1,150 research outputs found
Neutrino-driven wind and wind termination shock in supernova cores
The neutrino-driven wind from a nascent neutron star at the center of a
supernova expands into the earlier ejecta of the explosion. Upon collision with
this slower matter the wind material is decelerated in a wind termination
shock. By means of hydrodynamic simulations in spherical symmetry we
demonstrate that this can lead to a large increase of the wind entropy,
density, and temperature, and to a strong deceleration of the wind expansion.
The consequences of this phenomenon for the possible r-process nucleosynthesis
in the late wind still need to be explored in detail. Two-dimensional models
show that the wind-ejecta collision is highly anisotropic and could lead to a
directional dependence of the nucleosynthesis even if the neutrino-driven wind
itself is spherically symmetric.Comment: 6 pages, 3 figures, International Symposium on Nuclear Astrophysics -
Nuclei in the Cosmos - IX, CERN, Geneva, Switzerland, 25-30 June, 200
Testing Approximations of Thermal Effects in Neutron Star Merger Simulations
We perform three-dimensional relativistic hydrodynamical calculations of
neutron star mergers to assess the reliability of an approximate treatment of
thermal effects in such simulations by combining an ideal-gas component with
zero-temperature, micro-physical equations of state. To this end we compare the
results of simulations that make this approximation to the outcome of models
with a consistent treatment of thermal effects in the equation of state. In
particular we focus on the implications for observable consequences of merger
events like the gravitational-wave signal. It is found that the characteristic
gravitational-wave oscillation frequencies of the post-merger remnant differ by
about 50 to 250 Hz (corresponding to frequency shifts of 2 to 8 per cent)
depending on the equation of state and the choice of the characteristic index
of the ideal-gas component. In addition, the delay time to black hole collapse
of the merger remnant as well as the amount of matter remaining outside the
black hole after its formation are sensitive to the description of thermal
effects.Comment: 10 pages, 6 figures, 9 eps files; revised with minor additions due to
referee comments; accepted by Phys.Rev.
Neutrino Signal of Electron-Capture Supernovae from Core Collapse to Cooling
An 8.8 solar mass electron-capture supernova (SN) was simulated in spherical
symmetry consistently from collapse through explosion to nearly complete
deleptonization of the forming neutron star. The evolution time of about 9 s is
short because of nucleon-nucleon correlations in the neutrino opacities. After
a brief phase of accretion-enhanced luminosities (~200 ms), luminosity
equipartition among all species becomes almost perfect and the spectra of
electron antineutrinos and muon/tau antineutrinos very similar. We discuss
consequences for the neutrino-driven wind as a nucleosynthesis site and for
flavor oscillations of SN neutrinos.Comment: 4 pages, 4 eps figures; published as Physical Review Letters, vol.
104, Issue 25, id. 25110
Mass Limits For Black Hole Formation
We present a series of two-dimensional core-collapse supernova simulations
for a range of progenitor masses and different input physics. These models
predict a range of supernova energies and compact remnant masses. In
particular, we study two mechanisms for black hole formation: prompt collapse
and delayed collapse due to fallback. For massive progenitors above 20 solar
masses, after a hydrodynamic time for the helium core (a few minutes to a few
hours), fallback drives the compact object beyond the maximum neutron star mass
causing it to collapse into a black hole. With the current accuracy of the
models, progenitors more massive than 40 solar masses form black holes directly
with no supernova explosion (if rotating, these black holes may be the
progenitors of gamma-ray bursts). We calculate the mass distribution of black
holes formed, and compare these predictions to the observations, which
represent a small biased subset of the black hole population. Uncertainties in
these estimates are discussed.Comment: 15 pages total, 4 figures, Modifications in Conclusion, accepted by
Ap
Gamma-Ray Lines from Asymmetric Supernovae
We present 3-dimensional SPH simulations of supernova explosions from 100
seconds to 1 year after core-bounce. By extending our modelling efforts to a
3-dimensional hydrodynamics treatment, we are able to investigate the effects
of explosion asymmetries on mixing and gamma-ray line emergence in supernovae.
A series of initial explosion conditions are implemented, including jet-like
and equatorial asymmetries of varying degree. For comparison, symmetric
explosion models are also calculated. A series of time slices from the
explosion evolution are further analyzed using a 3-dimensional Monte Carlo
gamma-ray transport code. The emergent hard X- and gamma-ray spectra are
calculated as a function of both viewing angle and time, including trends in
the gamma-ray line profiles. We find significant differences in the velocity
distribution of radioactive nickel between the symmetric and asymmetric
explosion models. The effects of this spatial distribution change are reflected
in the overall high energy spectrum, as well as in the individual gamma-ray
line profiles.Comment: 32 pages, 14 figures, LAUR-02-6114, http://qso.lanl.gov/~clf
"Clumping Asymmetry" section revise
Global Anisotropies in Supernova Explosions and Pulsar Recoil
We show by two-dimensional and first three-dimensional simulations of
neutrino-driven supernova explosions that low (l=1,2) modes can dominate the
flow pattern in the convective postshock region on timescales of hundreds of
milliseconds after core bounce. This can lead to large global anisotropy of the
supernova explosion and pulsar kicks in excess of 500 km/s.Comment: 3 pages, 2 figures, contribution to Procs. 12th Workshop on Nuclear
Astrophysics, Ringberg Castle, March 22-27, 200
Instability of a stalled accretion shock: evidence for the advective-acoustic cycle
We analyze the linear stability of a stalled accretion shock in a perfect gas
with a parametrized cooling function L ~ rho^{beta-alpha} P^alpha. The
instability is dominated by the l=1 mode if the shock radius exceeds 2-3 times
the accretor radius, depending on the parameters of the cooling function. The
growth rate and oscillation period are comparable to those observed in the
numerical simulations of Blondin & Mezzacappa (2006). The instability mechanism
is analyzed by separately measuring the efficiencies of the purely acoustic
cycle and the advective-acoustic cycle. These efficiencies are estimated
directly from the eigenspectrum, and also through a WKB analysis in the high
frequency limit. Both methods prove that the advective-acoustic cycle is
unstable, and that the purely acoustic cycle is stable. Extrapolating these
results to low frequency leads us to interpret the dominant mode as an
advective-acoustic instability, different from the purely acoustic
interpretation of Blondin & Mezzacappa (2006). A simplified characterization of
the instability is proposed, based on an advective-acoustic cycle between the
shock and the radius r_nabla where the velocity gradients of the stationary
flow are strongest. The importance of the coupling region in this mechanism
calls for a better understanding of the conditions for an efficient
advective-acoustic coupling in a decelerated, nonadiabatic flow, in order to
extend these results to core-collapse supernovae.Comment: 29 pages, 18 figures, to appear in ApJ (1 new Section, 2 new Figures
Microscopic calculation of neutrino mean free path inside hot neutron matter
We calculate the neutrino mean free path and the Equation of State of pure
neutron matter at finite temperature within a selfconsistent scheme based on
the Brueckner--Hartree--Fock approximation. We employ the nucleon-nucleon part
of the recent realistic baryon-baryon interaction (model NSC97e) constructed by
the Nijmegen group. The temperatures considered range from 10 to 80 MeV. We
report on the calculation of the mean field, the residual interaction and the
neutrino mean free path including short and long range correlations given by
the Brueckner--Hartree--Fock plus Random Phase Approximation (BHF+RPA)
framework. This is the first fully consistent calculation in hot neutron matter
dedicated to neutrino mean free path. We compare systematically our results to
those obtain with the D1P Gogny effective interaction, which is independent of
the temperature. The main differences between the present calculation and those
with nuclear effective interactions come from the RPA corrections to BHF (a
factor of about 8) while the temperature lack of consistency accounts for a
factor of about 2
Radiation of Angular Momentum by Neutrinos from Merged Binary Neutron Stars
We study neutrino emission from the remnant of an inspiraling binary neutron
star following coalescence. The mass of the merged remnant is likely to exceed
the stability limit of a cold, rotating neutron star. However, the angular
momentum of the remnant may also approach or even exceed the Kerr limit, J/M^2
= 1, so that total collapse may not be possible unless some angular momentum is
dissipated. We find that neutrino emission is very inefficient in decreasing
the angular momentum of these merged objects and may even lead to a small
increase in J/M^2. We illustrate these findings with a post-Newtonian,
ellipsoidal model calculation. Simple arguments suggest that the remnant may
form a bar mode instability on a timescale similar to or shorter than the
neutrino emission timescale, in which case the evolution of the remnant will be
dominated by the emission of gravitational waves.Comment: 12 pages AASTeX, 2 figures, to appear in Ap
- …