147 research outputs found
Neutrino processes in partially degenerate neutron matter
We investigate neutrino processes for conditions reached in simulations of
core-collapse supernovae. Where neutrino-matter interactions play an important
role, matter is partially degenerate, and we extend earlier work that addressed
the degenerate regime. We derive expressions for the spin structure factor in
neutron matter, which is a key quantity required for evaluating rates of
neutrino processes. We show that, for essentially all conditions encountered in
the post-bounce phase of core-collapse supernovae, it is a very good
approximation to calculate the spin relaxation rates in the nondegenerate
limit. We calculate spin relaxation rates based on chiral effective field
theory interactions and find that they are typically a factor of two smaller
than those obtained using the standard one-pion-exchange interaction alone.Comment: 41 pages, 9 figures, NORDITA-2011-116; added comparison figures and
fit function for use in simulations, to appear in Astrophys.
Gravitational waves from supernova matter
We have performed a set of 11 three-dimensional magnetohydrodynamical core
collapse supernova simulations in order to investigate the dependencies of the
gravitational wave signal on the progenitor's initial conditions. We study the
effects of the initial central angular velocity and different variants of
neutrino transport. Our models are started up from a 15 solar mass progenitor
and incorporate an effective general relativistic gravitational potential and a
finite temperature nuclear equation of state. Furthermore, the electron flavour
neutrino transport is tracked by efficient algorithms for the radiative
transfer of massless fermions. We find that non- and slowly rotating models
show gravitational wave emission due to prompt- and lepton driven convection
that reveals details about the hydrodynamical state of the fluid inside the
protoneutron stars. Furthermore we show that protoneutron stars can become
dynamically unstable to rotational instabilities at T/|W| values as low as ~2 %
at core bounce. We point out that the inclusion of deleptonization during the
postbounce phase is very important for the quantitative GW prediction, as it
enhances the absolute values of the gravitational wave trains up to a factor of
ten with respect to a lepton-conserving treatment.Comment: 10 pages, 6 figures, accepted, to be published in a Classical and
Quantum Gravity special issue for MICRA200
Exploring the relativistic regime with Newtonian hydrodynamics: An improved effective gravitational potential for supernova simulations
We investigate the possibility to approximate relativistic effects in
hydrodynamical simulations of stellar core collapse and post-bounce evolution
by using a modified gravitational potential in an otherwise standard Newtonian
hydrodynamic code. Different modifications of the effective relativistic
potential introduced by Rampp & Janka (2002) are discussed. Corresponding
hydrostatic solutions are compared with solutions of the TOV equations, and
hydrodynamic simulations with two different codes are compared with fully
relativistic results. One code is applied for one- and two-dimensional
calculations with a simple equation of state, and employs either the modified
effective relativistic potential in a Newtonian framework or solves the general
relativistic field equations under the assumption of the conformal flatness
condition (CFC) for the three-metric. The second code allows for full-scale
supernova runs including a microphysical equation of state and neutrino
transport based on the solution of the Boltzmann equation and its moments
equations. We present prescriptions for the effective relativistic potential
for self-gravitating fluids to be used in Newtonian codes, which produce
excellent agreement with fully relativistic solutions in spherical symmetry,
leading to significant improvements compared to previously published
approximations. Moreover, they also approximate qualitatively well relativistic
solutions for models with rotation.Comment: 20 pages, 13 figures; corrected minor inaccuracies and added two
subsection
Conservative formulations of general relativistic kinetic theory
Experience with core-collapse supernova simulations shows that accurate
accounting of total particle number and 4-momentum can be a challenge for
computational radiative transfer. This accurate accounting would be facilitated
by the use of particle number and 4-momentum transport equations that allow
transparent conversion between volume and surface integrals in both
configuration and momentum space. Such conservative formulations of general
relativistic kinetic theory in multiple spatial dimensions are presented in
this paper, and their relevance to core-collapse supernova simulations is
described.Comment: 48 page
The Neutrino Signal in Stellar Core Collapse and Postbounce Evolution
General relativistic multi-group and multi-flavor Boltzmann neutrino
transport in spherical symmetry adds a new level of detail to the numerical
bridge between microscopic nuclear and weak interaction physics and the
macroscopic evolution of the astrophysical object. Although no supernova
explosions are obtained, we investigate the neutrino luminosities in various
phases of the postbounce evolution for a wide range of progenitor stars between
13 and 40 solar masses. The signal probes the dynamics of material layered in
and around the protoneutron star and is, within narrow limits, sensitive to
improvements in the weak interaction physics. Only changes that dramatically
exceed physical limitations allow experiments with exploding models. We discuss
the differences in the neutrino signal and find the electron fraction in the
innermost ejecta to exceed 0.5 as a consequence of thermal balance and weak
equilibrium at the masscut.Comment: 8 pages, 4 figures. Proceedings of the Nuclear Physics in
Astrophysics Conference, Debrecen, Hungary, 2002, to appear in Nuc. Phys. A.
Color figures added and reference actualize
2D and 3D Core-Collapse Supernovae Simulation Results Obtained with the CHIMERA Code
Much progress in realistic modeling of core-collapse supernovae has occurred
recently through the availability of multi-teraflop machines and the increasing
sophistication of supernova codes. These improvements are enabling simulations
with enough realism that the explosion mechanism, long a mystery, may soon be
delineated. We briefly describe the CHIMERA code, a supernova code we have
developed to simulate core-collapse supernovae in 1, 2, and 3 spatial
dimensions. We then describe the results of an ongoing suite of 2D simulations
initiated from a 12, 15, 20, and 25 solar mass progenitor. These have all
exhibited explosions and are currently in the expanding phase with the shock at
between 5,000 and 20,000 km. We also briefly describe an ongoing simulation in
3 spatial dimensions initiated from the 15 solar mass progenitor.Comment: 5 pages, 3 figure
A New Open-Source Code for Spherically-Symmetric Stellar Collapse to Neutron Stars and Black Holes
We present the new open-source spherically-symmetric general-relativistic
(GR) hydrodynamics code GR1D. It is based on the Eulerian formulation of GR
hydrodynamics (GRHD) put forth by Romero-Ibanez-Gourgoulhon and employs
radial-gauge, polar-slicing coordinates in which the 3+1 equations simplify
substantially. We discretize the GRHD equations with a finite-volume scheme,
employing piecewise-parabolic reconstruction and an approximate Riemann solver.
GR1D is intended for the simulation of stellar collapse to neutron stars and
black holes and will also serve as a testbed for modeling technology to be
incorporated in multi-D GR codes. Its GRHD part is coupled to various
finite-temperature microphysical equations of state in tabulated form that we
make available with GR1D. An approximate deleptonization scheme for the
collapse phase and a neutrino-leakage/heating scheme for the postbounce epoch
are included and described. We also derive the equations for effective rotation
in 1D and implement them in GR1D. We present an array of standard test
calculations and also show how simple analytic equations of state in
combination with presupernova models from stellar evolutionary calculations can
be used to study qualitative aspects of black hole formation in failing
rotating core-collapse supernovae. In addition, we present a simulation with
microphysical EOS and neutrino leakage/heating of a failing core-collapse
supernova and black hole formation in a presupernova model of a 40 solar mass
zero-age main-sequence star. We find good agreement on the time of black hole
formation (within 20%) and last stable protoneutron star mass (within 10%) with
predictions from simulations with full Boltzmann neutrino radiation
hydrodynamics.Comment: 25 pages, 6 figures, 2 appendices. Accepted for publication to the
Classical and Quantum Gravity special issue for MICRA2009. Code may be
downloaded from http://www.stellarcollapse.org Update: corrected title, small
modifications suggested by the referees, added source term derivation in
appendix
CASTRO: A New Compressible Astrophysical Solver. III. Multigroup Radiation Hydrodynamics
We present a formulation for multigroup radiation hydrodynamics that is
correct to order using the comoving-frame approach and the
flux-limited diffusion approximation. We describe a numerical algorithm for
solving the system, implemented in the compressible astrophysics code, CASTRO.
CASTRO uses an Eulerian grid with block-structured adaptive mesh refinement
based on a nested hierarchy of logically-rectangular variable-sized grids with
simultaneous refinement in both space and time. In our multigroup radiation
solver, the system is split into three parts, one part that couples the
radiation and fluid in a hyperbolic subsystem, another part that advects the
radiation in frequency space, and a parabolic part that evolves radiation
diffusion and source-sink terms. The hyperbolic subsystem and the frequency
space advection are solved explicitly with high-order Godunov schemes, whereas
the parabolic part is solved implicitly with a first-order backward Euler
method. Our multigroup radiation solver works for both neutrino and photon
radiation.Comment: accepted by ApJS, 27 pages, 20 figures, high-resolution version
available at https://ccse.lbl.gov/Publications/wqzhang/castro3.pd
The consequences of nuclear electron capture in core collapse supernovae
The most important weak nuclear interaction to the dynamics of stellar core
collapse is electron capture, primarily on nuclei with masses larger than 60.
In prior simulations of core collapse, electron capture on these nuclei has
been treated in a highly parameterized fashion, if not ignored. With realistic
treatment of electron capture on heavy nuclei come significant changes in the
hydrodynamics of core collapse and bounce. We discuss these as well as the
ramifications for the post-bounce evolution in core collapse supernovae.Comment: Accepted by PRL, 5 pages, 2 figure
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