Search

We begin an exploration of the capacity of the stationary accretion shock instability (SASI) to generate magnetic fields by adding a weak, stationary, and radial (but bipolar) magnetic field, and in some cases rotation, to an initially spherically symmetric fluid configuration that models a stalled shock in the post-bounce supernova environment. In axisymmetric simulations we find that cycles of latitudinal flows into and radial flows out of the polar regions amplify the field parallel to the symmetry axis, typically increasing the total magnetic energy by about two orders of magnitude. Nonaxisymmetric calculations result in fundamentally different flows and a larger magnetic energy increase: shearing associated with the SASI spiral mode contributes to a widespread and turbulent field amplification mechanism, boosting the magnetic energy by almost four orders of magnitude (a result which remains very sensitive to the spatial resolution of the numerical simulations). While the SASI may contribute to neutron star magnetization, these simulations do not show qualitatively new features in the global evolution of the shock as a result of SASI-induced magnetic field amplification., 62 pages (aastex, preprint), 24 figures. Rewritten with 3D simulations included. Accepted for publication in ApJ

We use time-dependent, axisymmetric, hydrodynamic simulations to study the linear stability of the stalled, spherical accretion shock that arises in the post-bounce phase of core-collapse supernovae. We show that this accretion shock is stable to radial modes, with decay rates and oscillation frequencies in close agreement with the linear stability analysis of Houck and Chevalier. For non-spherical perturbations we find that the l=1 mode is always unstable for parameters appropriate to core-collapse supernovae. We also find that the l=2 mode is unstable, but typically has a growth rate smaller than that for l=1. Furthermore, the l=1 mode is the only mode found to transition into a nonlinear stage in our simulations. This result provides a possible explanation for the dominance of an l=1 'sloshing' mode seen in many two-dimensional simulations of core-collapse supernovae.

Accurate neutrino transport has been built into spherically symmetric simulations of stellar core collapse and postbounce evolution. The results of such simulations agree that spherically symmetric models with standard microphysical input fail to explode by the delayed, neutrino-driven mechanism. Independent groups implemented fundamentally different numerical methods to tackle the Boltzmann neutrino transport equation. Here we present a direct and detailed comparison of such neutrino radiation-hydrodynamical simulations for two codes, Agile-Boltztran of the Oak Ridge-Basel group and Vertex of the Garching group. The former solves the Boltzmann equation directly by an implicit, general relativistic discrete angle method on the adaptive grid of a conservative implicit hydrodynamics code with second-order TVD advection. In contrast, the latter couples a variable Eddington factor technique with an explicit, moving-grid, conservative high-order Riemann solver with important relativistic effects treated by an effective gravitational potential. The presented study is meant to test both neutrino radiation-hydrodynamics implementations and to provide a data basis for comparisons and verifications of supernova codes to be developed in the future. Results are discussed for simulations of the core collapse and post-bounce evolution of a 13 solar mass star with Newtonian gravity and a 15 solar mass star with relativistic gravity., Comment: 23 pages, 13 figures, revised version, to appear in ApJ

We investigate the effect of progenitor rotation on the standing accretion shock instability (SASI) using two- and three-dimensional hydrodynamic simulations. We find that the growth rate of the SASI is a near-linearly increasing function of the specific angular momentum in the accreting gas. Both the growth rate and the angular frequency in the two-dimensional model with cylindrical geometry agree well with previous linear stability analyses. When excited by very small random perturbations, a one-armed spiral mode dominates the small rotation rates predicted by current stellar evolution models, while progressively higher-order modes are seen as the specific angular momentum increases.

We compare Newtonian three-flavor multigroup Boltzmann (MGBT) and (Bruenn's) multigroup flux-limited diffusion (MGFLD) neutrino transport in postbounce core-collapse supernova environments. We focus our study on quantities central to the postbounce neutrino heating mechanism for reviving the stalled shock. Stationary-state three-flavor neutrino distributions are developed in thermally and hydrodynamically frozen time slices obtained from core collapse and bounce simulations that implement Lagrangian hydrodynamics and MGFLD neutrino transport. We obtain distributions for time slices at 106 and 233 ms after core bounce for the core of a 15 M☉ progenitor, and at 156 ms after core bounce for a 25 M☉ progenitor. For both transport methods, the electron neutrino and antineutrino luminosities, rms energies, and mean inverse flux factors, all of which enter the neutrino heating rates, are computed as functions of radius and compared. The net neutrino heating rates are also computed as functions of radius and compared. Notably, we find significant differences in neutrino luminosities and mean inverse flux factors between the two transport methods for both precollapse models and for all three time slices. In each case, the luminosities for each transport method begin to diverge above the neutrinospheres, where the MGBT luminosities become larger than their MGFLD counterparts, finally settling to a constant difference maintained to the edge of the core. We find that the mean inverse flux factors, which describe the degree of forward peaking in the neutrino radiation field, also differ significantly between the two transport methods, with MGBT providing more isotropic radiation fields in the gain region. Most important, for a region above the gain radius we find net heating rates for MGBT that are as much as ~2 times the corresponding MGFLD rates, and we find net cooling rates below the gain radius that are typically ~0.8 times the MGFLD rates. These differences stem from differences in the neutrino luminosities and mean inverse flux factors, which can be as much as 11% and 24%, respectively. They are greatest at earlier postbounce times for a given progenitor mass and, for a given postbounce time, greater for greater progenitor mass. We discuss the ramifications that these new results have for the supernova mechanism.

We couple two-dimensional hydrodynamics to realistic one-dimensional multigroup flux-limited diffusion neutrino transport to investigate protoneutron star convection in core collapse supernovae, and more specifically, the interplay between its development and neutrino transport, for both 15 and 25 solar mass models. In the presence of neutrino transport, protoneutron star convection velocities are too small relative to bulk inflow velocities to result in any significant convective transport of entropy and leptons. A simple analytical model supports our numerical results, indicating that the inclusion of neutrino transport reduces the entropy-driven (lepton-driven) convection growth rates and asymptotic velocities by a factor of 3 (50) at the neutrinosphere and a factor 250 (1000) at a density of 10^{12} g/cm^{3}, for both our 15 and 25 solar mass models. Moreover, when transport is included, the initial postbounce entropy gradient is smoothed out by neutrino diffusion, whereas the initial lepton gradient is maintained by electron capture and neutrino escape near the neutrinosphere. Despite the maintenance of the lepton gradient, protoneutron star convection does not develop over the 100 ms duration typical of all our simulations, except in the instance where ``low-test'' initial conditions are used, which are generated by core collapse and bounce simulations that neglect neutrino-electron scattering and ion-ion screening corrections to neutrino- nucleus elastic scattering., 61 pages, 31 figures; accepted for publication in The Astrophysical Journal

We present four ab initio axisymmetric core-collapse supernova simulations for 12, 15, 20, and 25 $M_\odot$ progenitors. All of the simulations yield explosions and have been evolved for at least 1.2 seconds after core bounce and 1 second after material first becomes unbound. Simulations were computed with our Chimera code employing spectral neutrino transport, special and general relativistic transport effects, and state-of-the-art neutrino interactions. Continuing the evolution beyond 1 second allows explosions to develop more fully and the processes powering the explosions to become more clearly evident. We compute explosion energy estimates, including the binding energy of the stellar envelope outside the shock, of 0.34, 0.88, 0.38, and 0.70 B ($10^{51}$ ergs) and increasing at 0.03, 0.15, 0.19, and 0.52 B s$^{-1}$, respectively, for the 12, 15, 20, and 25 $M_\odot$ models. Three models developed pronounced prolate shock morphologies, while the 20 $M_\odot$ model, though exhibiting lobes and accretion streams like the other models, develops an approximately spherical, off-center shock as the explosion begins and then becomes moderately prolate $\sim$600 ms after bounce. This reduces the explosion energy relative to the other models by reducing mass accretion during the critical explosion power-up phase. We examine the growth of the explosion energy in our models through detailed analyses of the energy sources and flows. We find that the 12 and 20 $M_\odot$ models have explosion energies comparable to that of the lower range of observed explosion energies while the 15 and 25 $M_\odot$ models are within the range of observed explosion energies, particularly considering the rate at which their explosion energies are increasing. The ejected $^{56}$Ni masses given by our models are all within observational limits as are the proto-neutron star masses and kick velocities. (Truncated), Comment: 39 pages, 29 figures, accepted for publicatoin in The Astrophysical Journal, additional animations available at ChimeraSN.org

We extend our investigation of magnetic field evolution in three-dimensional flows driven by the stationary accretion shock instability (SASI) with a suite of higher-resolution idealized models of the post-bounce core-collapse supernova environment. Our magnetohydrodynamic simulations vary in initial magnetic field strength, rotation rate, and grid resolution. Vigorous SASI-driven turbulence inside the shock amplifies magnetic fields exponentially; but while the amplified fields reduce the kinetic energy of small-scale flows, they do not seem to affect the global shock dynamics. The growth rate and final magnitude of the magnetic energy are very sensitive to grid resolution, and both are underestimated by the simulations. Nevertheless our simulations suggest that neutron star magnetic fields exceeding $10^{14}$ G can result from dynamics driven by the SASI, \emph{even for non-rotating progenitors}., Comment: 28 pages, 17 figures, accepted for publication in the ApJ

We investigate prompt convection in core collapse supernovae and its consequences for the late-time shock evolution and supernova outcome. We examine the evolution of the core prior to the onset of convection and nd that the negative entropy gradients imprinted on the outer core by the weakening shock, which, along with the negative lepton gradient, drive the convection, are very sensitive to (1) the neutrino transport and microphysics included in the core collapse simulation and (2) the nuclear equation of state. We perform a number of detailed one-dimensional spherically symmetric simulations of prompt convection using a mixing length algorithm in a code coupling the core hydrodynamics with multigroup uxlimited di usion of neutrinos of all types. We nd that prompt convection does not have a signi cant e ect on the neutrino luminosities or spectra in the postshock region and, consequently, on the late-time postshock neutrino heating and shock evolution. Consequently, we do not nd that prompt convection is important for the supernova explosion mechanism. 2

Neutrino-electron scattering plays a major role in the deleptonization of the iron core during the gravitational collapse of a presupernova star and, hence, plays a major role in the success or failure of the shock ejection mechanism for Type II supernovae. In this paper we present the first simulation of realistic gravitational collapse in which neutrino-electron scattering is not aproximated in the neutrino transport equation with either a truncated Legendre series or a Fokker-Planck approximation. We begin with a 1.17 M ○. iron core extracted from a Nomoto-Hashimoto 13 Ma presupernova star

We present a numerical method to solve the neutrino Boltzmann equation coupled to stellar core collapse and lepton conservation, with no approximations made to the neutrino scattering kernels. Spherical symmetry is assumed. Our finite differencing of the Boltzmann equation is similar to its finite difference representation in the discrete ordinates method, but our auxiliary equations relating the zone-center and zone-edge distribution functions are different. We also differ from the discrete ordinates method in the way we solve the finite differenced Boltzmann equation

We present the results from a computer simulation of the gravitational collapse of a 1.17 M ○ . iron core extracted from a Nomoto-Hashimoto 13 M ○ . presupernova star. The results are obtained with a Newtonian gravity, O(v/c) Lagrangian hydrodynamics code coupled to an O(v/c) Boltzmann solver for the neutrino transport. We include electron capture on nuclei and free protons and electron neutrino absorption on nuclei and free neutrons. We also include conservative scattering of electron neutrinos on free protons and neutrons and conservative coherent scattering of electron-neutrinos on nuclei. We use the Baron-Cooperstein equation of state

The 3 + 1 Einstein equations (including a radiation component in the stress-energy tensor), the 3 + 1 equations for general relativistic hydrodynamics (including the radiation four-force density), and the 3 + 1 general relativistic Boltzmann equation, are all derived for time-dependent, spherically symmetric spacetimes. These equations are then specialized to maximally sliced spacetimes in the isotropic gauge. The implicit finite differencing of the Boltzmann equation and of the radiation contribution to the hydrodynamics equations is described, and a new implicit numerical scheme for the coupled collision term in the Boltzmann equation and the radiation four-force density in the hydrodynamics equations is presented. The results of a code test for this radiation code are given. 23 refs.