331 research outputs found
Two-Dimensional Hydrodynamic Core-Collapse Supernova Simulations with Spectral Neutrino Transport II. Models for Different Progenitor Stars
1D and 2D supernova simulations for stars between 11 and 25 solar masses are
presented, making use of the Prometheus/Vertex neutrino-hydrodynamics code,
which employs a full spectral treatment of the neutrino transport.
Multi-dimensional transport aspects are treated by the ``ray-by-ray plus''
approximation described in Paper I. Our set of models includes a 2D calculation
for a 15 solar mass star whose iron core is assumed to rotate rigidly with an
angular frequency of 0.5 rad/s before collapse. No important differences were
found depending on whether random seed perturbations for triggering convection
are included already during core collapse, or whether they are imposed on a 1D
collapse model shortly after bounce. Convection below the neutrinosphere sets
in about 40 ms p.b. at a density above 10**12 g/cm^3 in all 2D models, and
encompasses a layer of growing mass as time goes on. It leads to a more
extended proto-neutron star structure with accelerated lepton number and energy
loss and significantly higher muon and tau neutrino luminosities, but reduced
mean energies of the radiated neutrinos, at times later than ~100 ms p.b. In
case of an 11.2 solar mass star we find that low (l = 1,2) convective modes
cause a probably rather weak explosion by the convectively supported
neutrino-heating mechanism after ~150 ms p.b. when the 2D simulation is
performed with a full 180 degree grid, whereas the same simulation with 90
degree wedge fails to explode like all other models. This sensitivity
demonstrates the proximity of our 2D models to the borderline between success
and failure, and stresses the need of simulations in 3D, ultimately without the
axis singularity of a polar grid. (abridged)Comment: 42 pages, 44 figures; revised according to referee comments; accepted
to Astronomy & Astrophysic
Core-collapse supernova simulations: Variations of the input physics
Spherically symmetric simulations of stellar core collapse and post-bounce
evolution are used to test the sensitivity of the supernova dynamics to
different variations of the input physics. We consider a state-of-the-art
description of the neutrino-nucleon interactions, possible lepton-number
changing neutrino reactions in the neutron star, and the potential impact of
hydrodynamic mixing behind the supernova shock.Comment: 6 pages, 6 ps figures (in color), to appear in W. Hillebrandt and E.
Mueller, eds., Proceedings of the 11th Workshop on "Nuclear Astrophysics"
held at Ringberg Castle, February 11-16, 200
Core Collapse and Then? The Route to Massive Star Explosions
The rapidly growing base of observational data for supernova explosions of
massive stars demands theoretical explanations. Central of these is a
self-consistent model for the physical mechanism that provides the energy to
start and drive the disruption of the star. We give arguments why the delayed
neutrino-heating mechanism should still be regarded as the standard paradigm to
explain most explosions of massive stars and show how large-scale and even
global asymmetries can result as a natural consequence of convective overturn
in the neutrino-heating region behind the supernova shock. Since the explosion
is a threshold phenomenon and depends sensitively on the efficiency of the
energy transfer by neutrinos, even relatively minor differences in numerical
simulations can matter on the secular timescale of the delayed mechanism. To
enhance this point, we present some results of recent one- and two-dimensional
computations, which we have performed with a Boltzmann solver for the neutrino
transport and a state-of-the-art description of neutrino-matter interactions.
Although our most complete models fail to explode, the simulations demonstrate
that one is encouragingly close to the critical threshold because a modest
variation of the neutrino transport in combination with postshock convection
leads to a weak neutrino-driven explosion with properties that fulfill
important requirements from observations.Comment: 14 pages; 3 figures. Invited Review, in: ``From Twilight to
Highlight: The Physics of Supernovae'', Eds. W. Hillebrandt and B.
Leibundgut, Springer Series ``ESO Astrophysics Symposia'', Berli
Supernova neutrinos: Flavor-dependent fluxes and spectra
Transporting nu_mu and nu_tau in a supernova (SN) core involves several
processes that have been neglected in traditional simulations. Based on a Monte
Carlo study we find that the flavor-dependent spectral differences are much
smaller than is often stated in the literature. A full-scale SN simulation
using a Boltzmann solver and including all relevant neutrino reactions confirms
these results. The flavor-dependent flux differences are largest during the
initial accretion phase.Comment: Proceedings NOON 03, Kanazawa, 10-14 Feb 200
Magnetic Braking and Viscous Damping of Differential Rotation in Cylindrical Stars
Differential rotation in stars generates toroidal magnetic fields whenever an
initial seed poloidal field is present. The resulting magnetic stresses, along
with viscosity, drive the star toward uniform rotation. This magnetic braking
has important dynamical consequences in many astrophysical contexts. For
example, merging binary neutron stars can form "hypermassive" remnants
supported against collapse by differential rotation. The removal of this
support by magnetic braking induces radial fluid motion, which can lead to
delayed collapse of the remnant to a black hole. We explore the effects of
magnetic braking and viscosity on the structure of a differentially rotating,
compressible star, generalizing our earlier calculations for incompressible
configurations. The star is idealized as a differentially rotating, infinite
cylinder supported initially by a polytropic equation of state. The gas is
assumed to be infinitely conducting and our calculations are performed in
Newtonian gravitation. Though highly idealized, our model allows for the
incorporation of magnetic fields, viscosity, compressibility, and shocks with
minimal computational resources in a 1+1 dimensional Lagrangian MHD code. Our
evolution calculations show that magnetic braking can lead to significant
structural changes in a star, including quasistatic contraction of the core and
ejection of matter in the outermost regions to form a wind or an ambient disk.
These calculations serve as a prelude and a guide to more realistic MHD
simulations in full 3+1 general relativity.Comment: 20 pages, 19 figures, 3 tables, AASTeX, accepted by Ap
Core-Collapse Supernovae: Modeling between Pragmatism and Perfectionism
We briefly summarize recent efforts in Garching for modeling stellar core
collapse and post-bounce evolution in one and two dimensions. The transport of
neutrinos of all flavors is treated by iteratively solving the coupled system
of frequency-dependent moment equations together with a model Boltzmann
equation which provides the closure. A variety of progenitor stars, different
nuclear equations of state, stellar rotation, and global asymmetries due to
large-mode hydrodynamic instabilities have been investigated to ascertain the
road to finally successful, convectively supported neutrino-driven explosions.Comment: 8 pages, contribution to Procs. 12th Workshop on Nuclear
Astrophysics, Ringberg Castle, March 22-27, 200
Supernova Simulations with Boltzmann Neutrino Transport: A Comparison of Methods
Accurate neutrino transport has been built into spherically symmetric
simulations of stellar core collapse and postbounce evolution. The results of
such simulations agree that spherically symmetric models with standard
microphysical input fail to explode by the delayed, neutrino-driven mechanism.
Independent groups implemented fundamentally different numerical methods to
tackle the Boltzmann neutrino transport equation. Here we present a direct and
detailed comparison of such neutrino radiation-hydrodynamical simulations for
two codes, Agile-Boltztran of the Oak Ridge-Basel group and Vertex of the
Garching group. The former solves the Boltzmann equation directly by an
implicit, general relativistic discrete angle method on the adaptive grid of a
conservative implicit hydrodynamics code with second-order TVD advection. In
contrast, the latter couples a variable Eddington factor technique with an
explicit, moving-grid, conservative high-order Riemann solver with important
relativistic effects treated by an effective gravitational potential. The
presented study is meant to test both neutrino radiation-hydrodynamics
implementations and to provide a data basis for comparisons and verifications
of supernova codes to be developed in the future. Results are discussed for
simulations of the core collapse and post-bounce evolution of a 13 solar mass
star with Newtonian gravity and a 15 solar mass star with relativistic gravity.Comment: 23 pages, 13 figures, revised version, to appear in Ap
Improved Models of Stellar Core Collapse and Still no Explosions: What is Missing?
Two-dimensional hydrodynamic simulations of stellar core-collapse with and
without rotation are presented which for the first time were performed by
solving the Boltzmann equation for the neutrino transport including a
state-of-the-art description of neutrino interactions. Although convection
develops below the neutrinosphere and in the neutrino-heated region behind the
supernova shock, the models do not explode. This suggests missing physics,
possibly with respect to the nuclear equation of state and weak interactions in
the subnuclear regime. However, it might also indicate a fundamental problem of
the neutrino-driven explosion mechanism.Comment: PRL submitted; 3 eps figures, 1 colored, high-quality available upon
reques
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