The Role of Turbulence in Neutrino-Driven Core-Collapse Supernova Explosions

The neutrino-heated "gain layer" immediately behind the stalled shock in a core-collapse supernova is unstable to high-Reynolds-number turbulent convection. We carry out and analyze a new set of 19 high-resolution three-dimensional (3D) simulations with a three-species neutrino leakage/heating scheme and compare with spherically-symmetric (1D) and axisymmetric (2D) simulations carried out with the same methods. We study the postbounce supernova evolution in a $15$-$M_\odot$ progenitor star and vary the local neutrino heating rate, the magnitude and spatial dependence of asphericity from convective burning in the Si/O shell, and spatial resolution. Our simulations suggest that there is a direct correlation between the strength of turbulence in the gain layer and the susceptability to explosion. 2D and 3D simulations explode at much lower neutrino heating rates than 1D simulations. This is commonly explained by the fact that nonradial dynamics allows accreting material to stay longer in the gain layer. We show that this explanation is incomplete. Our results indicate that the effective turbulent ram pressure exerted on the shock plays a crucial role by allowing multi-D models to explode at a lower postshock thermal pressure and thus with less neutrino heating than 1D models. We connect the turbulent ram pressure with turbulent energy at large scales and in this way explain why 2D simulations are erroneously exploding more easily than 3D simulations.Comment: 13 pages, 8 figures, accepted by Ap

Aspherical Core-Collapse Supernovae in Red Supergiants Powered by Nonrelativistic Jets

We explore the observational characteristics of jet-driven supernovae by simulating bipolar-jet-driven explosions in a red supergiant progenitor. We present results of four models in which we hold the injected kinetic energy at a constant $10^{51}$ ergs across all jet models but vary the specific characteristics of the jets to explore the influence of the nature of jets on the structure of the supernova ejecta. We evolve the explosions past shock-breakout and into quasi-homologous expansion of the supernova envelope into a red supergiant wind. The oppositely-directed, nickel-rich jets give a large-scale asymmetry that may account for the non-spherical excitation and substructure of spectral lines such as H$\alpha$ and He I 10830\AA. Jets with a large fraction of kinetic to thermal energy punch through the progenitor envelope and give rise to explosions that would be observed to be asymmetric from the earliest epochs, inconsistent with spectropolarimetric measurements of Type II supernovae. Jets with higher thermal energy fractions result in explosions that are roughly spherical at large radii but are significantly elongated at smaller radii, deep inside the ejecta, in agreement with the polarimetric observations. We present shock breakout light curves that indicate that strongly aspherical shock breakouts are incompatible with recent {\it GALEX} observations of shock breakout from red supergiant stars. Comparison with observations indicates that jets must deposit their kinetic energy efficiently throughout the ejecta while in the hydrogen envelope. Thermal energy-dominated jets satisfy this criterion and yield many of the observational characteristics of Type II supernovae.Comment: 21 pages, 19 figures, submitted to ApJ on 4 Nov 200

Implicit large eddy simulations of anisotropic weakly compressible turbulence with application to core-collapse supernovae

(Abridged) In the implicit large eddy simulation (ILES) paradigm, the dissipative nature of high-resolution shock-capturing schemes is exploited to provide an implicit model of turbulence. Recent 3D simulations suggest that turbulence might play a crucial role in core-collapse supernova explosions, however the fidelity with which turbulence is simulated in these studies is unclear. Especially considering that the accuracy of ILES for the regime of interest in CCSN, weakly compressible and strongly anisotropic, has not been systematically assessed before. In this paper we assess the accuracy of ILES using numerical methods most commonly employed in computational astrophysics by means of a number of local simulations of driven, weakly compressible, anisotropic turbulence. We report a detailed analysis of the way in which the turbulent cascade is influenced by the numerics. Our results suggest that anisotropy and compressibility in CCSN turbulence have little effect on the turbulent kinetic energy spectrum and a Kolmogorov $k^{-5/3}$ scaling is obtained in the inertial range. We find that, on the one hand, the kinetic energy dissipation rate at large scales is correctly captured even at relatively low resolutions, suggesting that very high effective Reynolds number can be achieved at the largest scales of the simulation. On the other hand, the dynamics at intermediate scales appears to be completely dominated by the so-called bottleneck effect, \ie the pile up of kinetic energy close to the dissipation range due to the partial suppression of the energy cascade by numerical viscosity. An inertial range is not recovered until the point where relatively high resolution $\sim 512^3$, which would be difficult to realize in global simulations, is reached. We discuss the consequences for CCSN simulations.Comment: 17 pages, 9 figures, matches published versio

Revival of the Stalled Core-collapse Supernova Shock Triggered by Precollapse Asphericity in the Progenitor Star

Multi-dimensional simulations of advanced nuclear burning stages of massive stars suggest that the Si/O layers of presupernova stars harbor large deviations from the spherical symmetry typically assumed for presupernova stellar structure. We carry out three-dimensional core-collapse supernova simulations with and without aspherical velocity perturbations to assess their potential impact on the supernova hydrodynamics in the stalled-shock phase. Our results show that realistic perturbations can qualitatively alter the postbounce evolution, triggering an explosion in a model that fails to explode without them. This finding underlines the need for a multi-dimensional treatment of the presupernova stage of stellar evolution

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

Accretion onto Intermediate-Mass Black Holes in Dense Protogalactic Clouds

We present the first results from two-dimensional simulations of radiatively-efficient accretion of metal-free gas onto intermediate-mass black holes. We fix the shape of the spectral energy distribution of the radiation produced near the event horizon and study the structure of the irradiated low-angular-momentum accretion flow over three orders of magnitude in radius from the black hole, 10^{14}-10^{17} cm for a 100 M_sun black hole. The luminosity of the central source is made to be proportional to the rate at which gas accretes across the inner boundary, which we set just inside the sonic radius. We find that photoionization heating and radiation pressure modify the structure of the flow. When the ambient gas density is 10^7 cm^{-3}, accretion is intermittent and on average reduced to 32% of the Eddington-limited rate, two orders of magnitude below the "Bondi" rate evaluated ignoring radiation, in agreement with simplified theoretical models. Even if the vicinity of the black hole is supplied with high density gas, accretion is rendered inefficient through heating and radiation pressure.Comment: 4 pages, 3 figure