251 research outputs found
Relativistic Outflows in Gamma-Ray Bursts
The possibility that gamma-ray bursts (GRBs) were not isotropic emissions was
devised theoretically as a way to ameliorate the huge energetic budget implied
by the standard fireball model for these powerful phenomena. However, the
mechanism by which after the quasy-isotropic release of a few erg
yields a collimated ejection of plasma could not be satisfactory explained
analytically. The reason being that the collimation of an outflow by its
progenitor system depends on a very complex and non-linear dynamics. That has
made necessary the use of numerical simulations in order to shed some light on
the viability of some likely progenitors of GRBs. In this contribution I will
review the most relevant features shown by these numerical simulations and how
they have been used to validate the collapsar model (for long GRBs) and the
model involving the merger of compact binaries (for short GRBs).Comment: 8 pages, 1 figure. Proceedings of the conference: "Circumstellar
Media and Late Stages of Massive Stellar Evolution". Ensenada (Mexico). To be
published by Revista Mexicana de Astronomia y Astrofisic
Linear stability analysis of the Hall magnetorotational instability in a spherical domain
We investigate the stability of the Hall-MHD system and determine its
importance for neutron stars at their birth, when they still consist of
differentially rotating plasma permeated by extremely strong magnetic fields.
We solve the linearised Hall-MHD equations in a spherical shell threaded by a
homogeneous magnetic field. With the fluid/flow coupling and the Hall effect
included, the magnetorotational instability and the Hall effect are both acting
together. Results differ for magnetic fields aligned with the rotation axis and
anti-parallel magnetic fields. For a positive alignment of the magnetic field
the instability grows on a rotational time-scale for any sufficiently large
magnetic Reynolds number. Even the magnetic fields which are stable against the
MRI due to the magnetic diffusion are now susceptible to the shear-Hall
instability. In contrast, the negative alignment places strong restrictions on
the growth and the magnitude of the fields, hindering the effectiveness of the
Hall-MRI. While non-axisymmetric modes of the MRI can be suppressed by strong
enough rotation, there is no such restriction when the Hall effect is present.
The implications for the magnitude and the topology of the magnetic field of a
young neutron star may be significant.Comment: 7 pages, 10 figures, Astron. Nachr. 333, 202 (2012
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
Energetic particle acceleration and transport by Alfven/acoustic waves in tokamak-like Solar flares
Alfv´en/acoustic waves are ubiquitous in astrophysical as well as in laboratory plasmas. Their interplay with energetic ions is of crucial importance to understanding the energy
and particle exchange in astrophysical plasmas as well as to obtaining a viable energy source
in magnetically confined fusion devices. In magnetically confined fusion plasmas, an experimental phase-space characterisation of convective and diffusive energetic particle losses induced by Alfv´en/acoustic waves allows for a better understanding of the underlying physics. The relevance of these results in the problem of the anomalous heating of the solar corona is checked by MHD simulations of Tokamak-like Solar flare tubes
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