W David Arnett
- Regents Professor Emeritus
Contact
- (520) 621-9587
- STEWARD OBS, Rm. N208
- TUCSON, AZ 85721-0065
- wda@arizona.edu
Degrees
- Ph.D. Physics
- Yale University, New Haven, Connecticut, USA
- Physical Processes and Supernova Dynamics
- M.S. Physics
- Yale University, New Haven, Connecticut, USA
- NA
- B.S. Physics
- University of Kentucky, Lexington, Kentucky, USA
- NA
Work Experience
- Texas Institute for Advanced Study (2016 - 2017)
- Lawrence Livermore National Laboratory (2000 - 2012)
- Steward Observatory, University of AZ (1988 - Ongoing)
- Astrophysics, Physics, and Enrico Fermi Institute, Univ. of Chicago (1976 - 1988)
Awards
- Honorary Doctorate of Arts and Sciences
- University of Kentucky, Lexington KY, Spring 2016
Interests
Research
Theoretical physics, computational and nuclear astrophysics, supernovae, turbulence, stellar evolution, simulation of stellar fluid dynamics, black hole and neutron star formation
Courses
No activities entered.
Scholarly Contributions
Journals/Publications
- Green, E. M., Schindler, J., & Arnett, W. D. (2015). Exploring Stellar Evolution Models of sdB Stars using MESA. Astrophysical Journal, 806, 178. doi:10.1088/0004-637X/806/2/178
- {Arnett}, W., {Meakin}, C., {Viallet}, M., {Campbell}, S., {Lattanzio}, J., k}, M. (2015). Beyond Mixing-length Theory: A Step Toward 321D. \apj, 809, 30.
- {Couch}, S., {Chatzopoulos}, E., {Arnett}, W., , F. (2015). The Three-dimensional Evolution to Core Collapse of a Massive Star. \apjl, 808, L21.
- {Schindler}, J., {Green}, E., , W. (2015). Exploring Stellar Evolution Models of sdB Stars using MESA. \apj, 806, 178.
- {Viallet}, M., {Meakin}, C., {Prat}, V., , D. (2015). Toward a consistent use of overshooting parametrizations in 1D stellar evolution codes. \aap, 580, A61.
- Arnett, W. D., Meakin, C., & Viallet, M. (2014). Chaos and turbulent nucleosynthesis prior to a supernova explosion. AIP Advances, 4(4).More infoAbstract: Three-dimensional (3D), time dependent numerical simulations of flow of matter in stars, now have sufficient resolution to be fully turbulent. The late stages of the evolution of massive stars, leading up to core collapse to a neutron star (or black hole), and often to supernova explosion and nucleosynthesis, are strongly convective because of vigorous neutrino cooling and nuclear heating. Unlike models based on current stellar evolutionary practice, these simulations show a chaotic dynamics characteristic of highly turbulent flow. Theoretical analysis of this flow, both in the Reynolds-averaged Navier-Stokes (RANS) framework and by simple dynamic models, show an encouraging consistency with the numerical results. It may now be possible to develop physically realistic and robust procedures for convection and mixing which (unlike 3D numerical simulation) may be applied throughout the long life times of stars. In addition, a new picture of the presupernova stages is emerging which is more dynamic and interesting (i.e., predictive of new and newly observed phenomena) than our previous one. © 2014 Author(s).
- Smith, N., & Arnett, W. D. (2014). Preparing for an Explosion:Hydrodynamic instabilities and turbulence in presupernovae. Astrophysical Journal, 785, 12.
- Smith, N., & Arnett, W. D. (2014). Preparing for an explosion: Hydrodynamic instabilities and turbulence in presupernovae. Astrophysical Journal, 785(2).More infoAbstract: Both observations and numerical simulations are discordant with predictions of conventional stellar evolution codes for the latest stages of a massive star's life before core collapse. The most dramatic example of this disconnect is in the eruptive mass loss occurring in the decade preceding Type IIn supernovae. We outline the key empirical evidence that indicates severe pre-supernova instability in massive stars, and we suggest that the chief reason that these outbursts are absent in stellar evolution models may lie in the treatment of turbulent convection in these codes. The mixing length theory that is used ignores (1) finite amplitude fluctuations in velocity and temperature and (2) their nonlinear interaction with nuclear burning. Including these fluctuations is likely to give rise to hydrodynamic instabilities in the latest burning sequences, which prompts us to discuss a number of far-reaching implications for the fates of massive stars. In particular, we explore connections to enhanced pre-supernova mass loss, unsteady nuclear burning and consequent eruptions, swelling of the stellar radius that may trigger violent interactions with a companion star, and potential modifications to the core structure that could dramatically alter calculations of the core-collapse explosion mechanism itself. These modifications may also impact detailed nucleosynthesis and measured isotopic anomalies in meteorites, as well as the interpretation of young core-collapse supernova remnants. Understanding these critical instabilities in the final stages of evolution may make possible the development of an early warning system for impending core collapse, if we can identify their asteroseismological or eruptive signatures. © 2014. The American Astronomical Society. All rights reserved..
- Liebert, J., Fontaine, G., Young, P. A., Williams, K. A., & Arnett, D. (2013). The age and stellar parameters of the Procyon binary system. Astrophysical Journal, 769(1).More infoAbstract: The Procyon AB binary system (orbital period 40.838 yr, a newly refined determination) is near and bright enough that the component radii, effective temperatures, and luminosities are very well determined, although more than one possible solution to the masses has limited the claimed accuracy. Preliminary mass determinations for each component are available from Hubble Space Telescope imaging, supported by ground-based astrometry and an excellent Hipparcos parallax; we use these for our preferred solution for the binary system. Other values for the masses are also considered. We have employed the TYCHO stellar evolution code to match the radius and luminosity of the F5 IV-V primary star to determine the system's most likely age as 1.87 ± 0.13 Gyr. Since prior studies of Procyon A found its abundance indistinguishable from solar, the solar composition of Asplund, Grevesse, and Sauval (Z = 0.014) is assumed for the Hertzsprung-Russell diagram fitting. An unsuccessful attempt to fit using the older solar abundance scale of Grevesse & Sauval (Z = 0.019) is also reported. For Procyon B, 11 new sequences for the cooling of non-DA white dwarfs have been calculated to investigate the dependences of the cooling age on (1) the mass, (2) core composition, (3) helium layer mass, and (4) heavy-element opacities in the helium envelope. Our calculations indicate a cooling age of 1.19 ± 0.11 Gyr, which implies that the progenitor mass of Procyon B was 2.59+0.44-0.26 M⊙. In a plot of initial versus final mass of white dwarfs in astrometric binaries or star clusters (all with age determinations), the Procyon B final mass lies several σ below a straight line fit. © 2013. The American Astronomical Society. All rights reserved.
- Smith, N., Arnett, W. D., Bally, J., Ginsburg, A., & Filippenko, A. V. (2013). The ring nebula around the blue supergiant SBW1: Pre-explosion snapshot of an sn 1987A twin. Monthly Notices of the Royal Astronomical Society, 429(2), 1324-1341.More infoAbstract: SBW1 is a B-type supergiant surrounded by a ring nebula that is a nearby twin of SN 1987A's progenitor and its circumstellar ring. We present images and spectra of SBW1 obtained with the Hubble Space Telescope (HST), the Spitzer Space Telescope and Gemini South. HST images of SBW1 do not exhibit long Rayleigh-Taylor (RT) fingers, which are presumed to cause the 'hotspots' in the SN 1987A ring when impacted by the blast wave, but instead show a geometrically thin (Δ R/R ≲ 0.05) clumpy ring. The radial mass distribution and size scales of inhomogeneities in SBW1's ring closely resemble those in the SN 1987A ring, but the more complete disc expected to reside at the base of the RT fingers is absent in SBW1. This structure may explain why portions of the SN 1987A ring between the hotspots have not yet brightened, more than 15 years after the first hotspots appeared. The model we suggest does not require a fast wind colliding with a previous red supergiant wind, because a slowly expanding equatorial ring may be ejected by a rotating blue supergiant star or in a close binary system. More surprisingly, high-resolution images of SBW1 also reveal diffuse emission filling the interior of the ring seen in Hα and in thermal-infrared (IR) emission; ~190K dust dominates the 8-20 μm luminosity (but contains only 10-5M⊙ of dust). Cooler (~85 K) dust resides in the equatorial ring itself (and has a dust mass of at least 5 μ 10-3M⊙). Diffuse emission extends inward to ~1 arcsec from the central star, where a paucity of Hα and IR emission suggests an inner hole excavated by the B-supergiant wind. We propose that diffuse emission inside the ring arises from an ionized flow of material photoevaporated from the dense ring, and its pressure prevents the B-supergiant wind from advancing in the equatorial plane. This inner emission could correspond to a structure hypothesized to reside around Sk-69° 202 that was never directly detected. If this interpretation is correct, it would suggest that photoionization can play an important dynamical role in shaping the ring nebula, and we speculate that this might help explain the origin of the polar rings around SN 1987A. In effect, the photoevaporative flow shields the outer bipolar nebula at low latitudes, whereas the blue supergiant wind expands freely out the poles and clears away the polar caps of the nebula; the polar rings reside at the intersection of these two zones. © 2012 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.
- Viallet, M., Meakin, C., Arnett, D., & Mocák, M. (2013). Turbulent convection in stellar interiors. III. mean-field analysis and stratification effects. Astrophysical Journal, 769(1).More infoAbstract: We present three-dimensional implicit large eddy simulations of the turbulent convection in the envelope of a 5 M⊙ red giant star and in the oxygen-burning shell of a 23 M⊙ supernova progenitor. The numerical models are analyzed in the framework of one-dimensional Reynolds-Averaged Navier-Stokes equations. The effects of pressure fluctuations are more important in the red giant model, owing to larger stratification of the convective zone. We show how this impacts different terms in the mean-field equations. We clarify the driving sources of kinetic energy, and show that the rate of turbulent dissipation is comparable to the convective luminosity. Although our flows have low Mach numbers and are nearly adiabatic, our analysis is general and can be applied to photospheric convection as well. The robustness of our analysis of turbulent convection is supported by the insensitivity of the mean-field balances to linear mesh resolution. We find robust results for the turbulent convection zone and the stable layers in the oxygen-burning shell model, and robust results everywhere in the red giant model, but the mean fields are not well converged in the narrow boundary regions (which contain steep gradients) in the oxygen-burning shell model. This last result illustrates the importance of unresolved physics at the convective boundary, which governs the mixing there. © 2013. The American Astronomical Society. All rights reserved.
- Starrfield, S., Iliadis, C., Timmes, F. X., Hix, W. R., Arnett, W. D., Meakin, C., & Sparks, W. M. (2012). Theoretical studies of accretion of matter onto white dwarfs and the single degenerate scenario for supernovae of Type Ia. Bulletin of the Astronomical Society of India, 40(3), 419-442.More infoAbstract: We review our current knowledge about the thermonuclear processing that occurs during the evolution of accretion onto white dwarfs (WDs) both with and without the mixing of core with accreted material. We present a brief summary of the single degenerate scenario for the progenitors of Type Ia supernovae in which it is assumed that a low mass carbon-oxygen white dwarf is growing in mass as a result of accretion from a secondary star in a close binary system. The growth in mass requires that more material remain on a white dwarf after a thermonuclear runaway than is ejected by the explosion. Recent hydrodynamic simulations of accretion of solar material onto white dwarfs without mixing always produce a thermonuclear runaway and "steady burning" does not occur. For a broad range in WD mass (0.4 Mȯ to 1.35 Mȯ), the maximum ejected material occurs for the 1.25Mȯ sequences and then decreases as the white dwarf mass decreases. Therefore, the white dwarfs are growing in mass as a consequence of the accretion of solar material, and as long as there is no mixing of accreted material with core material. In contrast, a thermonuclear runaway in the accreted hydrogen-rich layers on the low luminosity WDs in close binary systems where mixing of core matter with accreted material has occurred is the outburst mechanism for classical (CN), recurrent, and symbiotic novae. The differences in characteristics of these systems is likely the WD mass and mass accretion rate. The high levels of enrichment of CN ejecta in elements ranging from carbon to sulphur confirm that there is dredge-up of matter from the core of the WD and enable them to contribute to the chemical enrichment of the interstellar medium. Therefore, studies of classical novae can lead to an improved understanding of Galactic nucleosynthesis, some sources of pre-solar grains, and the Extragalactic distance scale. The characteristics of the outburst depend on the white dwarf mass, luminosity, mass accretion rate, and the chemical composition of both the accreting material and WD material. The properties of the outburst also depends on when, how, and if the accreted layers are mixed with the WD core and the mixing mechanism is still unknown.
- Starrfield, S., Timmes, F. X., Iliadis, C., Hix, W. R., Arnett, W. D., Meakin, C., & Sparks, W. M. (2012). Hydrodynamic studies of the evolution of recurrent, symbiotic and dwarf novae: The white dwarf components are growing in mass. Baltic Astronomy, 21(1-2), 76-87.More infoAbstract: Symbiotic binaries are systems containing white dwarfs (WDs) and red giants. Symbiotic novae are those systems in which thermonuclear eruptions occur on the WD components. These are to be distinguished from events driven by accretion disk instabilities analogous to dwarf novae eruptions in cataclysmic variable outbursts. Another class of symbiotic systems are those in which the WD is extremely luminous and it seems likely that quiescent nuclear burning is ongoing on the accreting WD. A fundamental question is the secular evolution of the WD. Do the repeated outbursts or quiescent burning in these accreting systems cause the WD to gain or lose mass? If it is gaining mass, can it eventually reach the Chandrasekhar Limit and become a supernova (a SN Ia if it can hide the hydrogen and helium in the system)? In order to better understand these systems, we have begun a new study of the evolution of Thermonuclear Runaways (TNRs) in the accreted envelopes of WDs using a variety of initial WD masses, luminosities and mass accretion rates. We use our 1-D hydro code, NOVA, which includes the new convective algorithm of Arnett, Meakin and Young, the Hix and Thielemann nuclear reaction solver, the Iliadis reaction rate library, the Timmes equation of state, and the OPAL opacities. We assume a solar composition (Lodders abundance distribution) and do not allow any mixing of accreted material with core material. This assumption strongly influences our results. We report here (1) that the WD grows in mass for all simulations so that 'steady burning' does not occur, and (2) that only a small fraction of the accreted matter is ejected in some (but not all) simulations. We also find that the accreting systems, before thermonuclear runaway, are too cool to be seen in X-ray searches for SN Ia progenitors.
- Wang, X., Wang, L., Filippenko, A. V., Baron, E., Kromer, M., Jack, D., Zhang, T., Aldering, G., Antilogus, P., Arnett, W. D., Baade, D., Barris, B. J., Benetti, S., Bouchet, P., Burrows, A. S., Canal, R., Cappellaro, E., Carlberg, R. G., Carlo, E. D., , Challis, P. J., et al. (2012). Evidence for type Ia supernova diversity from ultraviolet observations with the hubble space telescope. Astrophysical Journal, 749(2).More infoAbstract: We present ultraviolet (UV) spectroscopy and photometry of four Type Ia supernovae (SNe 2004dt, 2004ef, 2005M, and 2005cf) obtained with the UV prism of the Advanced Camera for Surveys on the Hubble Space Telescope. This data set provides unique spectral time series down to 2000 Å. Significant diversity is seen in the near-maximum-light spectra (2000-3500 Å) for this small sample. The corresponding photometric data, together with archival data from Swift Ultraviolet/Optical Telescope observations, provide further evidence of increased dispersion in the UV emission with respect to the optical. The peak luminosities measured in the uvw1/F250W filter are found to correlate with the B-band light-curve shape parameter Δm 15(B), but with much larger scatter relative to the correlation in the broadband B band (e.g., 0.4mag versus 0.2mag for those with 0.8 mag < Δm 15(B) < 1.7mag). SN2004dt is found as an outlier of this correlation (at > 3σ), being brighter than normal SNe Ia such as SN2005cf by 0.9mag and 2.0mag in the uvw1/F250W and uvm2/F220W filters, respectively. We show that different progenitor metallicity or line-expansion velocities alone cannot explain such a large discrepancy. Viewing-angle effects, such as due to an asymmetric explosion, may have a significant influence on the flux emitted in the UV region. Detailed modeling is needed to disentangle and quantify the above effects. © 2012. The American Astronomical Society. All rights reserved.
- Arnett, W. D., & Meakin, C. (2011). Toward realistic progenitors of core-collapse supernovae. Astrophysical Journal, 733(2).More infoAbstract: Two-dimensional (2D) hydrodynamical simulations of progenitor evolution of a 23 M ⊙ star, close to core collapse (in ∼1 hr in one dimension (1D)), with simultaneously active C, Ne, O, and Si burning shells, are presented and contrasted to existing 1D models (which are forced to be quasi-static). Pronounced asymmetries and strong dynamical interactions between shells are seen in 2D. Although instigated by turbulence, the dynamic behavior proceeds to sufficiently large amplitudes that it couples to the nuclear burning. Dramatic growth of low-order modes is seen as well as large deviations from spherical symmetry in the burning shells. The vigorous dynamics is more violent than that seen in earlier burning stages in the three-dimensional (3D) simulations of a single cell in the oxygen burning shell, or in 2D simulations not including an active Si shell. Linear perturbative analysis does not capture the chaotic behavior of turbulence (e.g., strange attractors such as that discovered by Lorenz), and therefore badly underestimates the vigor of the instability. The limitations of 1D and 2D models are discussed in detail. The 2D models, although flawed geometrically, represent a more realistic treatment of the relevant dynamics than existing 1D models, and present a dramatically different view of the stages of evolution prior to collapse. Implications for interpretation of SN1987A, abundances in young supernova remnants, pre-collapse outbursts, progenitor structure, neutron star kicks, and fallback are outlined. While 2D simulations provide new qualitative insight, fully 3D simulations are needed for a quantitative understanding of this stage of stellar evolution. The necessary properties of such simulations are delineated. © 2011. The American Astronomical Society. All rights reserved.
- Arnett, W. D., & Meakin, C. (2011). Turbulent cells in stars: Fluctuations in kinetic energy and luminosity. Astrophysical Journal, 741(1).More infoAbstract: Three-dimensional (3D) hydrodynamic simulations of shell oxygen burning exhibit bursty, recurrent fluctuations in turbulent kinetic energy. These are shown to be due to a general instability of the convective cell, requiring only a localized source of heating or cooling. Such fluctuations are shown to be suppressed in simulations of stellar evolution which use the mixing-length theory. Quantitatively similar behavior occurs in the model of a convective roll (cell) of Lorenz, which is known to have a strange attractor that gives rise to chaotic fluctuations in time of velocity and, as we show, luminosity. Study of simulations suggests that the behavior of a Lorenz convective roll may resemble that of a cell in convective flow. We examine some implications of this simplest approximation and suggest paths for improvement. Using the Lorenz model as representative of a convective cell, a multiple-cell model of a convective layer gives total luminosity fluctuations which are suggestive of irregular variables (red giants and supergiants), and of the long secondary period feature in semiregular asymptotic giant branch variables. This "τ-mechanism" is a new source for stellar variability, which is inherently nonlinear (unseen in linear stability analysis), and one closely related to intermittency in turbulence. It was already implicit in the 3D global simulations of Woodward et al. This fluctuating behavior is seen in extended two-dimensional simulations of CNeOSi burning shells, and may cause instability which leads to eruptions in progenitors of core-collapse supernovae prior to collapse. © 2011. The American Astronomical Society. All rights reserved.
- Kuranz, C. C., Park, H. -., Remington, B. A., Drake, R. P., Miles, A. R., Robey, H. F., Kilkenny, J. D., Keane, C. J., Kalantar, D. H., Huntington, C. M., Krauland, C. M., Harding, E. C., Grosskopf, M. J., Marion, D. C., Doss, F. W., Myra, E., Maddox, B., Young, B., Kline, J. L., , Kyrala, G., et al. (2011). Astrophysically relevant radiation hydrodynamics experiment at the National Ignition Facility. Astrophysics and Space Science, 336(1), 207-211.More infoAbstract: The National Ignition Facility (NIF) is capable of creating new and novel high-energy-density (HED) systems relevant to astrophysics. Specifically, a system could be created that studies the effects of a radiative shock on a hydrodynamically unstable interface. These dynamics would be relevant to the early evolution after a core-collapse supernova of a red supergiant star. Prior to NIF, no HED facility had enough energy to perform this kind of experiment. The experimental target will include a 340 μm predominantly plastic ablator followed by a low-density SiO2 foam. The interface will have a specific, machined pattern that will seed hydrodynamic instabilities. The growth of the instabilities in a radiation-dominated environment will be observed. This experiment requires a ≥300 eV hohlraum drive and will be diagnosed using point projection pinhole radiography, which have both been recently demonstrated on NIF. © 2011 Springer Science+Business Media B.V.
- Meakin, C. A., Sukhbold, T., & Arnett, W. D. (2011). Presupernova structure of massive stars. Astrophysics and Space Science, 336(1), 123-128.More infoAbstract: Issues concerning the structure and evolution of core collapse progenitor stars, and stellar evolution in general, are discussed with an emphasis on interior evolution. We discuss some recent results that address quantifying the uncertainties inherent in modern stellar evolution calculations, and we describe a research effort aimed at investigating the transport and mixing processes associated with stellar turbulence, which is arguably the greatest source of uncertainty in supernova progenitor structure, besides mass loss, at the time of core collapse. We highlight the important role played by precision observations of stellar parameters in constraining theoretical models, as well as the physical insight that can be garnered from three-dimensional hydrodynamic simulation. © 2011 Springer Science+Business Media B.V.
- Starrfield, S., Timmes, F. X., Hix, W. R., Iliadis, C., Arnett, W. D., Meakin, C., & Sparks, W. M. (2011). Hydrodynamic studies of the evolution of recurrent novae to supernova Ia explosions. Proceedings of the International Astronomical Union, 7(S281), 166-171.More infoAbstract: We have begun new studies of the evolution of thermonuclear runaways (TNRs) in the accreted envelopes of white dwarfs (WDs). Here we focus on the recent outbursts of RS Oph (2006), U Sco (2010) and T Pyx (2011). U Sco explodes about every 10 years and the ejected material from the WD is helium rich. It has a short orbital period for recurrent novae (RNe) but the secondary is likely to be evolved. The WD is thought to be close in mass to the Chandrasekhar limit. T Pyx has just suffered its first outburst since 1966 and it was predicted to never experience another outburst. It has a short orbital period and has formed dust in the ejecta as this paper was being written. One important question is the secular evolution of the WD. Do the repeated outbursts cause the WD to gain or lose mass? If it is gaining mass, it could eventually reach the Chandrasekhar limit and become a Type Ia supernova (SNe Ia) if it can hide the hydrogen and helium in the system. Here, we report on our latest studies of TNRs in accreted envelopes on WDs using a variety of initial WD masses, luminosities, and mass accretion rates. Of great importance to our conclusions, we assume a solar composition (Lodders abundance distribution). We use our 1-D hydro code, NOVA, that includes the Hix and Thielemann nuclear reaction network, the Iliadis reaction rate library, the Timmes equation of state, OPAL opacities, and the new convection of Arnett, Meakin, and Young. We report on the amount of ejected mass, evolution time to explode, and whether or not the WD is growing or losing mass. Copyright © International Astronomical Union 2013.
- Arnett, D., Meakin, C., & Young, P. A. (2010). Convection theory and sub-photospheric stratification. Astrophysical Journal Letters, 710(2), 1619-1626.More infoAbstract: As a preliminary step toward a complete theoretical integration of three-dimensional compressible hydrodynamic simulations into stellar evolution, convection at the surface and sub-surface layers of the Sun is re-examined, from a restricted point of view, in the language of mixing-length theory (MLT). Requiring that MLT use a hydrodynamically realistic dissipation length gives a new constraint on solar models. While the stellar structure which results is similar to that obtained by Yale Rotational Evolution Code (Guenther et al.; Bahcall & Pinsonneault) and Garching models (Schlattl et al.), the theoretical picture differs. A new quantitative connection is made between macro-turbulence, micro-turbulence, and the convective velocity scale at the photosphere, which has finite values. The "geometric parameter" in MLT is found to correspond more reasonably with the thickness of the superadiabatic region (SAR), as it must for consistency in MLT, and its integrated effect may correspond to that of the strong downward plumes which drive convection (Stein & Nordlund), and thus has a physical interpretation even in MLT. If we crudely require the thickness of the SAR to be consistent with the "geometric factor" used in MLT, there is no longer a free parameter, at least in principle. Use of three-dimensional simulations of both adiabatic convection and stellar atmospheres will allow the determination of the dissipation length and the geometric parameter (i.e., the entropy jump) more realistically, and with no astronomical calibration. A physically realistic treatment of convection in stellar evolution will require substantial additional modifications beyond MLT, including nonlocal effects of kinetic energy flux, entrainment (the most dramatic difference from MLT found by Meakin & Arnett), rotation, and magnetic fields. © 2010. The American Astronomical Society. All rights reserved..
- Arnett, W. D., & Meakin, C. (2010). Time-dependent turbulence in stars. Proceedings of the International Astronomical Union, 6(S271), 205-212.More infoAbstract: Three-dimensional (3D) hydrodynamic simulations of shell oxygen burning by Meakin & Arnett (2007b) exhibit bursty, recurrent fluctuations in turbulent kinetic energy. These are shown to be due to a global instability in the convective region, which has been suppressed in simulations of stellar evolution which use mixing-length theory (MLT). Quantitatively similar behavior occurs in the model of a convective roll (cell) of Lorenz (1963), which is known to have a strange attractor that gives rise to random fluctuations in time. An extension of the Lorenz model, which includes Kolmogorov damping and nuclear burning, is shown to exhibit bursty, recurrent fluctuations like those seen in the 3D simulations. A simple model of a convective layer (composed of multiple Lorenz cells) gives luminosity fluctuations which are suggestive of irregular variables (red giants and supergiants, see Schwarzschild (1975). Details and additional discussion may be found in Arnett & Meakin (2011). Apparent inconsistencies between Arnett, Meakin, & Young (2009) and Nordlund, Stein, & Asplund (2009) on the nature of convective driving have been resolved, and are discussed. © International Astronomical Union 2011.
- Meakin, C. A., & Arnett, W. D. (2010). Some properties of the kinetic energy flux and dissipation in turbulent stellar convection zones. Astrophysics and Space Science, 328(1), 221-225.More infoAbstract: We investigate simulated turbulent flow within thermally driven stellar convection zones. Different driving sources are studied, including cooling at the top of the convectively unstable region, as occurs in surface convection zones; and heating at the base by nuclear burning. The transport of enthalpy and kinetic energy, and the distribution of turbulent kinetic energy dissipation are studied. We emphasize the importance of global constraints on shaping the quasi-steady flow characteristics, and present an analysis of turbulent convection which is posed as a boundary value problem that can be easily incorporated into standard stellar evolution codes for deep, efficient convection. Direct comparison is made between the theoretical analysis and the simulated flow and very good agreement is found. Some common assumptions traditionally used to treat quasi-steady turbulent flow in stellar models are briefly discussed. The importance and proper treatment of convective boundaries are indicated. © 2010 Springer Science+Business Media B.V.
- Perets, H. B., Gal-Yam, A., Mazzali, P. A., Arnett, D., Kagan, D., Filippenko, A. V., Li, W., Arcavi, I., Cenko, S. B., Fox, D. B., Leonard, D. C., Moon, D. -., Sand, D. J., Soderberg, A. M., Anderson, J. P., James, P. A., Foley, R. J., Ganeshalingam, M., Ofek, E. O., , Bildsten, L., et al. (2010). A faint type of supernova from a white dwarf with a helium-rich companion. Nature, 465(7296), 322-325.More infoPMID: 20485429;Abstract: Supernovae are thought to arise from two different physical processes. The cores of massive, short-lived stars undergo gravitational core collapse and typically eject a few solar masses during their explosion. These are thought to appear as type Ib/c and type II supernovae, and are associated with young stellar populations. In contrast, the thermonuclear detonation of a carbon-oxygen white dwarf, whose mass approaches the Chandrasekhar limit, is thought to produce type Ia supernovae. Such supernovae are observed in both young and old stellar environments. Here we report a faint type Ib supernova, SN 2005E, in the halo of the nearby isolated galaxy, NGC 1032. The g∼old environment near the supernova location, and the very low derived ejected mass (0.3 solar masses), argue strongly against a core-collapse origin. Spectroscopic observations and analysis reveal high ejecta velocities, dominated by helium-burning products, probably excluding this as a subluminous or a regular type Ia supernova. We conclude that it arises from a low-mass, old progenitor, likely to have been a helium-accreting white dwarf in a binary. The ejecta contain more calcium than observed in other types of supernovae and probably large amounts of radioactive 44 Ti. © 2010 Macmillan Publishers Limited. All rights reserved.
- Arnett, D., & Wallerstein, G. (2009). αβγ, Hoyle, and the history of nucleosynthesis. Physics Today, 62(5), 10-.
- Arnett, D., Meakin, C., & Young, P. A. (2009). Turbulent convection in stellar interiors. II. the velocity field. Astrophysical Journal Letters, 690(2), 1715-1729.More infoAbstract: We analyze stellar convection with the aid of three-dimensional (3D) hydrodynamic simulations, introducing the turbulent cascade into our theoretical analysis. We devise closures of the Reynolds-decomposed mean field equations by simple physical modeling of the simulations (we relate temperature and density fluctuations via coefficients); the procedure (Convection Algorithm Based on Simulations) is terrestrially testable and is amenable to systematic improvement. We develop a turbulent kinetic energy equation which contains both nonlocal and time-dependent terms, and is appropriate if the convective transit time is shorter than the evolutionary timescale. The interpretation of mixing-length theory (MLT) as generally used in astrophysics is incorrect; MLT forces the mixing length to be an imposed constant. Direct tests show that the damping associated with the flow is that suggested by Kolmogorov (εK ρ(u′)3rms/ℓ D, where ℓD is the size of the largest eddy and (u′)rms is the local rms turbulent velocity). This eddy size is approximately the depth of the convection zone ℓCZ in our simulations, and corresponds in some respects to the mixing length of MLT. New terms involving the local heating due to turbulent dissipation should appear in the stellar evolutionary equations, and are not guaranteed to be negligible. The enthalpy flux (stellar "convective luminosity") is directly connected to the buoyant acceleration, and hence to the scale of convective velocity. MLT tends to systematically underestimate the velocity scale, which affects estimates of chromospheric and coronal heating, mass loss, and wave generation. Quantitative comparison with a variety of 3D simulations reveals a previously unrecognized consistency. Extension of this approach to deal with rotational shear and MHD is indicated. Examples of application to stellar evolution will be presented in subsequent papers in this series. © 2009. The American Astronomical Society. All rights reserved.
- Arnett, W. D. (2009). Physics of stellar convection. AIP Conference Proceedings, 1111, 603-605.More infoAbstract: We review recent progress using numerical simulations as a testbed for development of a theory of stellar convection, much as envisaged by John von Newmann. Necessary features of the theory, non-locality and fluctuations, are illustrated by computer movies. It is found that the common approximation of convection as a diffusive process presents the wrong physical picture, and improvements are suggested. New observational results discussed at the conference are gratifying in their validation of some of our theoretical ideas, especially the idea that SNIb and SNIc events are related to the explosion of massive star cores which have been stripped by mass loss and binary interactions [1]. © 2009 American Institute of Physics.
- Grosskopf, M. J., Drake, R. P., Kuranz, C. C., Miles, A. R., Hansen, J. F., Plewa, T., Hearn, N., Arnett, D., & Wheeler, J. C. (2009). Modeling of multi-interface, diverging, hydrodynamic experiments for the National Ignition Facility. Astrophysics and Space Science, 322(1-4), 57-63.More infoAbstract: The National Ignition Facility (NIF) will soon provide experiments with far more than ten times the energy than has been previously available on laser facilities. In the context of supernova-relevant hydrodynamics, this will enable experiments in which hydrodynamic instabilities develop from multiple, coupled interfaces in a diverging explosion. This paper discusses the design of such blast-wave-driven explosions in which the relative masses of the layers are scaled to those within the star. It reports scaling simulations with CALE to model the global dynamics of such an experiment. CALE is a hybrid, Arbitrary Lagrangian-Eulerian code. The simulations probed the instability growth and multi-interface interactions in mass-scaled systems using different materials. The simulations assist in the target design process and in developing an experiment that can be diagnosed. © 2008 Springer Science+Business Media B.V.
- Kuranz, C. C., Drake, R. P., Grosskopf, M. J., Budde, A., Krauland, C., Marion, D. C., Visco, A. J., Ditmar, J. R., Robey, H. F., Remington, B. A., Miles, A. R., Cooper, A. B., Sorce, C., Plewa, T., Hearn, N. C., Killebrew, K. L., Knauer, J. P., Arnett, D., & Donajkowski, T. (2009). Three-dimensional blast-wave-driven Rayleigh-Taylor instability and the effects of long-wavelength modes. Physics of Plasmas, 16(4).More infoAbstract: This paper describes experiments exploring the three-dimensional (3D) Rayleigh-Taylor instability at a blast-wave-driven interface. This experiment is well scaled to the He/H interface during the explosion phase of SN1987A. In lhe experiments, ∼5 kJ of energy from the Omega laser was used to create a planar blast wave in a plastic disk, which is accelerated into a lower-density foam. These circumstances induce the Richtmyer-Meshkov instability and, after the shock passes the interface, the system quickly becomes dominated by the Rayleigh-Taylor instability. The plastic disk has an intentional pattern machined at the plastic/foam interface. This perturbation is 3D with a basic structure of two orthogonal sine waves with a wavelength of 71 μm and an amplitude of 2.5 μm. Additional long-wavelength modes with a wavelength of either 212 or 424 μm are added onto the single-mode pattern. The addition of the long-wavelength modes was motivated by the results of previous experiments where material penetrated unexpectedly to the shock front, perhaps due to an unintended structure. The current experiments and simulations were performed to explore the effects of this unintended structure: however, we were unable to reproduce the previous results. © 2009 American Institute of Physics.
- Kuranz, C. C., Drake, R. P., Harding, E. C., Grosskopf, M. J., Robey, H. F., Remington, B. A., Edwards, M. J., Miles, A. R., Perry, T. S., Blue, B. E., Plewa, T., Hearn, N. C., Knauer, J. P., Arnett, D., & Leibrandt, D. R. (2009). Two-dimensional blast-wave-driven Rayleigh-Taylor instability: Experiment and simulation. Astrophysical Journal, 696(1), 749-759.More infoAbstract: This paper shows results from experiments diagnosing the development of the Rayleigh-Taylor instability with two-dimensional initial conditions at an embedded, decelerating interface. Experiments are performed at the Omega Laser and use ∼5 kJ of energy to create a planar blast wave in a dense, plastic layer that is followed by a lower density foam layer. The single-mode interface has a wavelength of 50 μm and amplitude of 2.5 μm. Some targets are supplemented with additional modes. The interface is shocked then decelerated by the foam layer. This initially produces the Richtmyer-Meshkov instability followed and then dominated by Rayleigh-Taylor growth that quickly evolves into the nonlinear regime. The experimental conditions are scaled to be hydrodynamically similar to SN1987A in order to study the instabilities that are believed to occur at the He/H interface during the blast-wave-driven explosion phase of the star. Simulations of the experiment were performed using the FLASH hydrodynamics code. © 2009. The American Astronomical Society. All rights reserved. Printed in the U.S.A.
- Young, P. A., Ellinger, C. I., Arnett, D., Fryer, C. L., & Rockefeller, G. (2009). Finding tracers for supernova produced 26Al. Astrophysical Journal Letters, 699(2), 938-947.More infoAbstract: We consider the cospatial production of elements in supernova explosions to find observationally detectable proxies for enhancement of 26Al in supernova ejecta and stellar systems. Using four progenitors, we explore a range of one-dimensional explosions at different energies and an asymmetric three-dimensional explosion. We find that the most reliable indicator of the presence of 26Al in unmixed ejecta is a very low S/Si ratio (0.05). Production of N in O/S/Si-rich regions is also indicative. The biologically important element P is produced at its highest abundance in the same regions. Proxies should be detectable in supernova ejecta with high spatial resolution multiwavelength observations, but the small absolute abundance of material injected into a proto-planetary disk makes detection unlikely in existing or forming stellar/planetary systems. © 2009. The American Astronomical Society.
- Arnett, D. (2008). A "crib sheet" for supernova events. AIP Conference Proceedings, 1053, 237-242.More infoAbstract: This paper summarizes our theoretical understanding of supernova events in a "back of the envelope" way. It is intended to aid in the recognition and understanding of those events which are not "standard", and which may provide the most insight. © 2008 American Institute of Physics.
- Arnett, D., Meakin, C., Starrfield, S., Timmes, F., & Young, P. (2008). Theory and numerics: New results on convection in stars. AIP Conference Proceedings, 1001, 287-294.More infoAbstract: We have begun a new program to improve the treatment of turbulent convection in stars; it features Convection Algorithms Based on Simulations (CABS). We analyze stellar convection with the aid of 3D hydrodynamic simulations, introducing the turbulent cascade into our theoretical analysis. We devise closures of the Reynolds-decomposed mean field equations by simple physical modeling of the simulations. The procedure is testable and allows systematic improvement. The interpretation of mixing-length theory (MLT) as generally used in astrophysics is incorrect. Direct tests show that the damping associated with the turbulent cascade is that suggested by Kolmogorov [1] (εK≈(u′)3rms/lCZ), where lCZ is the depth of the convection zone. This implies that the mixing-length parameter α is a simple function of depth of the convection zone, and not freely adjustable. Appropriate adjustment of another parameter of MLT, the "geometric factor", can leave solar models almost unchanged, except for their velocity scale. This parameter adjusts the super-adiabatic region just below the photosphere, and represents a crude interpolation function for characterizing 3D hydrodynamic atmospheres. © 2008 American Institute of Physics.
- Jin, L., Arnett, W. D., Sui, N., & Wang, X. (2008). An interpretation of the anomalously low mass of mars. Astrophysical Journal, 674(2 PART 2), L105-L108.More infoAbstract: Conventionally the models of the solar nebula have been constructed so that the surface density of the nebula is a monotonic function of the heliocentric distance. This mass distribution is not consistent with the observed mass distribution of planets, in particular, the Mars drop (the anomalously low mass of Mars). In order to explain this Mars drop, some researchers adopt processes to take mass out of the Mars region, and the approach taken so far is from the point of view of planetary formation theory. Here we present our new calculations of the evolution of the solar nebula. We find that the surface density in the nebula is not monotonic and that there is a minimum in the Mars region. This naturally fits the planet mass distribution, especially the Mars drop. We suggest that the existence of this minimum leads to the low mass of Mars in the following three ways. (1) The low surface density of the Mars region gives a low mass supply, and (2) it gives a low rate of planetesimal formation from dust. (3) The low surface density in the Mars region preferentially makes Mars a leftover protoplanet without gaining much mass during chaotic growth, the last stage of planet formation. © 2008. The American Astronomical Society. All rights reserved. Printed in U.S.A.
- Young, P., Ellinger, C. I., Timmes, F., Arnett, D., Fryer, C. L., Rockefeller, G. R., Hungerford, A., Diehl, S., Bennet, M., Hirschi, R., Pignatari, M., Herwig, F., & Magkotsios, G. (2008). Spatial distribution of nucleosynthesis products in cassiopeia a: Comparison between observations and 3D explosion models. Proceedings of Science.More infoAbstract: We examine observed heavy element abundances in the Cassiopeia A supernova remnant as a constraint on the nature of the Cas A supernova. We compare bulk abundances from 1D and 3D explosion models and spatial distribution of elements in 3D models with those derived from X-ray observations. We also examine the cospatial production of 26Al with other species. We find that the most reliable indicator of the presence of 26Al in unmixed ejecta is a very low S/Si ratio (∼ 0.05). Production of N in O/S/Si-rich regions is also indicative. The biologically important element P is produced at its highest abundance in the same regions. Proxies should be detectable in supernova ejecta with high spatial resolution multiwavelength observations. © Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlikeLicence.
- Meakin, C. A., & Arnett, D. (2007). Anelastic and compressible simulations of stellar oxygen burning. Astrophysical Journal, 665(1 PART 1), 690-697.More infoAbstract: In this paper we compare fully compressible (Meakin & Arnett 2006, 2007) and anelastic (Kuhlen et al. 2003) simulations of stellar oxygen shell burning. It is found that the two models are in agreement in terms of the velocity scale (uc ∼ 107 cm s_1) and thermodynamic fluctuation amplitudes (e.g., ρ′/〈ρ〉 ∼2 × 10-3) in the convective flow. Large fluctuations (∼11%) arise in the compressible model, localized to the convective boundaries, and are due to internal waves excited in stable layers. Fluctuations on the several percent level are also present in the compressible model due to composition inhomogeneities from ongoing entrainment events at the convective boundaries. Comparable fluctuations (with amplitudes greater than ∼ 1%) are absent in the anelastic simulation, because they are due to physics not included in that model. We derive an analytic estimate for the expected density fluctuation amplitudes at convective boundaries by assuming that the pressure fluctuations due to internal waves at the boundary, ρ′w, balance the ram pressure of the convective motions, ρvc2. The predicted amplitudes agree well with the simulation data. The good agreement between the anelastic and the compressible solution within the convection zone and the agreement between the stable layer dynamics and analytic solutions to the nonradial wave equation indicate that the compressible hydrodynamic techniques used are robust for the simulated stellar convection model, even at the low Mach numbers found, M ∼ 0.01. © 2007. The American Astronomical Society. All rights reserved.
- Meakin, C. A., & Arnett, D. (2007). Turbulent convection in stellar interiors. I. Hydrodynamic simulation. Astrophysical Journal, 667(1 I), 448-475.More infoAbstract: We describe the results of 3D numerical simulations of oxygen shell burning and hydrogen core burning in a 23 M⊙ stellar model. A detailed comparison is made to stellar mixing-length theory (MLT) for the shell-burning model. Simulations in 2D are significantly different from 3D, in terms of both flow morphology and velocity amplitude. Convective mixing regions are better predicted using a dynamic boundary condition based on the bulk Richardson number than by purely local, static criteria like Schwarzschild or Ledoux, MLT gives a good description of the velocity scale and temperature gradient for shell convection; however, there are other important effects that it does not capture, mostly related to the dynamical motion of the boundaries between convective and nonconvective regions. There is asymmetry between upflows and downflows, so the net kinetic energy flux is not zero. The motion of convective boundaries is a source of gravity waves; this is a necessary consequence of the deceleration of convective plumes. Convective "overshooting" is best described as an elastic response by the convective boundary, rather than ballistic penetration of the stable layers by turbulent eddies. The convective boundaries are rife with internal and interfacial wave motions, and a variety of instabilities arise that induce mixing through a process best described as turbulent entrainment. We find that the rate at which material entrainment proceeds at the boundaries is consistent with analogous laboratory experiments and simulation and observation of terrestrial atmospheric mixing. In particular, the normalized entrainment rate E = uE/σH is well described by a power-law dependence on the bulk Richardson number RiB = ΔbL/ σ2H for the conditions studied, 20 ≲ Ri B ≲ 420. We find E = ARi-nB, with best-fit values log,A = 0.027 ±0.38 and n = 1.05 ±0.21. We discuss the applicability of these results to stellar evolution calculations. © 2007. The American Astronomical Society. All rights reserved.
- Arnett, D., Meakin, C., & Young, P. A. (2006). Stellar convection with nuclear burning. Proceedings of the International Astronomical Union, 2(S239), 247-257.More infoAbstract: Careful choice of of method, problem, and zoning has allowed us to do three-dimensional (3D) simulations of thermally relaxed, nearly adiabatic convection (with nuclear burning). The simulations are run long enough so that a robust statistical state is found. We find that 2D simulations are biased relative to 3D simulations: 2D shows larger velocities and less mixing than their 3D counterparts. Detailed theoretical analysis of these numerical experiments allows us to begin to build a simple theoretical model of turbulent convection in stars, which may be used in 1D calculations of stellar evolution. Implications for stellar evolution, will be discussed. Oxygen shell burning simulations in 3D, and multishell burning of C, Ne, O, and Si in 2D will be presented, as will aspherical distortion in supernovae progenitors (Meakin and Arnett, 2006a). Contact will be made with convective driving of waves, convective zone growth by entrainment, the velocity scale and the geometric parameters in mixing length theory, and the solar Ne abundance problem. Explicit comparisons of compressible and anelastic methods at modest Mach numbers (M 0.01 to 0.1), as well as solutions of the nonradial wave equations, are presented here. Additional detail is presented in the poster by Meakin. © 2007 International Astronomical Union.
- Meakin, C. A., & Arnett, D. (2006). Active carbon and oxygen shell burning hydrodynamics. Astrophysical Journal Letters, 637(1 II), L53-L56.More infoAbstract: We have simulated 2.5 × 103 s of the late evolution of a 23 M⊙ star with full hydrodynamic behavior. We present the first simulations of a multiple-shell burning epoch, including the concurrent evolution and interaction of an oxygen- and a carbon-burning shell. In addition, we have evolved a three-dimensional model of the oxygen-burning shell to sufficiently long times (300 s) to begin to assess the adequacy of the two-dimensional approximation. We summarize striking new results: (1) strong interactions occur between active carbon- and oxygen-burning shells; (2) hydrodynamic wave motions in nonconvective regions, generated at the convective-radiative boundaries, are energetically important in both two and three dimensions, with important consequences for compositional mixing; and (3) a spectrum of mixed p- and g-modes are unambiguously identified with corresponding adiabatic waves in these computational domains. We find that two-dimensional convective motions are exaggerated relative to three-dimensional ones because of vortex instability in three dimensions. We discuss the implications for supernova progenitor evolution and symmetry breaking in core collapse. © 2006. The American Astronomical Society. All rights reserved.
- Meakin, C., & Arnett, D. (2006). Si, O, Ne, and C shell burning. Proceedings of the International Astronomical Union, 2(S239), 296-297.More infoAbstract: We simulate the reactive-hydrodynamic flow for a variety of convective shell burning epochs in supernova progenitor models. The neutrino-cooled stages of carbon, neon, oxygen, and silicon burning are simulated in two and three dimensions. Even in the absence of rotation significant symmetry breaking occurs (10% in rms variation in thermodynamic variables such as temperature and density). These distortions are caused by turbulent convection interacting with stably stratified boundaries. Strong interactions of multiple active shells is seen; it is mediated by waves generated by convection. Some implications for supernova progenitors are presented. © 2007 International Astronomical Union.
- Young, P. A., Fryer, C. L., Hungerford, A., Arnett, D., Rockefeller, G., Timmes, F. X., Voit, B., Meakin, C., & Eriksen, K. A. (2006). Constraints on the progenitor of cassiopeia A. Astrophysical Journal Letters, 640(2 I), 891-900.More infoAbstract: We compare a suite of three-dimensional explosion calculations and stellar models incorporating advanced physics with observational constraints on the progenitor of Cassiopeia A. We consider binary and single stars from 16 to 40 M⊙ with a range of explosion energies and geometries. The parameter space allowed by observations of nitrogen-rich high-velocity ejecta, ejecta mass, compact remnant mass, and 44Ti and 56Ni abundances individually and as an ensemble is considered. A progenitor of 15-25 M⊙ that loses its hydrogen envelope to a binary interaction and undergoes an energetic explosion can match all the observational constraints. © 2006. The American Astronomical Society. All rights reserved.
- Arnett, D., Meakin, C., & Young, P. (2005). Improved nucleosynthetic yields. Proceedings of the International Astronomical Union, 1, 151-156.More infoAbstract: Theoretical estimates of nucleosynthesis yields are widely used to infer the meaning of abundance trends, and to predict behavior of abundances in the first stars. We show that the standard prescriptions are incomplete, and illustrate some simple improvements. We consider the effects of internal gravity waves (induced in radiative regions by convective zones) on the evolution of slowly rotating (nonrotating) stars. We demonstrate that such modifications to the standard evolutionary algorithms give better agreement with observation. They imply new phenomena: interations between carbon and oxygen burning shells, detached flames in convective layers, and intermittency, for example. We argue that such hydrodynamic behavior must be part of realistic stellar evolution. Some preliminary results for the Sun and Sirius are discussed. © 2005 International Astronomical Union.
- Kuranz, C. C., Drake, R. P., Leibrandt, D. R., Harding, E. C., Robey, H. F., Miles, A. R., Blue, B. E., Hansen, J. F., Louis, H., Bono, M., Knauer, J., Arnett, D., & Meakin, C. A. (2005). Progress toward the study of laboratory scale, astrophysically relevant, turbulent plasmas. Astrophysics and Space Science, 298(1-2), 9-16.More infoAbstract: Recent results from an ongoing series of Rayleigh-Taylor instability experiments being conducted on the Omega Laser are described. The goal of these experiments is to study, in a controlled laboratory setting, the mixing that occurs at an unstable interface subjected to an acceleration history similar to the explosion phase of a core-collapse supernova. In a supernova, the Reynolds number characterizing this mixing is extremely large (Re > 10 10) and is more than sufficient to produce a turbulent flow at the interface. In the laboratory experiment, by contrast, the spatial scales are much smaller, but are still sufficiently large (Re > 10 5) to support a turbulent flow and therefore recreate the conditions relevant to the supernova problem. The data from these experiments will be used to validate astrophysical codes as well as to better understand the transition to turbulence in such high energy density systems. The experimental results to date using two-dimensional initial perturbations demonstrate a clear visual transition from a well-ordered perturbation structure consisting of only a few modes to one with considerable modal content. Analysis of these results, however, indicates that while a turbulent spectrum visually appears to be forming, the layer has not yet reached the asymptotic growth rate characteristic of a fully turbulent layer. Recent advances in both target fabrication and diagnostic techniques are discussed as well. These advances will allow for the study of well-controlled 3D perturbations, increasing our ability to recreate the conditions occurring in the supernova. © Springer 2005.
- Liebert, J., Young, P. A., Arnett, D., Holberg, J. B., & Williams, K. A. (2005). The age and progenitor mass of Sirius B. Astrophysical Journal Letters, 630(1 II), L69-L72.More infoAbstract: The Sirius AB binary system has masses that are well determined from many decades of astrometric measurements. Because of the well-measured radius and luminosity of Sirius A, we employed the TYCHO stellar evolution code to determine the age of the Sirius AB binary system accurately, at 225-250 Myr, Note that this fit requires the assumption of solar abundance and the use of the new Asplund et al. primordial solar metallicity. No fit to Sirius A's position is possible using the old Grevesse & Sauval scale. Because the Sirius B white dwarf parameters have also been determined accurately from space observations, the cooling age could be determined from recent calculations by Fontaine et al. or Wood to be 124 ± 10 Myr. The difference in the two ages yields the nuclear lifetime and mass of the original primary star, 5.056-0.276+0.374M⊙. This result yields, in principle, the most accurate data point at relatively high masses for the initial-to-final mass relation. However, the analysis relies on the assumption that the primordial abundance of the Sinus stars was solar, based on membership in the Sirius supercluster. A recent study suggests that its membership in the group is by no means certain. © 2005. The American Astronomical Society. All rights reserved.
- Young, P. A., & Arnett, D. (2005). Observational tests and predictive stellar evolution. II. Nonstandard models. Astrophysical Journal Letters, 618(2 I), 908-918.More infoAbstract: We examine contributions of second-order physical processes to the results of stellar evolution calculations that are amenable to direct observational testing. In the first paper in the series, we established baseline results using only physics that were common to modern stellar evolution codes. In this paper we establish how much of the discrepancy between observations and baseline models is due to particular elements of new physics in the areas of mixing, diffusion, equations of state, and opacities. We then consider the impact of the observational uncertainties on the maximum predictive accuracy achievable by a stellar evolution code. The Sun is an optimal case because of the precise and abundant observations and the relative simplicity of the underlying stellar physics. The standard model is capable of matching the structure of the Sun as determined by helioseismology and gross surface observables to better than a percent. Given an initial mass and surface composition within the observational errors, and no current observables as additional constraints for which the models can be optimized, it is not possible to predict the Sun's current state to better than ∼7%. Convectively induced mixing in radiative regions, terrestrially calibrated by multidimensional numerical hydrodynamic simulations, dramatically improves the predictions for radii, luminosity, and apsidal motions of eclipsing binaries while simultaneously maintaining consistency with observed light element depletion and turnoff ages in young clusters. Systematic errors in core size for models of massive binaries disappear with more complete mixing physics, and acceptable fits are achieved for all of the binaries without calibration of free parameters. The lack of accurate abundance determinations for binaries is now the main obstacle to improving stellar models using this type of test.
- Young, P. A., Meakin, C., Arnett, D., & Fryer, C. L. (2005). The impact of hydrodynamic mixing on supernova progenitors. Astrophysical Journal Letters, 629(2 II), L101-L104.More infoAbstract: Recent multidimensional hydrodynamic simulations have demonstrated the importance of hydrodynamic motions in the convective boundary and radiative regions of stars to transport of energy, momentum, and composition. The impact of these processes increases with stellar mass. Stellar models that approximate this physics have been tested on several classes of observational problems. In this Letter, we examine the implications of the improved treatment for supernova progenitors. The improved models predict substantially different interior structures. We present presupernova conditions and simple explosion calculations from stellar models with and without the improved mixing treatment at 23 M⊙. The results differ substantially. © 2005. The American Astronomical Society. All rights reserved.
- Drake, R. P., Leibrandt, D. R., Harding, E. C., Kuranz, C. C., Blackburn, M. A., Robey, H. F., Remington, B. A., Edwards, M. J., Miles, A. R., Perry, T. S., Wallace, R. J., Louis, H., Knauer, J. P., & Arnett, D. (2004). Nonlinear mixing behavior of the three-dimensional Rayleigh-Taylor instability at a decelerating interface. Physics of Plasmas, 11(5 PART 2), 2829-2837.More infoAbstract: The evolution of the Rayleigh-Taylor (RT) instability from intentionally three-dimensional (3D) initial conditions at an embedded, decelerating interface in a high-Reynolds number flow was discussed. The study was carried out by using ∼ 5 kJ of laser energy to produce a blast wave shocked and then decelerated the perturbed interface between the first material and lower-density C foam. It was found that the interface caused the formation of a decelerating interface with an Atwood number∼ 2/3, producing a long term positive growth rate for the RT instability. The results show that the RT spikes appear to overtake the shock waves, moving at large fraction of the predeceleration.
- Reighard, A. B., Drake, R. P., Dannenberg, K., Perry, T. S., Robey, H. F., Remington, B. A., Wallace, R. J., Ryutov, D. D., Greenough, J., Knauer, J., Boehly, T., Bouquet, S., Calder, A., Rosner, R., Fryxell, B., Arnett, D., Koenig, M., & Stone, J. (2004). Collapsing radiative shocks in argon gas on the omega laser. Inertial Fusion Sciences and Applications 2003, 950-953.More infoAbstract: A number of astrophysical systems involve radiative shocks that collapse spatially in response to energy lost through radiation. Supernova remnants are an example of systems that cool enough to radiatively collapse. This is believed to produce thin, dense shells that are Vishniac unstable. This type of instability may be responsible for the convoluted structure of supernova remnants such as the Cygnus Loop. We are conducting experiments on the Omega laser intended to produce such collapsing shocks and to study their evolution. The experiments use the laser to accelerate a thin slab of driving material (beryllium) through 1.1 ATM of argon gas (∼1 mg/cc) at ∼100 km/sec. The simulations also predict that the dense layer will be pushed ahead of the dense beryllium by the leading edge of the expansion of this material. The experiment is diagnosed in two ways. X-ray radiography has detected the presence of the dense shocked layer. These data indicate that the shock velocity is ∼100 km/s. A unique, side-on application of the VISAR (Velocity Interferometer System for Any Reflector) technique is used to detect frequency shifts from ionization and any reflections from the edge of the dense shocked layer.
- Hlghberger, J. L., Thomson, K. J., Young, P. A., Arnett, D., & Zlurys, L. M. (2003). The salty scrambled egg: Detection of NaCl toward CRL 2688. Astrophysical Journal Letters, 593(1 I), 393-401.More infoAbstract: NaCl has been detected toward the circumstellar envelope of the post-AGB star CRL 2688 using the IRAM 30 m telescope, the first time this molecule has been identified in a source other than IRC +10216. The J = 7 → 6, 11 → 10, 12 → 11, and 18 → 17 transitions of NaCl at 1, 2, and 3 mm have been observed, as well as the J = 8 → 7 line of the 37C1 isotopomer. The J = 12 → 11 line was also measured at the ARO 12 m telescope. An unsuccessful search was additionally conducted for AlCl toward CRL 2688, although in the process new transitions of NaCN were observed. Both NaCl and NaCN were found to be present in the AGB remnant wind, as suggested by their U-shaped line profiles, indicative of emission arising from an optically thin, extended shell-like source of radius ∼10″-12″. These data contrast with past results in IRC +10216, where the distribution of both molecules is confined to within a few arcseconds of the star. A high degree of excitation is required for the transitions observed for NaCl and NaCN; therefore, these two species likely arise in the region where the high-velocity outflow has collided with the remnant wind. Here the effects of shocks and clumping due to Rayleigh-Taylor instabilities have raised the densities and temperatures significantly. The shell source is thus likely to be clumpy and irregular. The chemistry producing the sodium compounds is consequently more complex than simple LTE formation. Abundances of NaCl and NaCN, relative to H2, are f ∼ 1.6 × 10-10 and ∼5.2 × 10-9, respectively, while the upper limit to AlCl is f < 2 × 10-9. These values differ substantially from those in IRC 4-10216, where AlCl has an abundance near 10-7. The NaCl observations additionally indicate a chlorine isotope ratio of 35Cl/37Cl = 2.1 ± 0.8 in CRL2688, suggestive of s-process enhancement of chlorine 37.
- Young, P. A., Highberger, J. L., Arnett, D., & Ziurys, L. M. (2003). A model for the formation of high-density clumps in proto-planetary Nebulae. Astrophysical Journal Letters, 597(1 II), L53-L56.More infoAbstract: The detection of NaCl at large radii in the Egg Nebula, CRL 2688, requires densities of 107-108 cm-3 in a thick shell of r ∼ a few × 1017 cm. To explain these results, a mechanism is needed for producing high densities at a considerable distance from the central star. In two-dimensional simulations of the interaction of the fast wind with an ambient medium, the material becomes thermally unstable. The resulting clumps can achieve the requisite conditions for NaCl excitation. We present two-dimensional models with simple physics as proof-of-principle calculations to show that the clumping behavior is robust. Clumping is a natural outcome of cooling in the colliding wind model and comparable to that inferred from observations.
- Young, P. A., Knierman, K. A., Rigby, J. R., & Arnett, D. (2003). Stellar hydrodynamics in radiative regions. Astrophysical Journal Letters, 595(2 I), 1114-1123.More infoAbstract: We present an analysis of the response of a radiative region to waves generated by a convective region of the star; this wave treatment of the classical problem of "overshooting" gives extra mixing relative to the treatment traditionally used in stellar evolutionary codes. The interface between convectively stable and unstable regions is dynamic and nonspherical, so that the nonturbulent material is driven into motion, even in the absence of "penetrative overshoot." These motions may be described by the theory of nonspherical stellar pulsations and are related to motion measured by helioseismology. Multidimensional numerical simulations of convective flow show puzzling features, which we explain by this simplified physical model. Gravity waves generated at the interface are dissipated, resulting in slow circulation and mixing seen outside the formal convection zone. The approach may be extended to deal with rotation and composition gradients ("semiconvection"). Tests of this description in the stellar evolution code TYCHO produce carbon stars on the asymptotic giant branch (AGB), an isochrone age for the Hyades and three young clusters with lithium depletion ages from brown dwarfs, and lithium and beryllium depletion consistent with observations of the Hyades and Pleiades, all without tuning parameters. The potential insight into the different contributions of rotational and hydrodynamic mixing processes could have important implications for realistic simulation of supernovae and other questions in stellar evolution.
- Drake, R. P., Robey, H. F., Hurricane, O. A., Zhang, Y., Remington, B. A., Knauer, J., Glimm, J., Arnett, D., Kane, J. O., Budil, K. S., & Grove, J. (2002). Experiments to produce a hydrodynamically unstable, spherically diverging system of relevance to instabilities in supernovae. Astrophysical Journal Letters, 564(2 I), 896-908.More infoAbstract: Results of the first spherically diverging, hydrodynamically unstable laboratory experiments of relevance to supernovae (SNe) are reported. The experiments are accomplished by using laser radiation to explode a hemispherical capsule, having a perturbed outer surface, which is embedded within a volume of low-density foam. The evolution of the experiment, like that of a supernova, is well described by the Euler equations. We have compared the experimental results to those of two-dimensional simulations using both a radiation-hydrodynamics code and a pure hydrodynamics code with front tracking.
- Kane, J. O., Robey, H. F., Remington, B. A., Drake, R. P., Knauer, J., Ryutov, D. D., Louis, H., Teyssier, R., Hurricane, O., Arnett, D., Rosner, R., & Calder, A. (2001). Interface imprinting by a rippled shock using an intense laser. Physical Review E - Statistical, Nonlinear, and Soft Matter Physics, 63(5 II), 554011-554014.More infoAbstract: Interface imprinting by a rippled shock was studied using a laser hydrodynamics experiment. A planar shock was perturbed at Cu-polimide interface, resulting in an oscillating shock that imprinted a perturbation at CH(Br)-foam interface. Instability growth with opposite phases at the two interfaces was produced by chosen thickness of the second layer.
- Robey, H. F., Kane, J. O., Remington, B. A., Drake, R. P., Hurricane, O. A., Louis, H., Wallace, R. J., Knauer, J., Keiter, P., Arnett, D., & Ryutov, D. D. (2001). An experimental testbed for the study of hydrodynamic issues in supernovae. Physics of Plasmas, 8(5 II), 2446-2453.More infoAbstract: More than a decade after the explosion of supernova 1987A, unresolved discrepancies still remain in attempts to numerically simulate the mixing processes initiated by the passage of a very strong shock through the layered structure of the progenitor star. Numerically computed velocities of the radioactive 56Ni and 56Co, produced by shock-induced explosive burning within the silicon layer, for example, are still more than 50% too low as compared with the measured velocities. To resolve such discrepancies between observation and simulation, an experimental testbed has been designed on the Omega Laser for the study of hydrodynamic issues of importance to Supernovae (SNe). In this paper, results are presented from a series of scaled laboratory experiments designed to isolate and explore several issues in the hydrodynamics of supernova explosions. The results of the experiments are compared with numerical simulations and are generally found to be in reasonable agreement. © 2001 American Institute of Physics.
- Young, P. A., Mamajek, E. E., Arnett, D., & Liebert, J. (2001). Observational tests and predictive stellar evolution. Astrophysical Journal Letters, 556(1 PART 1), 230-244.More infoAbstract: We compare 18 binary systems with precisely determined radii and masses from 23 to 1.1 M⊙ and stellar evolution models produced with our newly revised code TYCHO. "Overshooting" and rotational mixing were suppressed in order to establish a baseline for isolating these and other hydrodynamic effects. Acceptable coeval fits are found for 16 pairs without optimizing for heavy-element or helium abundance. The precision of these tests is limited by the accuracies of the observed effective temperatures. High-dispersion spectra and detailed atmospheric modeling should give more accurate effective temperatures and heavy-element abundances. PV Cas, a peculiar early A system, EK Cep B, a known post-T Tauri star, and RS Cha, a member of a young OB association, are matched by pre-mainsequence models. Predicted mass loss agrees with upper limits from IUE for CW Cep A and B. Relatively poor fits are obtained for binaries having at least one component in the mass range 1.7 < M/M⊙ < 2.6, whose evolution is sensitive to mixing. These discrepancies are robust and consistent with additional mixing in real stars. The predicted apsidal motion implies that massive star models are systematically less centrally condensed than the real stars. If these effects are due to overshooting, then the overshooting parameter αov increases with stellar mass. The apsidal motion constants are controlled by radiative opacity under conditions close to those directly measured in laser experiments, making this test more stringent than possible before.
- Arnett, D. (2000). Explosive nucleosynthesis: Prospects. Physics Report, 333-334(4-6), 109-120.More infoAbstract: Explosive nucleosynthesis is a combination of the nuclear physics of thermonuclear reactions, and the hydrodynamics of the plasma in which the reactions occur. It depends upon the initial conditions - the stellar evolution up to the explosive instability, and the nature of the explosion mechanism. Some key issues for explosive nucleosynthesis are the interaction of burning with hydrodynamics, the degree of microscopic mixing in convective zones, and the breaking of spherical symmetry by convection and rotation. Recent experiments on high-intensity lasers provides new opportunities for laboratory testing of astrophysical hydrodynamic codes. Implications of supernovae 1987A and 1998bw (GRB980425?), and η Carina are discussed, as well as the formation of black holes or neutron stars. © 2000 Elsevier Science B.V.
- Arnett, D. (2000). Mixing and burning in stars. Annals of the New York Academy of Sciences, 898, 77-89.More infoAbstract: The role of hydrodynamic mixing in astrophysics is reviewed, emphasizing connections with simulations by computer and with laser physics experiments. Computer technology now allows two-dimensional (2D) simulations, with complex microphysics, of stellar hydrodynamics and evolutionary sequences, and holds the promise for 3D. Careful validation of astrophysical methods, by laboratory experiment, by critical comparison of numerical and analytical methods, and by observation are necessary for the development of simulation methods with reliable predictive capability. Recent and surprising results from isotopic patterns in presolar grains, 2D hydrodynamic simulations of stellar evolution, and laser tests and computer simulations of Richtmeyer-Meshkov and Rayleigh-Taylor instabilities are discussed and related to stellar evolution and supernovae.
- Arnett, D. (2000). The relevance of HED lasers to astrophysics. Comptes Rendus de l'Academie des Sciences - Series IV: Physics, Astrophysics, 1(6), 767-771.More infoAbstract: The role of high energy density (HED) lasers in astrophysics is reviewed, emphasizing connections with laser physics experiments and inertial confinement fusion (ICF). Careful validation of astrophysical methods, by laboratory experiment, by critical comparison of numerical and analytical methods, and by observation are necessary for the development of simulation methods with reliable predictive capability. Recent results from hydrodynamic simulations of stellar evolution, HED laser experiments, and computer simulations will be discussed, and related to stellar evolution and supernovae. © 2000 Académie des sciences/Éditions scientifiques et médicales Elsevier SAS.
- Arnett, D. (2000). The role of mixing in astrophysics. Astrophysical Journal, Supplement Series, 127(2), 213-217.More infoAbstract: The role of hydrodynamic mixing in astrophysics is reviewed, emphasizing its connections with laser physics experiments and inertial confinement fusion. Computer technology now allows us to use two-dimensional simulations, with complex microphysics, of stellar hydrodynamics and evolutionary sequences, and it holds the promise of simulations in three dimensions. Careful validation of astrophysical methods by laboratory experiment, by the critical comparison of numerical and analytical methods and by observation are necessary for the development of simulation methods with reliable predictive capabilities. The recent and surprising results from isotopic patterns in presolar grains, from two-dimensional hydrodynamic simulations of stellar evolution, and from laser tests and computer simulations of Richtmeyer-Meshkov and Rayleigh-Taylor instabilities will be discussed in relation to stellar evolution and supernovae.
- Asida, S. M., & Arnett, D. (2000). Further adventures: Oxygen burning in a convective shell. Astrophysical Journal Letters, 545(1 PART 1), 435-443.More infoAbstract: Two-dimensional hydrodynamical simulations of a convective oxygen burning shell in the presupernova evolution of a 20 M⊙ star are extended to later times. We used the VULCAN code to simulate longer evolution times than previously possible. Our results confirm the previous work of Bazàn & Arnett over their time span (400 s). However, at 1200 s we could identify a new steady state that is significantly different from the original one-dimensional model. There is considerable overshooting at both the top and bottom boundaries of the convection zone. Beyond the boundaries, the convective velocity falls off exponentially, with excitation of internal modes. The resulting mixing greatly affects the evolution of the simulations. Connections with other works of simulation of convection, in which such behavior is found in a different context, are discussed.
- Kane, J., Arnett, D., Remington, B. A., Glendinning, S. G., Bazan, G., Drake, R. P., & Fryxell, B. A. (2000). Supernova experiments on the Nova laser. Astrophysical Journal, Supplement Series, 127(2), 365-369.More infoAbstract: Supernova (SN) 1987A focused attention on the critical role of hydrodynamic instabilities in the evolution of supernovae. To test the modeling of these instabilities, we are developing laboratory experiments of hydrodynamic mixing under conditions relevant to supernovae. Initial results were reported by Kane et al. in a recent paper. The Nova laser is used to generate a 10-15 Mbar shock at the interface of a two-layer planar target, which triggers perturbation growth, due to the Richtmeyer-Meshkov instability, and to the Rayleigh-Taylor instability as the interface decelerates. This resembles the hydrodynamics of the He-H interface of a Type II supernova at intermediate times, up to a few times 103 s. The experiment is modeled using the hydrodynamics codes HYADES and CALE, and the supernova code PROMETHEUS. Results of the experiments and simulations are presented. We also present new analysis of the bubble velocity, a study of two-dimensional versus three-dimensional difference in growth at the He-H interface of SN 1987A, and designs for two-dimensional versus three-dimensional hydro experiments.
- Kane, J., Arnett, D., Remington, B. A., Glendinning, S. G., Bazán, G., Müller, E., Fryxell, B. A., & Teyssier, R. (2000). Two-dimensional versus three-dimensional supernova hydrodynamic instability growth. Astrophysical Journal Letters, 528(2 PART 1), 989-994.More infoAbstract: Numerical simulations using the SN hydrodynamics code PROMETHEUS are carried out to study the difference between growth of two-dimensional versus three-dimensional single-mode perturbations at the He-H and O-He interfaces of SN 1987A. We find that in the rest frame of an unperturbed one-dimensional interface, a three-dimensional single-mode perturbation grows ≈30%-35% faster than a two-dimensional single-mode perturbation, when the wavelengths are chosen to give the same linear stage growth in the planar limit. In simulations where we impose single-mode density perturbations in the O layer of the initial model and random velocity perturbations in the postshock fluid near the He-H interface, we find that both axisymmetric O spikes and three-dimensional O spikes penetrate significantly further than two-dimensional O spikes. The difference between two dimensions and three dimensions predicted by our calculations is not enough to account for the difference between observed 56Co velocities in SN 1987A and the results of previous two-dimensional simulations of SN 1987A, but our results suggest that the real three-dimensional hydrodynamics are noticeably different than the two-dimensional simulations predict.
- Remington, B. A., Drake, R. P., Takabe, H., & Arnett, D. (2000). A review of astrophysics experiments on intense lasers. Physics of Plasmas, 7(5 II), 1641-1652.More infoAbstract: Astrophysics has traditionally been pursued at astronomical observatories and on theorists' computers. Observations record images from space, and theoretical models are developed to explain the observations. A component often missing has been the ability to test theories and models in an experimental setting where the initial and final states are well characterized. Intense lasers are now being used to recreate aspects of astrophysical phenomena in the laboratory, allowing the creation of experimental testbeds where theory and modeling can be quantitatively tested against data. We describe here several areas of astrophysics - supernovae, supernova remnants, gamma-ray bursts, and giant planets - where laser experiments are under development to test our understanding of these phenomena. © 2000 American Institute of Physics.
- Arnett, D. (1999). The first supernovae: A Challenge to Astrophysicists. Astrophysics and Space Science, 265(1-4), 29-35.More infoAbstract: What were the first supernova like, and what nuclei did they synthesize? What limits our ability to accurately simulate their behavior? These and related questions are critically reviewed.
- Kane, J., Arnett, D., Remington, B. A., Glendinning, S. G., Bazan, G., Drake, R. P., Fryxell, B. A., Teyssier, R., & Moore, K. (1999). Scaling supernova hydrodynamics to the laboratory. Physics of Plasmas, 6(5 I), 2065-2071.More infoAbstract: Supernova (SN) 1987A focused attention on the critical role of hydrodynamic instabilities in the evolution of supernovae. To test the modeling of these instabilities, we are developing laboratory experiments of hydrodynamic mixing under conditions relevant to supernovae. Initial results were reported in J. Kane et al. [Astrophys. J. 478, L75 (1997) and B. A. Remington et al., Phys. Plasmas 4, 1994 (1997)]. The Nova laser is used to generate a 10-15 Mbar shock at the interface of a two-layer planar target, which triggers perturbation growth due to the Richtmyer-Meshkov instability, and to the Rayleigh-Taylor instability as the interface decelerates. This resembles the hydrodynamics of the He-H interface of a Type II supernova at intermediate times, up to a few × 103 s. The scaling of hydrodynamics on microscopic laser scales to the SN-size scales is presented. The experiment is modeled using the hydrodynamics codes HYADES [J. T. Larson and S. M. Lane, J. Quant. Spect. Rad. Trans. 51, 179 (1994)] and CALE [R. T. Barton, Numerical Astrophysics (Jones and Bartlett, Boston, 1985), pp. 482-497], and the supernova code PROMETHEUS [P. R. Woodward and P. Collela, J. Comp. Phys. 54, 115 (1984)]. Results of the experiments and simulations are presented. Analysis of the spike-and-bubble velocities using potential flow theory and Ott thin-shell theory is presented, as well as a study of 2D versus 3D differences in perturbation growth at the He-H interface of SN 1987A. © 1999 American Institute of Physics.
- Remington, B. A., Arnett, D., Drake, R. P., & Takabe, H. (1999). Modelling astrophysical phenomena in the laboratory with intense lasers. Science, 284(5419), 1488-1493.More infoAbstract: Astrophysical research has traditionally been divided into observations and theoretical modeling or a combination of both. A component sometimes missing has been the ability to quantitatively test the observations and models in an experimental setting where the initial and final states are well characterized. Intense lasers are now being used to recreate aspects of astrophysical phenomena in the laboratory, allowing the creation of experimental test beds where observations and models can be quantitatively compared with laboratory data. Experiments are under development at intense laser facilities to test and refine our understanding of phenomena such as supernovae, supernova remnants, gamma-ray bursts, and giant planets.
- Timmes, F. X., & Arnett, D. (1999). The accuracy, consistency, and speed of five equations of state for stellar hydrodynamics. Astrophysical Journal, Supplement Series, 125(1), 277-294.More infoAbstract: We compare the thermodynamic properties and execution speed of five independent equations of state. A wide range of temperatures, densities, and compositions are considered - conditions appropriate for modeling the collapse of a cloud of hydrogen gas (or an exploding supernova) to the outer layers of a neutron star. The pressures and specific thermal energies calculated by each equation-of-state routine are reasonably accurate (typically 0.1% error or less) and agree remarkably well with each other, despite the different approaches and approximations used in each routine. The derivatives of the pressure and specific thermal energies with respect to the temperature and density generally show less accuracy (typically 1% error or less) and more disagreement with one another. Thermodynamic consistency, as measured by deviations from the appropriate Maxwell relations, shows that the Timmes equation of state and the Nadyozhin equation of state achieve thermodynamic consistency to a high degree of precision. The execution speed of the five equation-of-state routines - evaluated across several different machine architectures, compiler options, and modes of operation - differ dramatically. The Arnett equation of state is the fastest of the five routines, with the Nadyozhin equation of state close behind.
- Bazán, G., & Arnett, D. (1998). Two-dimensional hydrodynamics of pre-core collapse: Oxygen shell burning. Astrophysical Journal Letters, 496(1 PART I), 316-332.More infoAbstract: By direct hydrodynamic simulation, using the piecewise parabolic method code PROMETHEUS, we study the properties of a convective oxygen-burning shell in a SN 1987A progenitor star (20 M⊙) prior to collapse. The convection is too heterogeneous and dynamic to be well approximated by one-dimensional diffusion-like algorithms that have previously been used for this epoch. Qualitatively new phenomena are seen. The simulations are two-dimensional, with good resolution in radius and angle, and used a large (90°) slice centered at the equator. The microphysics and the initial model were carefully treated. Many of the qualitative features of previous multidimensional simulations of convection are seen, including large kinetic and acoustic energy fluxes, which are not accounted for by mixing length theory. Small but significant amounts of 12C are mixed nonuniformly into the oxygen-burning convection zone, resulting in hot spots of nuclear energy production that are more than an order of magnitude more energetic than the oxygen flame itself. Density perturbations (up to 8%) occur at the "edges" of the convective zone and are the result of gravity waves generated by interaction of penetrating flows into the stable region. Perturbations of temperature and Ye (or neutron excess η) at the base of the convective zone are of sufficient magnitude to create angular inhomogeneities in explosive nucleosynthesis products and need to be included in quantitative estimates of yields. Combined with the plumelike velocity structure arising from convection, the perturbations will contribute to the mixing of 56Ni throughout supernovae envelopes. Runs of different resolution and angular extent were performed to test the robustness of these simulations. © 1998. The American Astronomical Society. All rights reserved.
- Arnett, D., & Bazan, G. (1997). Nucleosynthesis in stars: Recent developments. Science, 276(5317), 1359-1362.More infoPMID: 9161994;Abstract: The development of new observational, experimental, and computational technologies is changing our understanding of the origins of the elements by thermonuclear burning in stars. Gamma-ray lines from newly made radioactive nuclei have been identified using instruments onboard low-Earth orbiting satellites. Grains in meteorites have isotopic anomalies which suggest that the grains were put together in a stellar explosion such as a supernova. Computer simulations allow such anomalies to be used to probe how these events happen. The simulations are being independently tested by experiments with high-energy density lasers. These developments are beginning to provide a quantitative diagnostic of galactic evolution, and of the epoch of formation of the first stars and galaxies.
- Bazán, G., & Arnett, D. (1997). Large nuclear networks in presupernova models. Nuclear Physics A, 621(1-2), 607C-610C.More infoAbstract: We outline the role of multidimensional hydrodynamics coupled to large nuclear networks in the case of core silicon burning in massive stars. Using an implementation of the Piecewise Parabolic Method (PPM) of solving the Euler equations for mass, momentum, and total energy, we examine the differences and similarities between a 1-D hydrostatic stellar evolution model and a 2-D hydrodynamical model at two resolutions. We find that 2-D models exhibit significantly less vigorous convection than 1-D hydrostatic models, and that the core compensates for the lack of energy production by increasing temperatures and densities through contraction. Equilibration between the Si-burning and convective timescales appears to occur. Including an 123 isotope network from Ye to 56Ge to the hydrodynamic code leads to similar global behaviors as the 2-D model with the simplified burning algorithm used in the 1-D models. However, significant inhomogeneity in iron peak isotope composition occurs, which could have important consequences to energy losses via electron captures onto G-T resonances and the local energetics which drive convective silicon burning.
- Clayton, D. D., Arnett, D., Kane, J., & Meyer, B. S. (1997). Type X silicon carbide presolar grains: Type Ia supernova condensates?. Astrophysical Journal Letters, 486(2 PART I), 824-834.More infoAbstract: In terms of nucleosynthesis issues alone, we demonstrate that the type X silicon carbide particles have chemical and isotopic compositions resembling those from explosive helium burning in 14N-rich matter. These particles are extracted chemically from meteorites and were once interstellar particles. They have already been identified by their discoverers as supernova particles on the basis of their isotopic compositions, but we argue that they are from supernovae of Type Ia that explode with a cap of helium atop their CO structure. The relative abundances of the isotopes of C and Si and trace N, Mg, and Ca match those in the X particles without need of complicated and arbitrary mixing postulates. Furthermore, both C and Si abundances are enhanced and more abundant than O, which suggests that SiC is in fact the natural condensate of such matter. We also briefly address special issues relevant to the growth of dust within Type la interiors during their expansions. © 1997. The American Astronomical Society. All rights reserved.
- Kane, J., Arnett, D., Remington, B. A., Glendinning, S. G., Castor, J., Wallace, R., Rubenchik, A., & Fryxell, B. A. (1997). Supernova-relevant hydrodynamic instability experiments on the Nova Laser. Astrophysical Journal Letters, 478(2 PART II), L75-L78.More infoAbstract: Supernova 1987A focused attention on the critical role of hydrodynamic instabilities in the evolution of supernovae. To test the modeling of these instabilities, we are developing laboratory experiments of hydrodynamic mixing under conditions relevant to supernovae. The target consists of a two-layer planar package composed of 85 μm Cu backed by 500 μm CH2, having a single-mode sinusoidal perturbation at the interface, with λ = 200 μm and η0 = 20 μm. The Nova Laser is used to generate a 10-15 Mbar [(10-15) × 1012 dyne cm2] shock at the interface, which triggers perturbation growth as a result of the Richtmyer-Meshkov instability, followed by the Rayleigh-Taylor instability as the interface decelerates. This resembles the hydrodynamics of the He-H interface of a Type II supernova at intermediate times, up to a few times 103s. The experiment is modeled using the hydrodynamics codes HYADES and CALE and the supernova code PROMETHEUS. Results of the experiments and simulations are presented. © 1997. The American Astronomical Society. All rights reserved.
- Remington, B. A., Kane, J., Drake, R. P., Glendinning, S. G., Estabrook, K., London, R., Castor, J., Wallace, R. J., Arnett, D., Liang, E., McCray, R., Rubenchik, A., & Fryxell, B. (1997). Supernova hydrodynamics experiments on the Nova laser. Physics of Plasmas, 4(5 /2), 1994-2003.More infoAbstract: Experiments have been developed to (1) investigate hydrodynamic instabilities relevant to core-collapse supernova at intermediate times, (2) and plasma flow dynamics relevant to the SN ejecta-ambient plasma interactions during the early stages of remnant formation. Initial results from both experiments are generally promising.
- Arnett, D. (1995). Explosive nucleosynthesis revisited: Yields. Annual Review of Astronomy and Astrophysics, 33(1), 115-132.More infoAbstract: Three new, independent projects that provide detailed predictions of nucleosynthesis yields (up to Z = 36) from massive stars are examined and compared. The general agreement is surprisingly good. Preliminary comparison with abundance data, especially for elemental abundances in extreme Population II stars, is promising. Implications of these comparisons and critical uncertainties are discussed.
- Livne, E., & Arnett, D. (1995). Explosions of sub-chandrasekhar mass white dwarfs in two dimensions. Astrophysical Journal Letters, 452(1), 62-74.More infoAbstract: The hydrodynamics of detonations of white dwarfs, having a carbon-oxygen core and a helium envelope, are examined in a two-dimensional geometry. Various combinations of the masses of the CO core and the overlying He layer are examined. Nuclear burning is directly computed with an alpha network from 12C to 56Ni as a part of the hydrodynamic calculation. The nucleosynthesis yield is similar to that needed for Type Ia supernovae to explain the variation in alpha elements relative to iron in Population II stars. Significant amounts of radioactive 44Ti are produced. The yields are similar to those predicted from one-dimensional computations, despite the very nonspherical hydrodynamic behavior. Light curves are simulated and compared to observations of Type Ia supernovae. The observed variation in peak absolute luminosity and in post-maximum decline rate is reproduced in a natural way. A rough comparison of predicted velocities with observations of Ca II H and K, Mg II λ4481 and Si II λ6355 in SN 1989B (in NGC 3627) is promising; implications for the use of Type Ia supernovae as distance indicators are discussed, and the key importance of spectral synthesis is stressed.
- Martin, C. L., & Arnett, D. (1995). The origin of the rings around SN 1987A: An evaluation of the interacting-winds model. Astrophysical Journal Letters, 447(1), 378-390.More infoAbstract: The origin of the nebulosity around SN 1987A is investigated using two-dimensional, hydrodynamic calculations of the interaction of the progenitor's stellar winds. The collision of the final blue supergiant (BSG) wind with a highly flattened red supergiant (RSG) wind expelled earlier produces a double-lobed bubble of shocked BSG wind confined by a shell of shocked, swept-up RSG wind. Assuming the supernova ionizes a portion of this structure, we derive emission-measure images from our models. They provide a direct comparison between the interacting-winds model and the Hubble Space Telescope (HST) images, which reveal three elliptical rings of emission around SN 1987A. The similarity of the overall morphology of our images and the HST images suggests that the interacting-winds scenario is the correct explanation for these puzzling rings. This model is subsequently used to predict the nebula's appearance in the promptly reflected continuum light from the supernova. The simulated image for day 750 is consistent with a recently published 6120 Å continuum image, but our day 1028 image does not resemble the observations as closely. The discrepancy is traced to a small difference in the extent of the bubble along the polar axis and can be resolved by increasing the pole-to-equator density gradient in the RSG wind. Finally, we present stellar evolution models for the progenitor of SN 1987A. The timescales of the late evolutionary stages correspond closely to hydrodynamic timescales in the interacting-winds model. We emphasize that the densest layers of the nebula are not well resolved in our calculations, and clumping on smaller scales will affect the emission. Hence, we compare the images quantitatively only to demonstrate the plausibility of the model and to motivate additional work on the clumping. The extreme asymmetries inferred for the RSG wind and the subtle discrepancies between the images are discussed to motivate further work with the interacting-winds model. In the Appendix, we illustrate how grid resolution affects the radiative cooling and discuss why it should be a concern in calculations of this type.
- Arnett, D. (1994). Oxygen-burning hydrodynamics. I. Steady shell burning. Astrophysical Journal Letters, 427(2), 932-946.More infoAbstract: With new hydrodynamic techniques, the relatively fast evolutionary stages of a star prior to core collapse may be explicitly computed in two spatial dimensions, with a treatment of the microphysics (e.g., nuclear reactions, equation of state, neutrino cooling) which is comparable to typical one-dimensional simulations. The nature of shell oxygen burning in a massive star, prior to core collapse, is used as a first example; it is of particular interest because it is (1) the region in which 56Ni will be produced by the supernova shock, (2) the region of the "mass cut," which will separate the collapsed core from the ejected mantle, (3) the site of much of the explosive nucleosynthesis, and (4) a suggested source of symmetry breaking to drive mixing instabilities which were observed in SN 1987A. The nature of the shell burning affects the size of the core which will collapse. The method is illustrated on this test case, and the character of the convection is examined.
- Arnett, D., & Livne, E. (1994). The delayed-detonation model of Type Ia supernovae. II. The detonation phase. Astrophysical Journal Letters, 427(1), 330-341.More infoAbstract: The investigation, by use of two-dimensional numerical hydrodynamics simulations, of the "delayed detonation" mechanism of Khokhlov for the explosion of Type Ia supernovae is continued. Previously we found that the deflagration is insufficient to unbind the star. Expansion shuts off the flame; much of this small production of iron group nuclei occurs at lower densities, which reduces the electron-capture problem. Because the degenerate star has an adiabatic exponent only slightly above 4/3, the energy released by deflagration drives a pulsation of large amplitude. During the first expansion phase, adiabatic cooling shuts off the burning, and a Rayleigh-Taylor instability then gives mixing of high-entropy ashes with low-entropy fuel. During the first contraction phase, compressional heating reignites the material. The burning was allowed to develop into a detonation in these nonspherical models. The detonation grows toward spherical symmetry at late times. At these densities (ρ ∼ 107 to 108 g cm-3), either 56Ni or nuclei of the Si-Ca group are the dominant products of the burning. The bulk yields are sensitive to the density of the star when the transition to detonation occurs. The relevance of the abundances, velocities, mixing, and total energy release to the theory and interpretation of Type Ia supernovae is discussed.
- Arnett, D., & Livne, E. (1994). The delayed-detonation model of a Type Ia supernovae. I. The deflagration phase. Astrophysical Journal Letters, 427(1), 315-329.More infoAbstract: The nature of the "delayed detonation" mechanism of Khokhlov for the explosion of Type Ia supernovae is investigated by using two-dimensional numerical hydrodynamics simulations. A new algorithm is used to treat the deflagration front. Assuming that it propagates locally at the laminar flame speed, the deflagration is insufficient to unbind the star. Expansion shuts off the flame; much of this small production of iron group nuclei occurs at lower densities, which reduces the electron-capture problem. The burning front does become wrinkled, but the wavelength of the instability is much larger than the computational grid size and is resolved; this is consistent with previous analysis. Because the degenerate star has an adiabatic exponent only slightly above 4/3, the energy released by deflagration drives a pulsation of large amplitude. During the first expansion phase, adiabatic cooling shuts off the burning, and a Rayleigh-Taylor instability then gives mixing of high-entropy ashes with low-entropy fuel. During the first contraction phase, compressional heating reignites the material. This paper deals with the deflagration phase, from the onset of burning, through expansion and quenching of the flame, to the first contraction.
- Bazan, G., & Arnett, D. (1994). Convection, nucleosynthesis, and core collapse. Astrophysical Journal Letters, 433(1 PART 2), L41-L43.More infoAbstract: We use a piecewise parabolic method hydrodynamics code (PROMETHEUS) to study convective burning in two dimensions in an oxygen shell prior to core collapse. Significant mixing beyond convective boundaries determined by mixing-length theory brings fuel (12C) into the convective regon, causing hot spots of nuclear burning. Plumes dominate the velocity structure. Finite perturbations arise in a region in which 16O will be explosively burned to 56Ni when the star explodes; the resulting instabilities and mixing are likely to distribute 56Ni throughout the supernova envelope. Inhomogeneities in Ye may be large enough to affect core collapse and will affect explosive nucleosynthesis. The nature of convective burning is dramatically different from that assumed in one-dimensional simulations; quantitative estimates of nucleosynthetic yields, core masses, and the approach to core collapse will be affected.
- Ruffert, M., & Arnett, D. (1994). Three-dimensional hydrodynamic Bondi-Hoyle accretion. II. Homogeneous medium at Mach 3 with γ = 5/3. Astrophysical Journal Letters, 427(1), 351-376.More infoAbstract: We investigate the hydrodynamics of three-dimensional classical Bondi-Hoyle accretion. Totally absorbing spheres of varying sizes (from 10 down to 0.01 accretion radii) move at Mach 3 relative to a homogeneous and slightly perturbed medium, which is taken to be an ideal gas (γ = 5/3). To accommodate the long-range gravitational forces, the extent of the computational volume is 323 accretion radii. We examine the influence of numerical procedure on physical behavior. The hydrodynamics is modeled by the "piecewise parabolic method." No energy sources (nuclear burning) or sinks (radiation, conduction) are included. The resolution in the vicinity of the accretor is increased by multiply nesting several (5-10) grids around the sphere, each finer grid being a factor of 2 smaller in zone dimension that the next coarser grid. The largest dynamic range (ratio of size of the largest grid to size of the finest zone) is 16,384. This allows us to include a coarse model for the surface of the accretor (vacuum sphere) on the finest grid, while at the same time evolving the gas on the coarser grids. Initially (at time t = 0-10), a shock front is set up, a Mach cone develops, and the accretion column is observable. Eventually the flow becomes unstable, destroying axisymmetry. This happens approximately when the mass accretion rate reaches the values (±10%) predicted by the Bondi-Hoyle accretion formula (factor of 2 included). However, our three-dimensional models do not show the highly dynamic flip-flop flow so prominent in two-dimensional calculations performed by other authors. The flow, and thus the accretion rate of all quantities, shows quasi-periodic (P ≈ 5) cycles between quiescent and active states. The interpolation formula proposed in an accompanying paper is found to follow the collected numerical data to within approximately 30%. The specific angular momentum accreted is of the same order of magnitude as the values previously found for two-dimensional flows.
- Shankar, A., & Arnett, D. (1994). Thermonuclear runaways in nova outbursts. II. Effect of strong, instantaneous, local fluctuations. Astrophysical Journal Letters, 433(1), 216-228.More infoAbstract: In an attempt to understand the manner in which nova outbursts are initiated on the surface of a white dwarf, we investigate the effects fluctuations have on the evolution of a thermonuclear runaway. Fluctuations in temperature, density, or the composition of material in the burning shell may arise due to the chaotic flow field generated by convection when it occurs, or by the accretion process itself. With the aid of two-dimensional reactive flow calculations, we consider cases where a strong fluctuation in temperature arises during the early, quiescent accretion phase or during the later, more dynamic, explosion phase. In all cases we find that an instantaneous, local temperature fluctuation causes the affected material to become Rayleigh-Taylor unstable. The rapid rise and subsequent expansion of matter immediately cools the hot blob, which prevents the lateral propagation of burning. This suggests that local temperature fluctuations do not play a significant role in directly initiating the runaway, especially during the early stages. However, they may provide an efficient mechanism of mixing core material into the envelope (thereby pre-enriching the fuel for subsequent episodes of explosive hydrogen burning) and of mixing substantial amounts of the radioactive nucleus 13N into the surface layers, making novae potential gamma-ray sources. This suggests that it is the global, not the local, evolution of the core-envelope interface to high temperatures which dominates the development of the runaway. We also present a possible new scenario for the initiation of nova outbursts based on our results.
- Grossman, S. A., Narayan, R., & Arnett, D. (1993). A theory of nonlocal mixing-length convection. I. The moment formalism. Astrophysical Journal Letters, 407(1), 284-315.More infoAbstract: Nonlocal theories of convection that have been developed for the study of convective overshooting often make unwarranted assumptions which preordain the conclusions. We develop a flexible and potentially powerful theory of convection, based on the mixing length picture, which is designed to make unbiased, self-consistent predictions about overshooting and other complicated phenomena in convection. In this paper we set up the basic formalism and demonstrate the power of the method by showing that a simplified version of the theory reproduces all the standard results of local convection. The mathematical technique we employ is a moment method, where we develop a Boltzmann transport theory for turbulent fluid elements. We imagine that a convecting fluid consists of a large number of independent fluid blobs. The ensemble of blobs is described by a distribution function, fA(t, z, v, T), where t is the time, z is the vertical position of a blob, v is its vertical velocity, and T is its temperature. The distribution function satisfies a Boltzmann equation, where the physics of the interactions of blobs is introduced through dynamical equations for v and T. We assume horizontal pressure equilibrium. The equation for v̇ includes terms due to buoyancy, microscopic viscosity, and turbulent viscosity. The latter effect is modeled with a turbulent viscosity coefficient, vturb = σℓw, where σ is the local velocity dispersion of the blobs and ℓw is the mixing length corresponding to momentum exchange between blobs. Similarly, the equation for T includes adiabatic heating, radiative diffusion of heat, and turbulent diffusion of heat, which is modeled through a diffusion coefficient, χturb = σℓθ, where ℓθ is the thermal mixing length. By taking various moments of the Boltzmann equation, we generate a series of equations which describe the evolution of the mean fluid and various moments of the turbulent fluctuations. The equations, taken to various orders, are useful for describing turbulent fluids at corresponding levels of complexity. In this picture, fluid blobs define the background, and the background tells the blobs how to move through v-T phase space. We consider the second-order equations of our theory in the limit of a steady state and vanishing third moments, and show that they reproduce all the standard results of local mixing-length convection. We find that there is a particular value of the superadiabatic gradient, Δ∇Tcrit, below which the only possible steady state of a fluid is nonconvecting. Above this critical value, a fluid is convectively unstable. We identify two distinct regimes of convection which we identify as efficient and inefficient convection. The equations we derive for convection in these regimes are very similar to the standard equations employed in stellar astrophysics. We also develop the theory of local convection in a composition-stratified fluid. We reproduce the various known regimes of convection in this problem, including semiconvection and the "salt fingers" phenomena. Surprisingly, we find that the well-known Ledoux criterion has no bearing at all on the physics of convection in a stratified medium, except in certain limits that are not of interest in astrophysics. To investigate nonlocal effects like convective overshooting, it is necessary to consider third-order equations. We write down the appropriate equations and see that they involve fourth moments in a nontrivial way. Closure relations for the fourth moments and the solution of the full nonlocal equations are the topics of future papers.
- Livne, E., & Arnett, D. (1993). On the instability of deflagration fronts in white dwarfs. Astrophysical Journal Letters, 415(2 PART 2), L107-L109.More infoAbstract: We discuss the stability of a deflagration front in white dwarfs in the context of burning mechanisms for Type Ia Supernovae. It is shown that the basic characteristics of the instability of the deflagration front are different frorn those of Rayleigh-Taylor instability, on which the deflagration models rely. In view of this, together with resuts from two-dimensional simulations, we argue that the deflagration model needs reconsideration, as Rayleigh-Taylor instability cannot explain the required high flame speed required for producing an explosion similar to Type Ia Supernovae. On the other hand, the same arguments imply that delayed detonations is acceptable and that the transition to detonation must occur in the very low density regime, where good agreement between observations and numerical results is obtained.
- Shankar, A., Arnett, D., & Fryxell, B. A. (1992). Thermonuclear runaways in nova outbursts. Astrophysical Journal Letters, 394(1 PART 2), L13-L15.More infoAbstract: We present results of exploratory, two-dimensional numerical calculations of a local thermonuclear runaway on the surface of a white dwarf. We find that the energy released by the runaway can induce a significant amount of vorticity near the burning region. Such mass motions account naturally for mixing of core matter into the envelope during the explosion. A new mechanism for the lateral spread of nuclear burning is also discussed.
- Arnett, D. (1991). Massive star evolution and SN 1987A. Astrophysical Journal Letters, 383(1), 295-307.More infoAbstract: Our understanding of the evolution of massive stars has been challenged by SN 1987A, which may well have been the normal evolution of a single star of ∼20 M⊙ of LMC abundances; there seems to be no argument based upon stellar evolution which contradicts that simple hypothesis. A set of stellar evolutionary sequences, for mass M/M⊙ = 15, 20, and 25 and metallicity z = 0.002, 0.005, 0.007, 0.010, and 0.20 are presented; semiconvection is restricted to operating slower than the local thermal time scale (as previously done). Using these sequences, simple models of the massive star content of the LMC agree moderately well with the new observational data of Fitzpatrick and Garmany. They are at least as successful as interpretations based upon use of the Schwarzschild criterion to describe semiconvection; they imply that we have found few of the main-sequence supergiants in the LMC. Untangling the effects of various uncertainties will require more rigor in both observation and theory. Despite the varieity of envelope behavior, the structure of the core at collapse is rather similar for the stars of a given mass. Variations due to different rates of mass loss are likely to be larger than those due to composition. There is an increasing tendency for dredge-up of helium core material with increasing heavy-element abundance.
- Arnett, D. (1991). New implications of SN1987A. Annals of the New York Academy of Sciences, 647, 1-10.
- Fryxell, B., Müller, E., & Arnett, D. (1991). Instabilities and clumping in SN 1987A. I. Early evolution in two dimensions. Astrophysical Journal Letters, 367(2), 619-634.More infoAbstract: Two-dimensional hydrodynamic calculations of SN 1987A (Arnett, Fryxell, and Müller 1989) have been confirmed and extended. A systematic sequence of calculations, differing only in resolution, is presented in order to permit differentiation between physical and numerical effects. This sequence also makes possible comparison with other work done with different numerical techniques. Shortly after the shock reaches the surface of the presupernova, the spherical symmetry of the mantle is severely fractured as a result of Rayleigh-Taylor instabilities. No special asymmetry in the explosion is required. This fracturing appears long before significant decay of 56Ni occurs. The presence of the instability seems to be rather insensitive to the initial presupernova structure (provided it is reasonably accurate). The amount of mixing of various elements is determined by including 10 separate fluids in the calculations enabling us to determine the distribution of each element in velocity space. The amount of mixing and clumping is somewhat sensitive to the choice of seed perturbation. However, it appears that perturbations of physically reasonable amplitude will provide sufficient mixing and clumping to explain the earlier than expected observations of X-rays and γ-rays and the shape of the light curve. The velocities of heavy elements are lower than observed for an explosion energy of 1.0 × 1051 ergs, but are interestingly close for 2.0 × 1051 ergs, and will increase after 56Ni decay heats the inner regions. The degree of clumping obtained would also affect estimates of mass from modeling the light curves of SN Ib.
- Hamrin Jr., C. E., Arnett, W. D., Angelis, R. D., Ding, X. X., & Ehmann, W. D. (1989). Starting materials and superconductor stoichiometry. Solid State Communications, 69(11), 1063-1066.More infoAbstract: Oxygen contents of as-received La2O3 and Nd2O3 were determined by fast neutron activation analysis. X-ray diffraction was used to determine the phases present in these materials. Results of these analyses show that significant amounts of hydroxides (in one case over 90 wt%) were present in all oxide samples. These results carry tremendous implications for the stoichiometry of the superconductor formation reaction and the composition of the final product. The most important conclusion to be drawn from this study is that many of the bulk superconducting ceramics produced to date may be of approximate composition at best. © 1989.
- Arnett, W. D., Zint, D. R., Hamrin Jr., C. E., Angelis, R. D., Okazaki, K., Kim, D. K., Ding, X. X., Ehmann, W. D., Brill, J. W., O'Neal, T., DeLong, L. E., Osofsky, M. S., Tourneau, V. L., & Wolf, S. A. (1988). Characterization of doped Nd-based superconductors. Journal of Superconductivity, 1(4), 427-440.More infoAbstract: Y-, Dy-, and Gd-doped neodymium (Nd) superconductors (RE0.1Nd0.9Ba2-Cu3Oy) exhibited sharp resistivity transitions at 92 K. DC susceptibility measurements gave a common curve with diamagnetic onsets at 93 K and transitions complete at ≈50 K. Lattice constants for an assumed orthorhombic structure were calculated from X-ray diffraction measurements. These values were used to create a simulated diffraction pattern with 2 θ values and relative intensities that agreed well with observed values. Oxygen contents determined by instrumental neutron activation analysis, ranged from 14.7 to 15.1±0.1 wt%(6.59≤y in Oy≤6.73). This range of oxygen content for YBa2Cu3Oy gives ≈50-60 K transitions in marked contrast to the 92 K (midpoint) values for the materials of this study. © 1988 Plenum Publishing Corporation.
- Angelis, R. D., Brill, J. W., Chung, M., Arnett, W. D., Xiang, X. -., Minton, G., Rice, L. A., & Hamrin Jr., C. E. (1987). Studies of the superconducting oxides NdxY1-xBa2Cu3Oy. Solid State Communications, 64(11), 1353-1357.More infoAbstract: We have prepared polycrystalline samples of the superconducting oxides NdxY1-xBa2Cu3Oy with x=0, 0.1, 0.5, 0.9, and 1.0. A SEM investigation indicates, for neodymium containing samples, 20 micron size particles in the interior and a "basket weave" morphology on the pressed surface. EDX and x-ray diffraction reveal no indication of rare earth segregation in the alloys. The in-plane lattice constants increase by 1% with x, but ther is no variation in the orthorhombic splitting (b/a-1 = 2%). All samples have transition (50% resistance) temperatures between 91 K and 92 K. © 1987.
- Arnett, W. D., & Rosner, J. L. (1987). Neutrino mass limits from SN1987A. Physical Review Letters, 58(18), 1906-1909.More infoAbstract: A neutrino signal from the supernova SN1987A is used to place an upper limit on the neutrino mass. If most of the neutrinos must have been emitted within several seconds, as suggested by astrophysical models, the last three of the eleven events observed by the Kamioka detector must correspond to noise or to the tail of a distribution in emission times. If the remaining eight events (which arrived within two seconds of one another) are due to neutrinos emitted within four seconds (a conservative upper limit), bound &12 eV/c2 is obtained. The Irvine-Michigan-Brookhaven data, with a higher energy threshold, primarily provide information regarding the total duration of the burst. © 1987 The American Physical Society.
- Arnett, W. (1985). COMPUTATIONAL ASTROPHYSICS.. Communications of the ACM, 28(4), 354-357.More infoAbstract: As computers become more powerful and sophisticated, computational astrophysicists will be able to find out more about stellar evolution and other astronomical phenomena. An attempt is made to answer the questions of why we are using supercompouters today, and how new hardware and software may help in the future.
- Arnett, W. D., Branch, D., & Wheeler, J. C. (1985). Hubble's constant and exploding carbon-oxygen white dwarf models for Type I supernovae. Nature, 314(6009), 337-338.More infoAbstract: The immediate progenitor of a Type I supernova (SN I) is thought to be a mass-accreting carbon-oxygen (C-O) white dwarf in a binary system. When the mass of the white dwarf approaches the Chandrasekhar mass (1.4 M⊙) the C-O nuclear fuel ignites, part of the star is incinerated to radioactive 56Ni, and the thermonuclear energy completely disrupts the star. The optical luminosity results from the trapping and thermalization of the γ rays and positrons emitted by the decay of 56Ni through 56Co to stable 56Fe. The amount of 56Ni synthesized, MNi, and the corresponding peak luminosity, L max, can be used with the observed Hubble diagram for SN I to determine the value of Hubble's constant, H0. We argue here that if this model is correct, MNi is in the range 0.4-1.4 M ⊙, the best estimate being 0.6 M⊙, and that H0 is in the range 39-73 km s-1 Mpc-1 with a best estimate of 59 km s-1 Mpc-1. This line of reasoning does not require knowledge of the temperature of the supernova and, therefore, is not subject to the uncertainties associated with attempts to determine supernova luminosities and distances by the Baade method1. It relies on the physical correctness of the model, which is subject to independent tests. © 1985 Nature Publishing Group.
- Bond, J. R., Carr, B. J., & Arnett, W. D. (1983). Can Population III stars generate primordial helium?. Nature, 304(5926), 514-515.More infoAbstract: Only stars with masses in excess of Mc∼200 M ⊙ can generate a cosmologically significant amount of helium1-3 without overproducing either metals2,3 or radiation2. Such very massive objects (VMOs) would also leave a large number of black hole remnants. Recent stellar evolution calculations 4,5 show that the helium-rich envelope of VMOs could undergo explosive ejection. We describe here a viable model of primordial helium synthesis which can be constructed after recombination, providing heavy elements are not dredged up and ejected at the same time. © 1983 Nature Publishing Group.
- Schramm, D. N., & Arnett, W. D. (1975). Neutral currents and supernovas. Physical Review Letters, 34(2), 113-116.More infoAbstract: It is shown that if more accurate neutrino opacities (including effects of electron degeneracy) are used in a gravitational collapse calculation, then the effects of neutral currents and coherent scattering may be considerably greater than was previously thought. It is also shown that a careful inclusion of the electron-capture neutrinos should increase the importance of the region near densities of ∼ 2 × 1011 g/cm3. © 1975 The American Physical Society.
- Woosley, S. W., Arnett, W. D., & Clayton, D. D. (1972). Astrophysical importance of the reaction 16O(p, α) 13N. Physics Letters B, 38(4), 196-198.More infoAbstract: The rate of the nuclear reaction 16O(p,α) 13N is shown to be very important in determining nucleosynthesis that occurs in the oxygen rich layers of an exploding star. The value of the cross section for this reaction in a specified energy range will help to determine the nature of explosions that have produced the abundant elements between 28Si and 42Ca. © 1972.
- Arnett, D. W. (1971). Receptive field organization of units in the first optic ganglion of diptera. Science, 173(4000), 929-931.More infoPMID: 5572167;Abstract: Centripetal spike potentials were recorded from two classes of units (transient and sustained) in the intermediate chiasma of flies. On-off units were characterized by a transient discharge after the onset and cessation of a light spot presented within its elliptical receptive field. Receptive fields of sustaining units were composed of three roughly circular regions arranged adjacently along a line; stimulation of the center region elicited a sustainied discharge, whereas stimulation of either adjacent region elicited an off discharge. Adjacent regions antagonized the central region, for stimulation of either inhibited the discharge resulting from stimulatiotn of the central region.
- Howard, W. M., Arnett, W. D., Clayton, D. D., & Woosley, S. E. (1971). Thermonclear origin of rare neutron-rich isotopes. Physical Review Letters, 27(23), 1607-1610.More infoAbstract: Many rare neutron-rich isotopes in the range 16
- Truran, J. W., & Arnett, W. (1971). Explosive nucleosynthesis and the composition of metal-poor stars. Astrophysics and Space Science, 11(3), 430-442.More infoAbstract: The character of the nuclear abundances synthesized under explosive carbon, oxygen and silicon burning conditions is demonstrated to be dependent upon the initial metal content of the star. Specifically, for metal-poor stars, the calculated abundances both of odd-Z nuclei and of the neutron rich isotopes of even-Z nuclei are found to be substantially reduced. This odd-even effect in Z is consistent with spectral studies of metal deficient stars. © 1971 D. Reidel Publishing Company.
- Woosley, S. E., Arnett, W. D., & Clayton, D. D. (1971). Astrophysical importance of the reaction C12 + O16. Physical Review Letters, 27(4), 213-216.More infoAbstract: The nuclear reactions between C12 and O16 are shown to be vitally important during explosive oxygen burning because they regulate the number of α particles produced per Si28 nucleus. Since it appears that explosive oxygen burning produces the observed abundances of the nuclei between Si28 and Ca42, the value of the C12 + O16 total reaction cross section will help determine the nature of the explosions that have produced the elements. © 1971 The American Physical Society.
- Woosley, S. E., Arnett, W. D., & Clayton, D. D. (1971). Erratum: Astrophysical importance of the reaction C12 + O16. (Physical Review Letters (1971) 27, 10 (700)). Physical Review Letters, 27(10), 700-.
- Arnett, W. D., & Clayton, D. D. (1970). Explosive nucleosynthesis in stars. Nature, 227(5260), 780-784.More infoAbstract: Recent calculations provide convincing evidence that the naturally occurring nuclei were produced in explosions. The required temperature, density and expansion rate strongly suggest that before the explosion the objects were ordinary evolved massive stars. We review these new developments and present a new table indicating our hypothesis concerning the origin of the nuclei in the mass range 20≤A≤62. © 1970 Nature Publishing Group.
- Arnett, W. (1969). A possible model of supernovae: Detonation of12C. Astrophysics and Space Science, 5(2), 180-212.More infoAbstract: Stars of intermediate mass (4 M⊙≲M≲9 M⊙) may ignite the12C+12C reaction explosively because of the high degree of electron degeneracy in their central regions. After the exhaustion of helium burning in the core of such stars, a helium-burning shell develops which is thermally unstable. Approximating this shell by suitable boundary conditions, the subsequent evolution of the core is examined quantitatively by standard techniques. An explosive instability due to ignition and detonation of12C+12C develops at a central density ρ{variant}c ∼ 2 × 109. Subsequent hydrodynamic expansion is computed; final velocities of expansion up to v∼20 000 km/sec are found. The star is totally disrupted; no condensed remnant is left. Such an explosion may be a plausible model for a significant fraction of supernovae. Investigation of the relevant nuclear reaction network shows that the entire core (Mcore∼1.37 M⊙) is processed through12C burning,16O burning and silicon burning. Significant amounts of56Ni are produced. This nucleosynthesis is critically sensitive to the exact central density at which the12C+12C reaction ignites; several factors which affect this critical density are discussed. A brief summary of other supernovae thories which have been expounded in detail is presented for comparison. © 1969 D. Reidel Publishing Company.
- Arnett, W. D. (1969). Pulsars and neutron star formation [2]. Nature, 222(5191), 359-361.More infoAbstract: THE current popularity of the rotating neutron star hypothesis as an explanation of pulsars suggests that a re-examination of the problem of the formation of neutron stars is appropriate. In particular it is of interest to know the predictions of the theory of stellar evolution concerning the formation of neutron stars and the type of object from which they are formed. I present here a few plausible assumptions that lead to some surprising]y general predictions. It will be suggested that pulsars are formed from massive O stars; observational tests of this hypothesis are possible and will be discussed. © 1969 Nature Publishing Group.
- Arnett, W. D. (1968). Explosive ignition of carbon in stars of intermediate mass. Nature, 219(5161), 1344-1346.More infoAbstract: Hoyle and Fowler1 have suggested that type I supernovae are the result of an explosion following the ignition of degenerate nuclear fuel in an evolved star. The purpose of this communication is to place this idea within the context of current stellar evolutionary calculations and to examine the development of the instability. The growth of carbon-oxygen core as a result of helium shell burning in stars of intermediate mass has been studied by Hayashi, Hoshi and Sugimoto2,3, Kippenhahn, Thomas and Weigert4, and Weigert5. For a carbon-oxygen core of mass Mc ∼ 0.7 M, these authors find that inclusion of energy loss by neutrino emission prevents the carbon flash from occurring, and that a cold degenerate core of potentially explosive carbon and oxygen slowly grows. If energy loss by neutrino emission is included, a carbon star cannot ignite by the 12C + 12C reaction if its mass is less than about 1 M (unpublished results of T. Murai, D. Sugimoto, R. Hoshi and C. Hayashi, and of G. Beaudet and E. E. Salpeter). This implies that all stars which have a less massive carbon-oxygen core at the onset of the helium shell burning stage will not ignite carbon burning by the comparatively gentle carbon flash. In what follows I consider how this ignition might occur. © 1968 Nature Publishing Group.
- Truran, J. W., Arnett, W. D., Tsuruta, S., & Cameron, A. G. (1968). Rapid neutron capture in supernova explosions. Astrophysics and Space Science, 1(1), 129-146.More infoAbstract: The implications of recent studies of the dynamics of the cores of highly evolved massive stars are considered with regard to the general problems of nucleosynthesis. The typical conditions estimated for these models are shown to be very promising for the process of element synthesis by neutron capture on a fast time scale (the r-process of Burbidgeet al., 1957). © 1968 D. Reidel Publishing Company.
Proceedings Publications
- Schindler, J., Green, E., & Arnett, D. (2014, apr). Exploring Stellar Evolution Models of sdB Stars Using MESA With Convective Overshoot. In Astronomical Society of the Pacific Conference Series, 481, 197.
- Arnett, D. (2013, jan). Henry Norris Russell Lecture: Thinking and Computing. In American Astronomical Society Meeting Abstracts, 221, #138.01.