248 research outputs found
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
Spherically Symmetric Simulation with Boltzmann Neutrino Transport of Core Collapse and Post-Bounce Evolution of a 15 Solar Mass Star
We present a spherically symmetric, Newtonian core-collapse simulation of a
15 solar mass star with a 1.28 solar mass iron core. The time-, energy-, and
angle-dependent transport of electron neutrinos and antineutrinos was treated
with a new code which iteratively solves the Boltzmann equation and the
equations for neutrino number, energy and momentum to order O(v/c) in the
velocity v of the stellar medium. The supernova shock expands to a maximum
radius of 350 km instead of only about 240 km as in a comparable calculation
with multi-group flux-limited diffusion (MGFLD) by Bruenn, Mezzacappa, & Dineva
(1995). This may be explained by stronger neutrino heating due to the more
accurate transport in our model. Nevertheless, after 180 ms of expansion the
shock finally recedes to a radius around 250 km (compared to about 170 km in
the MGFLD run). The effect of an accurate neutrino transport is helpful, but
not large enough to cause an explosion of the considered 15 solar mass star.
Therefore postshock convection and/or an enhancement of the core neutrino
luminosity by convection or reduced neutrino opacities in the neutron star seem
necessary for neutrino-driven explosions of such stars. We find an electron
fraction Y_e > 0.5 in the neutrino-heated matter, which suggests that the
overproduction problem of neutron-rich nuclei with mass numbers around A = 90
in exploding models may be absent when a Boltzmann solver is used for the
electron neutrino and antineutrino transport.Comment: 6 pages, LaTex, 3 encapsulated postscript figures, revised and
shortened version. Astrophys. J., Letters, accepte
Differential Rotation in Neutron Stars: Magnetic Braking and Viscous Damping
Diffferentially rotating stars can support significantly more mass in
equilibrium than nonrotating or uniformly rotating stars, according to general
relativity. The remnant of a binary neutron star merger may give rise to such a
``hypermassive'' object. While such a star may be dynamically stable against
gravitational collapse and bar formation, the radial stabilization due to
differential rotation is likely to be temporary. Magnetic braking and viscosity
combine to drive the star to uniform rotation, even if the seed magnetic field
and the viscosity are small. This process inevitably leads to delayed collapse,
which will be accompanied by a delayed gravitational wave burst and, possibly,
a gamma-ray burst. We provide a simple, Newtonian, MHD calculation of the
braking of differential rotation by magnetic fields and viscosity. The star is
idealized as a differentially rotating, infinite cylinder consisting of a
homogeneous, incompressible conducting gas. We solve analytically the simplest
case in which the gas has no viscosity and the star resides in an exterior
vacuum. We treat numerically cases in which the gas has internal viscosity and
the star is embedded in an exterior, low-density, conducting medium. Our
evolution calculations are presented to stimulate more realistic MHD
simulations in full 3+1 general relativity. They serve to identify some of the
key physical and numerical parameters, scaling behavior and competing
timescales that characterize this important process.Comment: 11 pages. To appear in ApJ (November 20, 2000
General-Relativistic MHD for the Numerical Construction of Dynamical Spacetimes
We assemble the equations of general relativistic magnetohydrodynamics (MHD)
in 3+1 form. These consist of the complete coupled set of Maxwell equations for
the electromagnetic field, Einstein's equations for the gravitational field,
and the equations of relativistic MHD for a perfectly conducting ideal gas. The
adopted form of the equations is suitable for evolving numerically a
relativistic MHD fluid in a dynamical spacetime characterized by a strong
gravitational field.Comment: 8 pages; scheduled for March 10 issue of 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
About the detection of gravitational wave bursts
Several filtering methods for the detection of gravitational wave bursts in
interferometric detectors are presented. These are simple and fast methods
which can act as online triggers. All methods are compared to matched filtering
with the help of a figure of merit based on the detection of supernovae signals
simulated by Zwerger and Muller.Comment: 5 pages, proceedings of GWDAW99 (Roma, Dec. 1999), to appear in Int.
J. Mod. Phys.
The bar-mode instability in differentially rotating neutron stars: Simulations in full general relativity
We study the dynamical stability against bar-mode deformation of rapidly
spinning neutron stars with differential rotation. We perform fully
relativistic 3D simulations of compact stars with , where is
the total gravitational mass and the equatorial circumferential radius. We
adopt an adiabatic equation of state with adiabatic index . As in
Newtonian theory, we find that stars above a critical value of (where is the rotational kinetic energy and the gravitational
binding energy) are dynamically unstable to bar formation. For our adopted
choices of stellar compaction and rotation profile, the critical value of
is , only slightly smaller than the
well-known Newtonian value for incompressible Maclaurin spheroids.
The critical value depends only very weakly on the degree of differential
rotation for the moderate range we surveyed. All unstable stars form bars on a
dynamical timescale. Models with sufficiently large subsequently form
spiral arms and eject mass, driving the remnant to a dynamically stable state.
Models with moderately large do not develop spiral
arms or eject mass but adjust to form dynamically stable ellipsoidal-like
configurations. If the bar-mode instability is triggered in supernovae collapse
or binary neutron star mergers, it could be a strong and observable source of
gravitational waves. We determine characteristic wave amplitudes and
frequencies.Comment: 17 pages, accepted for publication in AP
Electron Neutrino Pair Annihilation: A New Source for Muon and Tau Neutrinos in Supernovae
We show that in a supernova core the annihilation process nu_e nu_e-bar ->
nu_{mu,tau} nu_{mu,tau}-bar is always more important than the traditional
reaction e^+ e^- -> nu_{mu,tau} nu_{mu,tau}-bar as a source for muon and tau
neutrino pairs. We study the impact of the new process by means of a Monte
Carlo transport code with a static stellar background model and by means of a
self-consistent hydrodynamical simulation with Boltzmann neutrino transport.
Nucleon bremsstrahlung NN -> NN nu_{mu,tau} nu_{mu,tau}-bar is also included as
another important source term. Taking into account nu_e nu_e-bar -> nu_{mu,tau}
nu_{mu,tau}-bar increases the nu_mu and nu_tau luminosities by as much as 20%
while the spectra remain almost unaffected. In our hydrodynamical simulation
the shock was somewhat weakened. Elastic nu_{mu,tau} nu_e and nu_{mu,tau} nu_e
scattering is not negligible but less important than nu_{mu,tau} e^+ or e^-
scattering. Its influence on the nu_{mu,tau} fluxes and spectra is small after
all other processes have been included.Comment: 11 pages, 9 eps-figs, submitted to Ap
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
- âŠ