Eric Stephen Myra

Neutrinos from type II supernovae - The first 100 milliseconds

The collapse of a 1.17 solar mass iron core is numerically followed through infall to 100 ms past core bounce, and the emergent neutrino spectra during each phase are highlighted. It is found that, even with fairly optimistic conditions for producing a strong, sustained core-bounce shock wave, the prompt shock stalls within 9 ms of core bounce at a radius of less than 250 km. It appears that a radical change in the character of the progenitor core or in our understanding of the relevant physics of stellar collapse is needed before the direct mechanism for type II supernovae can become viable. Expanding the number of neutrino types from one to six magnifies the debilitating effect of neutrino loss on shock propagation. At shock breakout, prompt bursts of all neutrino types are observed. The luminosities of the nonelectron types show a sudden turn-on in luminosity while that of the electron neutrinos steadily increases throughout infall as a result of accelerating electron capture.

A Numerical Algorithm for Modeling Multigroup Neutrino-Radiation Hydrodynamics in Two Spatial Dimensions

It is now generally agreed that multidimensional, multigroup, neutrino-radiation hydrodynamics (RHD) is an indispensable element of any realistic model of stellar-core collapse, core-collapse supernovae, and proto-neutron star instabilities. We have developed a new, two-dimensional, multigroup algorithm that can model neutrino-RHD flows in core-collapse supernovae. Our algorithm uses an approach similar to the ZEUS family of algorithms, originally developed by Stone and Norman. However, this completely new implementation extends that previous work in three significant ways: first, we incorporate multispecies, multigroup RHD in a flux-limited-diffusion approximation. Our approach is capable of modeling pair-coupled neutrino-RHD, and includes effects of Pauli blocking in the collision integrals. Blocking gives rise to nonlinearities in the discretized radiation-transport equations, which we evolve implicitly in time. We employ parallelized Newton-Krylov methods to obtain a solution of these nonlinear, implicit equations. Our second major extension to the ZEUS algorithm is the inclusion of an electron conservation equation that describes the evolution of electron-number density in the hydrodynamic flow. This permits calculating deleptonization of a stellar core. Our third extension modifies the hydrodynamics algorithm to accommodate realistic, complex equations of state, including those having nonconvex behavior. In this paper, we present a description of our complete algorithm, giving sufficient details to allow others to implement, reproduce, and extend our work. Finite-differencing details are presented in appendices. We also discuss implementation of this algorithm on state-of-the-art, parallel-computing architectures. Finally, we present results of verification tests that demonstrate the numerical accuracy of this algorithm on diverse hydrodynamic, gravitational, radiation-transport, and RHD sample problems. We believe our methods to be of general use in a variety of model settings where radiation transport or RHD is important. Extension of this work to three spatial dimensions is straightforward.

Multigroup models of the convective epoch in core collapse supernovae

Understanding the explosion mechanism of core collapse supernovae is a problem that has plagued nuclear astrophysicists since the first computational models of this phenomenon were carried out in the 1960s. Our current theories of this violent phenomenon center around multi-dimensional effects involving radiation-hydrodynamic flows of hot, dense matter and neutrinos. Modeling these multi-dimensional radiative flows presents a computational challenge that will continue to stress high-performance computing beyond the teraflops to the petaflop level. In this paper we describe a few of the scientific discoveries that we have made via terascale computational simulations of supernovae under the auspices of the SciDAC-funded Terascale Supernova Initiative.

Neutrino transport in stellar collapse

Abstract A multigroup flux-limited diffusion approximation to the equations of neutrino transport is outlined. Because the approximation requires no assumptions about the functional form of the neutrino distribution, it enables detailed tracing of the evolution of neutrinos when applied, as it is here, as part of a description of a collapsing stellar core. Presented are some results of such collapse calculations made using this method. They show that during the crucial period of lepton loss from the system, the neutrinos are not accurately modelled by a thermal distribution.

ADVANCES IN MULTI-DIMENSIONAL SIMULATION OF CORE-COLLAPSE SUPERNOVAE

We discuss recent advances in the radiative-hydrodynamic modeling of core collapse supernovae in multi-dimensions. A number of earlier attempts at fully radiation-hydrodynamic models utilized either the grey approximation to describe the neutrino distribution or utilized more sophisticated multigroup transport methods restricted to radial rays. In both cases these models have also neglected the O(v/c) terms that couple the radiation and matter strongly in the optically thick regions of the collapsed core. In this paper we present some recent advances that resolve some shortcomings of earlier models.

Spectral shifts near compact objects

We show that radiation emitted from material freely falling toward a black hole or neutron star cannot be blue-shifted as recently claimed by Cohen and Struble. The relativistic corrections to the classical apparent limb angle are given explicitly for spherical sources in collapse.