Stephen W. Bruenn

Stellar core collapse: Numerical model and infall epoch

A numerical model based on hydrodynamics coupled to radiation transport of all neutrino types is developed for calculating stellar core collapse. General relativistic hydrodynamic equations for spherically symmetric systems including neutrino flow are obtained and presented in a form paralleling the adiabatic equations of May and White (1967). A multigroup, flux-limited diffusion scheme, used to evolve ..nu../sub e/s, nu-bar/sub e/s, and ..nu../sub ..mu../ and ..nu../sub tau/ pairs independently, is derived from the neutrino Boltzmann equation. Expressions for the zeroth and first Legendre moments of all important neutrino interactions are derived from the standard model of electroweak interactions. Numerical values are obtained from these expressions (by numerical integration when necessary) during core-collapse calculations. An implicit numerical scheme for directly solving the neutrino equations, including the neutrino-electron scattering and neutrino thermal production terms, is presented. This scheme includes a consistent treatment of neutrino-matter decoupling in the neutrino transparent regimes. At the present time, the equation of state of Lamb et al. (1978, 1981) up to nuclear density, and that of Friedman and Pandharipande (1981) above nucleon density, are incorporated in the numerical model.

Probing the gravitational well: No supernova explosion in spherical symmetry with general relativistic Boltzmann neutrino transport

We report on the stellar core collapse, bounce, and postbounce evolution of a 13 M{sub 0} star in a self-consistent general relativistic spherically symmetric simulation based on Boltzmann neutrino transport. We conclude that approximations to exact neutrino transport and the omission of general relativistic effects were not alone responsible for the failure of numerous preceding attempts to model supernova explosions in spherical symmetry. Compared to simulations in Newtonian gravity, the general relativistic simulation results in a smaller shock radius. We however argue that the higher neutrino luminosities and rms energies in the general relativistic case could lead to a larger supernova explosion energy.

Simulation of the Spherically Symmetric Stellar Core Collapse, Bounce, and Postbounce Evolution of a Star of 13 Solar Masses with Boltzmann Neutrino Transport, and Its Implications for the Supernova Mechanism

With exact three-flavor Boltzmann neutrino transport, we simulate the stellar core collapse, bounce, and postbounce evolution of a 13M star in spherical symmetry, the Newtonian limit, without invoking convection. In the absence of convection, prior spherically symmetric models, which implemented approximations to Boltzmann transport, failed to produce explosions. We consider exact transport to determine if these failures were due to the transport approximations made and to answer remaining fundamental questions in supernova theory. The model presented here is the first in a sequence of models beginning with different progenitors. In this model, a supernova explosion is not obtained.

A Finite Difference Representation of Neutrino Radiation Hydrodynamics in Spherically Symmetric General Relativistic Spacetime

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. A comoving frame in the momentum phase space facilitates the evaluation and tabulation of neutrino-matter interaction cross sections but produces a multitude of observer corrections in the transport equation. 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. Half a second 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 μ- and τ-neutrino luminosities and rms energies, however, continue to rise because previously shock-heated material with a nondegenerate 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. Adhering to our tradition, we compare the evolution of the 13 M⊙ progenitor star to corresponding simulations with the multigroup flux-limited diffusion 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.

A numerical method for solving the neutrino Boltzmann equation coupled to spherically symmetric stellar core collapse

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

Stellar core collapse : a Boltzmann treatment of neutrino-electron scattering

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

Type II supernovae and Boltzmann neutrino transport: The Infall phase

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 development of explosions in axisymmetric ab initio core-collapse supernova simulations of 12–25 M⊙ stars

We present four ab initio axisymmetric core-collapse supernova simulations for 12, 15, 20, and 25

Three dimensional core-collapse supernova simulated using a 15 M⊙ progenitor

M_\odot

Neutrino - nucleus interactions in core collapse supernovae

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 (