O abundance ratio was found to increase with decreasing extinction, suggesting selective photodissociation of C18O by the ambient UV radiation field. The effect is particularly pronounced in the vicinity of an embedded cluster where the UV radiation appears to have penetrated deeply (i.e., ≲ 15 mag) into the cloud. We derived the cloud-averaged X-factor to be X = 2.53 × 1020 , a value somewhat higher than the Milky Way average. On sub-parsec scales we find there is no single empirical value of the 12CO X-factor that can characterize the molecular gas in cold (T ≲ 15 K) cloud regions, with X ∝ for 3 mag. However, in regions containing relatively hot (T 25 K) molecular gas we find a clear correlation between W(12CO) and over a large (3 ≲ ≲ 25 mag) range of extinction. This results in a constant X = 1.5 × 1020 for the hot gas, a lower value than either the average for the CMC or the Milky Way. Overall we find an (inverse) correlation between X and T in the cloud with X ∝ T. This correlation suggests that the global X-factor of a giant molecular cloud may depend on the relative amounts of hot gas contained within the cloud.
• Tan, J., Kong, S., Butler, M., Caselli, P., & Fontani, F. (2013). The dynamics of massive starless cores with alma. Astrophysical Journal, 779(2). doi:10.1088/0004-637x/779/2/96
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  How do stars that are more massive than the Sun form, and thus how is the stellar initial mass function (IMF) established? Such intermediate- and high-mass stars may be born from relatively massive pre-stellar gas cores, which are more massive than the thermal Jeans mass. The turbulent core accretion model invokes such cores as being in approximate virial equilibrium and in approximate pressure equilibrium with their surrounding clump medium. Their internal pressure is provided by a combination of turbulence and magnetic fields. Alternatively, the competitive accretion model requires strongly sub-virial initial conditions that then lead to extensive fragmentation to the thermal Jeans scale, with intermediate- and high-mass stars later forming by competitive Bondi-Hoyle accretion. To test these models, we have identified four prime examples of massive (∼100 MO) clumps from mid-infrared extinction mapping of infrared dark clouds. Fontani et al. found high deuteration fractions of N2H+ in these objects, which are consistent with them being starless. Here we present ALMA observations of these four clumps that probe the N2D+ (3-2) line at 2.″3 resolution. We find six N2D+ cores and determine their dynamical state. Their observed velocity dispersions and sizes are broadly consistent with the predictions of the turbulent core model of self-gravitating, magnetized (with Alfvén Mach number mA ∼ 1) and virialized cores that are bounded by the high pressures of their surrounding clumps. However, in the most massive cores, with masses up to ∼60 MO, our results suggest that moderately enhanced magnetic fields (so that mA ≃ 0.3) may be needed for the structures to be in virial and pressure equilibrium. Magnetically regulated core formation may thus be important in controlling the formation of massive cores, inhibiting their fragmentation, and thus helping to establish the stellar IMF. © 2013. The American Astronomical Society. All rights reserved.
• Kong, S., & Wu, Y. (2010).

  Inclination angle of the outflow in IRAS 05553+1631: a method to correct the projection effect

  . Monthly Notices of the Royal Astronomical Society. doi:10.1111/j.1365-2966.2010.18112.x
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  A mapping study of IRAS 05553+1631 was performed with 12CO J=3-2 and 13CO J=2-1 lines observed by the KOSMA 3 m-telescope. A core with a size of 0.65 pc and with a LTE mass of 120 M\odot was defined by the mapping with 13CO J=2-1 line. We have identified a bipolar outflow with 12CO J=3-2. For accuracy in the calculation of outflow parameters, overcoming the projection effect is important. We propose a new method to directly calculate the inclination-angle {\theta}. We establish two basic equations with the help of outflow contour diagram and finally obtain the "angle function" and the "angle equation" to derive {\theta}. We apply our method to the outflow of IRAS 05553+1631, finding that {\theta}blue is 73\circ and {\theta}red is 78\circ. Compared to the parameters initially estimated under an assumption of 45\circ inclination-angle, the newly derived parameters are changed with different factors. For instance, the timescales for the blue and the red lobes are reduced by 0.31 and 0.21, respectively. Larger influences apply to mechanical luminosity, driving force, and mass-loss rate. The comparisons between parameters before and after the correction show that the effect of the inclination-angle cannot be neglected.

Proceedings Publications

• Hamden, E., Schiminovich, D., Turner, N., Burkhart, B., Haworth, T., Arulanantham, N., Chung, H., Kong, S., Hoadley, K., Willacy, K., Dharmawardena, T., Kim, J., Bialy, S., Lee, M., Smith, M., & Luthman, E. (2024). Eos: a FUV spectroscopic mission to observe molecular hydrogen in molecular clouds. In Space Telescopes and Instrumentation 2024: Ultraviolet to Gamma Ray, 13093.
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  Eos is a mission concept to be proposed to the expected 2025 NASA Small Explorers Announcement of Opportunity (SMEX AO). Eos observes molecular clouds in our galaxy and nearby planet forming disks to understand the link between star and planet formation and molecular hydrogen in galactic star forming regions. Eos does this using very long-slit, high resolution spectroscopy of far ultraviolet (FUV) emission from fluorescent molecular hydrogen (H2), a powerful and underutilized FUV diagnostic. H2 is the most abundant molecule in the universe, but is typically observed in the infrared (IR) or inferred via proxies such as CO. Eos will directly observe H2 via fluorescence, which can be stimulated from a range of sources (shocks, interstellar UV radiation, bright stars, etc). Here we briefly describe the science objectives of Eos, as well as the instrument implementation.