1,176 research outputs found
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.
Measuring neutron-star properties via gravitational waves from binary mergers
We demonstrate by a large set of merger simulations for symmetric binary
neutron stars (NSs) that there is a tight correlation between the frequency
peak of the postmerger gravitational-wave (GW) emission and the physical
properties of the nuclear equation of state (EoS), e.g. expressed by the radius
of the maximum-mass Tolman-Oppenheimer-Volkhoff configuration. Therefore, a
single measurement of the peak frequency of the postmerger GW signal will
constrain the NS EoS significantly. For plausible optimistic merger-rate
estimates a corresponding detection with Advanced LIGO is likely to happen
within an operation time of roughly a year.Comment: 5 pages, 4 figures, accepted by Phys. Rev. Lett., revised version
including referee comment
Neutrino oscillation signatures of oxygen-neon-magnesium supernovae
We discuss the flavor conversion of neutrinos from core collapse supernovae
that have oxygen-neon-magnesium (ONeMg) cores. Using the numerically calculated
evolution of the star up to 650 ms post bounce, we find that, for the normal
mass hierarchy, the electron neutrino flux in a detector shows signatures of
two typical features of an ONeMg-core supernova: a sharp step in the density
profile at the base of the He shell and a faster shock wave propagation
compared to iron core supernovae. Before the shock hits the density step (t ~
150 ms), the survival probability of electron neutrinos is about 0.68, in
contrast to values of 0.32 or less for an iron core supernova. The passage of
the shock through the step and its subsequent propagation cause a decrease of
the survival probability and a decrease of the amplitude of oscillations in the
Earth, reflecting the transition to a more adiabatic propagation inside the
star. These changes affect the lower energy neutrinos first; they are faster
and more sizable for larger theta_13. They are unique of ONeMg-core supernovae,
and give the possibility to test the speed of the shock wave. The time
modulation of the Earth effect and its negative sign at the neutronization peak
are the most robust signatures in a detector.Comment: 14 pages, 10 figures (16 figure files). Text and graphics added for
illustration and clarification; Results unchanged. Version accepted for
publication in Physical Review
Light curve analysis of ordinary type IIP supernovae based on neutrino-driven explosion simulations in three dimensions
Type II-plateau supernovae (SNe IIP) are the most numerous subclass of
core-collapse SNe originating from massive stars. In the framework of the
neutrino-driven explosion mechanism, we study the SN outburst properties for a
red supergiant progenitor model and compare the corresponding light curves with
observations of the ordinary Type IIP SN 1999em. Three-dimensional (3D)
simulations of (parametrically triggered) neutrino-driven explosions are
performed with the (explicit, finite-volume, Eulerian, multifluid
hydrodynamics) code PROMETHEUS, using a presupernova model of a 15 Msun star as
initial data. At approaching homologous expansion, the hydrodynamical and
composition variables of the 3D models are mapped to a spherically symmetric
configuration, and the simulations are continued with the (implicit, Lagrangian
radiation-hydrodynamics) code CRAB to follow the blast-wave evolution during
the SN outburst. Our 3D neutrino-driven explosion model with an explosion
energy of about 0.5x10^51 erg produces Ni-56 in rough agreement with the amount
deduced from fitting the radioactively powered light-curve tail of SN 1999em.
The considered presupernova model, 3D explosion simulations, and light-curve
calculations can explain the basic observational features of SN 1999em, except
for those connected to the presupernova structure of the outer stellar layers.
Our 3D simulations show that the distribution of Ni-rich matter in velocity
space is asymmetric with a strong dipole component that is consistent with the
observations of SN 1999em. The monotonic luminosity decline from the plateau to
the radioactive tail in ordinary SNe IIP is a manifestation of the intense
turbulent mixing at the He/H composition interface.Comment: 16 pages, 13 figures, 2 tables; added figure, discussions, and
references; accepted for publication in Ap
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
Nucleosynthesis and Clump Formation in a Core Collapse Supernova
High-resolution two-dimensional simulations were performed for the first five
minutes of the evolution of a core collapse supernova explosion in a 15 solar
mass blue supergiant progenitor. The computations start shortly after bounce
and include neutrino-matter interactions by using a light-bulb approximation
for the neutrinos, and a treatment of the nucleosynthesis due to explosive
silicon and oxygen burning. We find that newly formed iron-group elements are
distributed throughout the inner half of the helium core by Rayleigh-Taylor
instabilities at the Ni+Si/O and C+O/He interfaces, seeded by convective
overturn during the early stages of the explosion. Fast moving nickel mushrooms
with velocities up to about 4000 km/s are observed. This offers a natural
explanation for the mixing required in light curve and spectral synthesis
studies of Type Ib explosions. A continuation of the calculations to later
times, however, indicates that the iron velocities observed in SN 1987 A cannot
be reproduced because of a strong deceleration of the clumps in the dense shell
left behind by the shock at the He/H interface.Comment: 8 pages, LaTeX, 2 postscript figures, 2 gif figures, shortened and
slightly revised text and references, accepted by ApJ Letter
Acquire information about neutrino parameters by detecting supernova neutrinos
We consider the supernova shock effects, the Mikheyev-Smirnov-Wolfenstein
(MSW) effects, the collective effects, and the Earth matter effects in the
detection of type II supernova neutrinos on the Earth. It is found that the
event number of supernova neutrinos depends on the neutrino mass hierarchy, the
neutrino mixing angle , and neutrino masses. Therefore, we propose
possible methods to identify the mass hierarchy and acquire information about
and neutrino masses by detecting supernova neutrinos. We apply
these methods to some current neutrino experiments.Comment: 21 pages, 11 figure
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
Neutrino Oscillations and the Supernova 1987A Signal
We study the impact of neutrino oscillations on the interpretation of the
supernova (SN) 1987A neutrino signal by means of a maximum-likelihood analysis.
We focus on oscillations between with or
with those mixing parameters that would solve the solar
neutrino problem. For the small-angle MSW solution (, ), there are no
significant oscillation effects on the Kelvin-Helmholtz cooling signal; we
confirm previous best-fit values for the neutron-star binding energy and
average spectral temperature. There is only marginal overlap
between the upper end of the 95.4\% CL inferred range of and the lower end of the range of theoretical
predictions. Any admixture of the stiffer spectrum by
oscillations aggravates the conflict between experimentally inferred and
theoretically predicted spectral properties. For mixing parameters in the
neighborhood of the large-angle MSW solution (, ) the oscillations in the SN are adiabatic,
but one needs to include the regeneration effect in the Earth which causes the
Kamiokande and IMB detectors to observe different spectra. For
the solar vacuum solution (,
) the oscillations in the SN are nonadiabatic; vacuum
oscillations take place between the SN and the detector. If either of the
large-angle solutions were borne out by the upcoming round of solar neutrino
experiments, one would have to conclude that the SN~1987A
and/or spectra had been much softer than predicted by currentComment: Final version with very minor wording changes, to be published in
Phys. Rev.
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