155 research outputs found
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 - 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 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 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 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 , 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
Core-Collapse Supernova Simulations including Neutrino Interactions from the Virial EOS
Core-collapse supernova explosions are driven by a central engine that
converts a small fraction of the gravitational binding energy released during
core collapse to outgoing kinetic energy. The suspected mode for this energy
conversion is the neutrino mechanism, where a fraction of the neutrinos emitted
from the newly formed protoneutron star are absorbed by and heat the matter
behind the supernova shock. Accurate neutrino-matter interaction terms are
crucial for simulating these explosions. In this proceedings for IAUS 331, SN
1987A, 30 years later, we explore several corrections to the neutrino-nucleon
scattering opacity and demonstrate the effect on the dynamics of the
core-collapse supernova central engine via two dimensional
neutrino-radiation-hydrodynamics simulations. Our results reveal that the
explosion properties are sensitive to corrections to the neutral-current
scattering cross section at the 10-20% level, but only for densities at or
above g cmComment: 6 pages, 3 figures, appears in Proc. IAU Symposium 331, SN 1987A, 30
years later - Cosmic Rays and Nuclei from Supernovae and Their Aftermath
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
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
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