The Three Dimensional Evolution to Core Collapse of a Massive Star

We present the first three dimensional (3D) simulation of the final minutes of iron core growth in a massive star, up to and including the point of core gravitational instability and collapse. We self-consistently capture the development of strong convection driven by violent Si burning in the shell surrounding the iron core. This convective burning builds the iron core to its critical (Chandrasekhar) mass and collapse ensues, driven by electron capture and photodisintegration. The non-spherical structure and motion (turbulent fluctuations) generated by 3D convection is substantial at the point of collapse. We examine the impact of such physically-realistic 3D initial conditions on the core-collapse supernova mechanism using 3D simulations including multispecies neutrino leakage. We conclude that non-spherical progenitor structure should not be ignored, and has a significant and favorable impact on the likelihood for neutrino-driven explosions.Comment: 7 pages, 5 figures, accepted for publication in ApJ Letters. Movies may be viewed at http://flash.uchicago.edu/~smc/progen3

Two-Dimensional Hydrodynamics of Pre-Core Collapse: Oxygen Shell Burning

By direct hydrodynamic simulation, using the Piecewise Parabolic Method (PPM) code PROMETHEUS, we study the properties of a convective oxygen burning shell in a SN 1987A progenitor star prior to collapse. The convection is too heterogeneous and dynamic to be well approximated by one-dimensional diffusion-like algorithms which 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 use a large (90-degree) slice centered at the equator. The microphysics and the initial model were carefully treated. Many of the qualitative features of previous multi-dimensional 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 carbon-12 are mixed non-uniformly into the oxygen burning convection zone, resulting in hot spots of nuclear energy production which 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 electron fraction 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 plume-like velocity structure arising from convection, the perturbations will contribute to the mixing of nickel-56 throughout supernovae envelopes. Runs of different resolution, and angular extent, were performed to test the robustness of theseComment: For mpeg movies of these simulations, see http://www.astrophysics.arizona.edu/movies.html Submitted to the Astrophysical Journa

A Finite Difference Representation of Neutrino Radiation Hydrodynamics in Spherically Symmetric General Relativistic Space-Time

We present an implicit finite difference representation for general relativistic radiation hydrodynamics in spherical symmetry. Our code, Agile-Boltztran, solves the Boltzmann transport equation for the angular and spectral neutrino distribution functions in self-consistent simulations of stellar core collapse and postbounce evolution. It implements a dynamically adaptive grid in comoving coordinates. Most macroscopically interesting physical quantities are defined by expectation values of the distribution function. We optimize the finite differencing of the microscopic transport equation for a consistent evolution of important expectation values. We test our code in simulations launched from progenitor stars with 13 solar masses and 40 solar masses. ~0.5 s after core collapse and bounce, the protoneutron star in the latter case reaches its maximum mass and collapses further to form a black hole. When the hydrostatic gravitational contraction sets in, we find a transient increase in electron flavor neutrino luminosities due to a change in the accretion rate. The muon- and tauon-neutrino luminosities and rms energies, however, continue to rise because previously shock-heated material with a non-degenerate electron gas starts to replace the cool degenerate material at their production site. We demonstrate this by supplementing the concept of neutrinospheres with a more detailed statistical description of the origin of escaping neutrinos. We compare the evolution of the 13 solar mass progenitor star to simulations with the MGFLD approximation, based on a recently developed flux limiter. We find similar results in the postbounce phase and validate this MGFLD approach for the spherically symmetric case with standard input physics.Comment: reformatted to 63 pages, 24 figures, to be published in ApJ

Collapsars - Gamma-Ray Bursts and Explosions in "Failed Supernovae"

Using a two-dimensional hydrodynamics code (PROMETHEUS), we study the continued evolution of rotating massive helium stars whose iron core collapse does not produce a successful outgoing shock, but instead forms a black hole. We study the formation of a disk, the associated flow patterns, and the accretion rate for disk viscosity parameter, alpha ~ 0.001 and 0.1. For the standard 14 solar mass model the average accretion rate for 15 s is 0.07 solar masses per second and the total energy deposited along the rotational axes by neutrino annihilation is (1 - 14) x 10**51 erg, depending upon the evolution of the Kerr parameter and uncertain neutrino efficiencies. Simulated deposition of this energy in the polar regions results in strong relativistic outflow - jets beamed to about 1.5% of the sky. The jets remain highly focused, and are capable of penetrating the star in 5 - 10 s. After the jet breaks through the surface of the star, highly relativistic flow can commence. Because of the sensitivity of the mass ejection and jets to accretion rate, angular momentum, and disk viscosity, and the variation of observational consequences with viewing angle, a large range of outcomes is possible ranging from bright GRBs like GRB 971214 to faint GRB-supernovae like SN 1998bw. X-ray precursors are also possible as the jet first breaks out of the star. While only a small fraction of supernovae make GRBs, we predict that all GRBs longer than a few seconds will make supernovae similar to SN 1998bw. However, hard, energetic GRBs shorter than a few seconds will be difficult to make in this model.Comment: Latex, 66 pages including 27 figures (9 color), Submitted to The Astrophysical Journal, latex uses aaspp4.sty. Figures also available at http://www.ucolick.org/~andre

Theoretical Studies of Accretion of Matter onto White Dwarfs and the Single Degenerate Scenario for Supernovae of Type Ia

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. 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 Solar masses to 1.35 Solar Masses), the maximum ejected material occurs for the 1.25 Solar Mass 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, 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 sulfur 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 CNe 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.Comment: 25 Pages, Bulletin of the Astronomical Society of India (BASI) in pres

The Evolution of Oxygen and Magnesium in the Bulge and Disk of the Milky Way

We show that the Galactic bulge and disk share a similar, strong, decline in [O/Mg] ratio with [Mg/H]. The similarity of the [O/Mg] trend in these two, markedly different, populations suggests a metallicity-dependent modulation of the stellar yields from massive stars, by mass loss from winds, and related to the Wolf-Rayet phenomenon, as proposed by McWilliam & Rich (2004). We have modified existing models for the chemical evolution of the Galactic bulge and the solar neighborhood with the inclusion of metallicity-dependent oxygen yields from theoretical predictions for massive stars that include mass loss by stellar winds. Our results significantly improve the agreement between predicted and observed [O/Mg] ratios in the bulge and disk above solar metallicity; however, a small zero-point normalization problem remains to be resolved. The zero-point shift indicates that either the semi-empirical yields of Francois et al. (2004) need adjustment, or that the bulge IMF is not quite as flat as found by Ballero et al. (2007); the former explanation is preferred. Our result removes a previous inconsistency between the interpretation of [O/Fe] and [Mg/Fe] ratios in the bulge, and confirms the conclusion that the bulge formed more rapidly than the disk, based on the over-abundances of elements produced by massive stars. We also provide an explanation for the long-standing difference between [Mg/Fe] and [O/Fe] trends among disk stars more metal-rich than the sun.Comment: 22 pages including 5 figures. Submitted to the Astronomical Journa

Nucleosynthesis in Type Ia Supernovae

Among the major uncertainties involved in the Chandrasekhar mass models for Type Ia supernovae are the companion star of the accreting white dwarf (or the accretion rate that determines the carbon ignition density) and the flame speed after ignition. We present nucleosynthesis results from relatively slow deflagration (1.5 - 3 % of the sound speed) to constrain the rate of accretion from the companion star. Because of electron capture, a significant amount of neutron-rich species such as ^{54}Cr, ^{50}Ti, ^{58}Fe, ^{62}Ni, etc. are synthesized in the central region. To avoid the too large ratios of ^{54}Cr/^{56}Fe and ^{50}Ti/^{56}Fe, the central density of the white dwarf at thermonuclear runaway must be as low as \ltsim 2 \e9 \gmc. Such a low central density can be realized by the accretion as fast as \dot M \gtsim 1 \times 10^{-7} M_\odot yr^{-1}. These rapidly accreting white dwarfs might correspond to the super-soft X-ray sources.Comment: 10 page LaTeX, 7 PostScript figures, to appear in Nuclear Physics A, Vol. A621 (1997

The Type II supernovae 2006V and 2006au: two SN 1987A-like events

Supernova 1987A revealed that a blue supergiant (BSG) star can end its life as a core-collapse supernova (SN). SN 1987A and other similar objects exhibit properties that distinguish them from ordinary Type II Plateau (IIP) SNe, whose progenitors are believed to be red supergiants (RSGs). Similarities among 1987A-like events include a long rise to maximum, early luminosity fainter than that of normal Type IIP SNe, and radioactivity acting as the primary source powering the light curves. We present and analyze two SNe monitored by the Carnegie Supernova Project that are reminiscent of SN 1987A. Optical and near-infrared (NIR) light curves, and optical spectroscopy of SNe 2006V and 2006au are presented. These observations are compared to those of SN 1987A, and are used to estimate properties of their progenitors. Both objects exhibit a slow rise to maximum and light curve evolution similar to that of SN 1987A. At the earliest epochs, SN 2006au also displays an initial dip which we interpret as the signature of the adiabatic cooling phase that ensues shock break- out. SNe 2006V and 2006au are both found to be bluer, hotter and brighter than SN 1987A. Spectra of SNe 2006V and 2006au are similar to those of SN 1987A and other normal Type II objects, although both consistently exhibit expansion velocities higher than SN 1987A. Semi-analytic models are fit to the UVOIR light curve of each object from which physical properties of the progenitors are estimated. This yields ejecta mass estimates of about 20 solar masses, explosion energies of 2 - 3 x 10^51 erg, and progenitor radii of 75 - 100 solar radii for both SNe. The progenitors of SNe 2006V and 2006au were most likely BSGs with a larger explosion energy as compared to that of SN 1987A.Comment: 21 pages,15 figures, accepted for publication in A&A, 25 October 201

Presupernova Structure of Massive Stars

Issues concerning the structure and evolution of core collapse progenitor stars are discussed with an emphasis on interior evolution. We describe a program designed to investigate the transport and mixing processes associated with stellar turbulence, arguably the greatest source of uncertainty in progenitor structure, besides mass loss, at the time of core collapse. An effort to use precision observations of stellar parameters to constrain theoretical modeling is also described.Comment: Proceedings for invited talk at High Energy Density Laboratory Astrophysics conference, Caltech, March 2010. Special issue of Astrophysics and Space Science, submitted for peer review: 7 pages, 3 figure

Collisional Dark Matter and the Origin of Massive Black Holes

If the cosmological dark matter is primarily in the form of an elementary particle which has cross section and mass for self-interaction having a ratio similar to that of ordinary nuclear matter, then seed black holes (formed in stellar collapse) will grow in a Hubble time, due to accretion of the dark matter, to a mass range 10^6 - 10^9 solar masses. Furthermore, the dependence of the final black hole mass on the galaxy velocity dispersion will be approximately as observed and the growth rate will show a time dependence consistent with observations. Other astrophysical consequences of collisional dark matter and tests of the idea are noted.Comment: 7 pages, no figures, LaTeX2e, Accepted for publication in Phys. Rev. Lett. Changed conten