101 research outputs found
A new multidimensional, energy-dependent two-moment transport code for neutrino-hydrodynamics
We present the new code ALCAR developed to model multidimensional, multi
energy-group neutrino transport in the context of supernovae and neutron-star
mergers. The algorithm solves the evolution equations of the 0th- and 1st-order
angular moments of the specific intensity, supplemented by an algebraic
relation for the 2nd-moment tensor to close the system. The scheme takes into
account frame-dependent effects of order O(v/c) as well as the most important
types of neutrino interactions. The transport scheme is significantly more
efficient than a multidimensional solver of the Boltzmann equation, while it is
more accurate and consistent than the flux-limited diffusion method. The
finite-volume discretization of the essentially hyperbolic system of moment
equations employs methods well-known from hydrodynamics. For the time
integration of the potentially stiff moment equations we employ a scheme in
which only the local source terms are treated implicitly, while the advection
terms are kept explicit, thereby allowing for an efficient computational
parallelization of the algorithm. We investigate various problem setups in one
and two dimensions to verify the implementation and to test the quality of the
algebraic closure scheme. In our most detailed test, we compare a fully
dynamic, one-dimensional core-collapse simulation with two published
calculations performed with well-known Boltzmann-type neutrino-hydrodynamics
codes and we find very satisfactory agreement.Comment: 30 pages, 12 figures. Revised version: several additional comments
and explanations, results remain unchanged. Accepted for publication in MNRA
Three-Dimensional Core-Collapse Supernova Simulations with Multi-Dimensional Neutrino Transport Compared to the Ray-by-Ray-plus Approximation
Self-consistent, time-dependent supernova (SN) simulations in three spatial
dimensions (3D) are conducted with the Aenus-Alcar code, comparing, for the
first time, calculations with fully multi-dimensional (FMD) neutrino transport
and the ray-by-ray-plus (RbR+) approximation, both based on a two-moment solver
with algebraic M1 closure. We find good agreement between 3D results with FMD
and RbR+ transport for both tested grid resolutions in the cases of a 20
solar-mass progenitor, which does not explode with the employed simplified set
of neutrino opacities, and of an exploding 9 solar-mass model. This is in stark
contrast to corresponding axisymmetric (2D) simulations, which confirm previous
claims that the RbR+ approximation can foster explosions in 2D in particular in
models with powerful axial sloshing of the stalled shock due to the standing
accretion shock instability (SASI). However, while local and instantaneous
variations of neutrino fluxes and heating rates can still be considerably
higher with RbR+ transport in 3D, the time-averaged quantities are very similar
to FMD results because of the absence of a fixed, artificial symmetry axis that
channels the flow. Therefore, except for stochastic fluctuations, the neutrino
signals and the post-bounce evolution of 3D simulations with FMD and RbR+
transport are also very similar, in particular for our calculations with the
better grid resolution. Higher spatial resolution has clearly a more important
impact than the differences by the two transport treatments. Our results back
up the use of the RbR+ approximation for neutrino transport in 3D SN modeling.Comment: 25 pages, 16 figures; referee comments included, new appendix added;
accepted by Ap
Neutron-star merger ejecta as obstacles to neutrino-powered jets of gamma-ray bursts
We present the first special relativistic, axisymmetric hydrodynamic
simulations of black hole-torus systems (approximating general relativistic
gravity) as remnants of binary-neutron star (NS-NS) and neutron star-black hole
(NS-BH) mergers, in which the viscously driven evolution of the accretion torus
is followed with self-consistent energy-dependent neutrino transport and the
interaction with the cloud of dynamical ejecta expelled during the NS-NS
merging is taken into account. The modeled torus masses, BH masses and spins,
and the ejecta masses, velocities, and spatial distributions are adopted from
relativistic merger simulations. We find that energy deposition by neutrino
annihilation can accelerate outflows with initially high Lorentz factors along
polar low-density funnels, but only in mergers with extremely low baryon
pollution in the polar regions. NS-BH mergers, where polar mass ejection during
the merging phase is absent, provide sufficiently baryon-poor environments to
enable neutrino-powered, ultrarelativistic jets with terminal Lorentz factors
above 100 and considerable dynamical collimation, favoring short gamma-ray
bursts (sGRBs), although their typical energies and durations might be too
small to explain the majority of events. In the case of NS-NS mergers, however,
neutrino emission of the accreting and viscously spreading torus is too short
and too weak to yield enough energy for the outflows to break out from the
surrounding ejecta shell as highly relativistic jets. We conclude that neutrino
annihilation alone cannot power sGRBs from NS-NS mergers.Comment: 7 pages, 4 figures, minor revisions compared to original version,
accepted for publication in ApJ Letter
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