O.E.B. Messer

Electron capture rates on nuclei and implications for stellar core collapse

Supernova simulations to date have assumed that during core collapse electron captures occur dominantly on free protons, while captures on heavy nuclei are Pauli blocked and are ignored. We have calculated rates for electron capture on nuclei with mass numbers A=65-112 for the temperatures and densities appropriate for core collapse. We find that these rates are large enough so that, in contrast to previous assumptions, electron capture on nuclei dominates over capture on free protons. This leads to significant changes in core collapse simulations.

A Comparison of Boltzmann and Multigroup Flux-limited Diffusion Neutrino Transport during the Postbounce Shock Reheating Phase in Core-Collapse Supernovae

We compare Newtonian three-flavor multigroup Boltzmann (MGBT) and (Bruenns) 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.

Neutrino Transport in Core Collapse Supernovae

Neutrino production, transport, and interaction is arguably the single-most important component of a core collapse supernova model. Neutrinos are believed to be responsible for powering these supernovae, in part or entirely, and their production and transport set the stage for the radiation magnetohydrodynamics of stellar core collapse and bounce, which provides the initial conditions for the post-stellar-core-bounce dynamics. Neutrino transport is governed by multidimensional, phase-space, integro-partial differential kinetic equations. The solution of these equations dominates the computational challenge in simulating this supernova class. We present the neutrino transport and neutrino radiation hydrodynamics equations involved, and their finite differencing, and briefly discuss their solution. We use the spherically symmetric (spatially one-dimensional) case to illustrate the equations and the issues involved, but give the general formalism for the spatially multidimensional case as well. We conclude by briefly discussing the implications of the now experimentally measured nonzero neutrino masses.

Near Real-time Data Analysis of Core-Collapse Supernova Simulations With Bellerophon

Abstract We present an overview of a software system, Bellerophon, built to support a production-level HPC application called CHIMERA, which simulates core-collapse supernova events at the petascale. Developed over the last four years, Bellerophon enables CHIMERAs geographically dispersed team of collaborators to perform data analysis in near real-time. Its n-tier architecture provides an encapsulated, end-to-end software solution that enables the CHIMERA team to quickly and easily access highly customizable animated and static views of results from anywhere in the world via a web- deliverable, cross-platform desktop application. In addition, Bellerophon addresses software engineering tasks for the CHIMERA team by providing an automated mechanism for performing regression testing on a variety of supercomputing platforms. Elements of the teams workflow management needs are met with software tools that dynamically generate code repository statistics, access important online resources, and monitor the current status of several supercomputing resources.

The Multi-Dimensional Character of Core-Collapse Supernovae

Core-collapse supernovae, the culmination of massive stellar evolution, are spectacular astronomical events and the principle actors in the story of our elemental origins. Our understanding of these events, while still incomplete, centers around a neutrino-driven central engine that is highly hydrodynamically unstable. Increasingly sophisticated simulations reveal a shock that stalls for hundreds of milliseconds before reviving. Though brought back to life by neutrino heating, the development of the supernova explosion is inextricably linked to multi-dimensional fluid flows. In this paper, the outcomes of three-dimensional simulations that include sophisticated nuclear physics and spectral neutrino transport are juxtaposed to learn about the nature of the three dimensional fluid flow that shapes the explosion. Comparison is also made between the results of simulations in spherical symmetry from several groups, to give ourselves confidence in the understanding derived from this juxtaposition.

A Multitier System for the Verification, Visualization and Management of CHIMERA

Abstract CHIMERA is a multi-dimensional radiation hydrodynamics code designed to study core-collapse supernovae. The code is made up of three essentially independent parts: a hydrodynamics module, a nuclear burning module, and a neutrino transport solver combined within an operator-split approach. Given CHIMERAs complexity and pace of ongoing development, a new support system, Bellerophon, has been designed and implemented to perform automated verification, visualization and management tasks while integrating with other workflow systems utilized by CHIMERAs development group. In order to achieve these goals, a multitier approach has been adopted. By integrating supercomputing platforms, visualization clusters, a dedicated web server and a client-side desktop application, this system attempts to provide an encapsulated, end-to-end solution to these needs.

Advancements in Modeling Self-Consistent Core-Collapse Supernovae with CHIMERA

We discuss advancements in modeling core-collapse supernovae with our code CHIMERA. We describe the status and details of our tracer particle method and its uses for post-processing nucleosynthesis and as a tool for broad core-collapse supernova (CCSN) model analyses. We also introduce our progress towards expanding a nuclear reaction network beyond the alphanetwork for the purpose of accurate in-situ nucleosynthesis, not only for core-collapse supernovae (CCSNe) in general, but for a special sub-class of supernovae called electron capture supernovae (ECSNe), which stem from progenitors stars between 8 and 10 M⊙. By using an advanced nuclear reaction network, our 2D and 3D code will allow for unparalleled studies of CCSNe and ECSNe ejecta.

Ash-detonation interactions in multi-dimensional simulation of Type Ia supernovae

Thermonuclear supernovae (Type Ia SNe) are believed to result from the complete disruption of a near-Chandrasekhar-mass white dwarf (WD) by explosive thermonuclear runaway. Their use as cosmological distance indicators as well as comprehension of their role in galactic chemical evolution ultimately depends on a reliable and robust understanding of the explosion mechanism. We have undertaken a study of turbulent thermonuclear combustion in these events, focusing on the effects of increased physical fidelity in simulations, through improved resolution and increased dimensionality. We begin with a series of detonation simulations at low WD densities that we will use for baseline metrics against which subsequent simulations will be measured. We have performed these simulations using a version of the FLASH code[1]. We show progress on a set of simulations in one, two, and three spatial dimensions designed to explore the impact of dimensionality on the evolution of the detonations, and thereby clearly delineate the role of the nuclear kinetics. By performing a suite of simulations incorporating an alpha network at several levels of maximum mesh refinement, we will be able to quantify the effects of resolution and network size on future results.

General relativistic simulations of stellar core collapse and postbounce evolution with Boltzmann neutrino transport

We present self-consistent general relativistic simulations of stellar core collapse, bounce, and postbounce evolution for 13, 15, and 20 solar mass progenitors in spherical symmetry. Our simulations implement three-flavor Boltzmann neutrino transport and standard nuclear physics. The results are compared to our corresponding simulations with Newtonian hydrodynamics and O(v/c) Boltzmann transport.