1,112 research outputs found
Delayed neutrino-driven supernova explosions aided by the standing accretion-shock instability
We present results of 2D hydrodynamic simulations of stellar core collapse,
which confirm that the neutrino-heating mechanism remains viable for the
explosion of a wider mass range of supernova progenitors with iron cores. We
used an energy-dependent treatment of the neutrino transport based on the
"ray-by-ray plus" approximation, in which the number, energy, and momentum
equations are closed with a variable Eddington factor obtained by iteratively
solving a model Boltzmann equation. We focus on the evolution of a 15 Msun
progenitor and show that shock revival and the explosion are initiated at about
600 ms post bounce, powered by neutrino energy deposition. Similar to previous
findings for an 11.2 Msun star, but significantly later, the onset of the
explosion is fostered by the standing accretion shock instability (SASI). This
instability exhibits highest growth rates for the dipole and quadrupole modes,
which lead to large-amplitude bipolar shock oscillations and push the shock to
larger radii, thus increasing the time accreted matter is exposed to neutrino
heating in the gain layer. Therefore also convective overturn behind the shock
is strengthened. A "soft" nuclear equation of state that causes a rapid
contraction and a smaller radius of the forming neutron star and thus a fast
release of gravitational binding energy, seems to be more favorable for an
explosion. Rotation has the opposite effect because it leads to a more extended
and cooler neutron star and thus lower neutrino luminosities and mean energies
and overall less neutrino heating. Neutron star g-mode oscillations and the
acoustic mechanism play no important role in our simulations. (abridged)Comment: 46 pages, 20 figures, 59 eps files; submitted to ApJ; significantly
extended and revised version to account for referee comments; high-resolution
images can be obtained upon reques
Revealing the high-density equation of state through binary neutron star mergers
We present a novel method for revealing the equation of state of high-density
neutron star matter through gravitational waves emitted during the postmerger
phase of a binary neutron star system. The method relies on a small number of
detections of the peak frequency in the postmerger phase for binaries of
different (relatively low) masses, in the most likely range of expected
detections. From such observations, one can construct the derivative of the
peak frequency versus the binary mass, in this mass range. Through a detailed
study of binary neutron star mergers for a large sample of equations of state,
we show that one can extrapolate the above information to the highest possible
mass (the threshold mass for black hole formation in a binary neutron star
merger). In turn, this allows for an empirical determination of the maximum
mass of cold, nonrotating neutron stars to within 0.1 M_sun, while the
corresponding radius is determined to within a few percent. Combining this with
the determination of the radius of cold, nonrotating neutron stars of 1.6 M_sun
(to within a few percent, as was demonstrated in Bauswein et al., PRD, 86,
063001, 2012), allows for a clear distinction of a particular candidate
equation of state among a large set of other candidates. Our method is
particularly appealing because it reveals simultaneously the moderate and very
high-density parts of the equation of state, enabling the distinction of
mass-radius relations even if they are similar at typical neutron star masses.
Furthermore, our method also allows to deduce the maximum central energy
density and maximum central rest-mass density of cold, nonrotating neutron
stars with an accuracy of a few per cent.Comment: 14 pages, 12 figures, 2 tables, accepted for publication in Phys.
Rev.
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.
New Two-Dimensional Models of Supernova Explosions by the Neutrino-Heating Mechanism: Evidence for Different Instability Regimes in Collapsing Stellar Cores
The neutrino-driven explosion mechanism for core-collapse supernovae in its
modern flavor relies on the additional support of hydrodynamical instabilities
in achieving shock revival. Two possible candidates, convection and the
so-called standing accretion shock instability (SASI), have been proposed for
this role. In this paper, we discuss new successful simulations of supernova
explosions that shed light on the relative importance of these two
instabilities. While convection has so far been observed to grow first in
self-consistent hydrodynamical models with multi-group neutrino transport, we
here present the first such simulation in which the SASI grows faster while the
development of convection is initially inhibited. We illustrate the features of
this SASI-dominated regime using an explosion model of a 27 solar mass
progenitor, which is contrasted with a convectively-dominated model of an 8.1
solar mass progenitor with subsolar metallicity, whose early post-bounce
behavior is more in line with previous 11.2 and 15 solar mass explosion models.
We analyze the conditions discriminating between the two different regimes,
showing that a high mass-accretion rate and a short advection time-scale are
conducive for strong SASI activity. We also briefly discuss some important
factors for capturing the SASI-driven regime, such as general relativity, the
progenitor structure, a nuclear equation of state leading to a compact
proto-neutron star, and the neutrino treatment. Finally, we evaluate possible
implications of our findings for 2D and 3D supernova simulations. Our results
show that a better understanding of the SASI and convection in the non-linear
regime is required.Comment: 12 pages, 13 figures; revised version accepted for publication in Ap
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
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
Two-dimensional, Time-dependent, Multi-group, Multi-angle Radiation Hydrodynamics Test Simulation in the Core-Collapse Supernova Context
We have developed a time-dependent, multi-energy-group, and multi-angle
(S) Boltzmann transport scheme for radiation hydrodynamics simulations, in
one and two spatial dimensions. The implicit transport is coupled to both 1D
(spherically-symmetric) and 2D (axially-symmetric) versions of the explicit
Newtonian hydrodynamics code VULCAN. The 2D variant, VULCAN/2D, can be operated
in general structured or unstructured grids and though the code can address
many problems in astrophysics it was constructed specifically to study the
core-collapse supernova problem. Furthermore, VULCAN/2D can simulate the
radiation/hydrodynamic evolution of differentially rotating bodies. We
summarize the equations solved and methods incorporated into the algorithm and
present results of a time-dependent 2D test calculation. A more complete
description of the algorithm is postponed to another paper. We highlight a 2D
test run that follows for 22 milliseconds the immediate post-bounce evolution
of a collapsed core. We present the relationship between the anisotropies of
the overturning matter field and the distribution of the corresponding flux
vectors, as a function of energy group. This is the first 2D multi-group,
multi-angle, time-dependent radiation/hydro calculation ever performed in core
collapse studies. Though the transport module of the code is not gray and does
not use flux limiters (however, there is a flux-limited variant of VULCAN/2D),
it still does not include energy redistribution and most velocity-dependent
terms.Comment: 19 pages, plus 13 figures in JPEG format. Submitted to the
Astrophysical Journa
Neutrino-driven explosions twenty years after SN1987A
The neutrino-heating mechanism remains a viable possibility for the cause of
the explosion in a wide mass range of supernova progenitors. This is
demonstrated by recent two-dimensional hydrodynamic simulations with detailed,
energy-dependent neutrino transport. Neutrino-driven explosions were not only
found for stars in the range of 8-10 solar masses with ONeMg cores and in case
of the iron core collapse of a progenitor with 11 solar masses, but also for a
``typical'' progenitor model of 15 solar masses. For such more massive stars,
however, the explosion occurs significantly later than so far thought, and is
crucially supported by large-amplitude bipolar oscillations due to the
nonradial standing accretion shock instability (SASI), whose low (dipole and
quadrupole) modes can develop large growth rates in conditions where convective
instability is damped or even suppressed. The dominance of low-mode deformation
at the time of shock revival has been recognized as a possible explanation of
large pulsar kicks and of large-scale mixing phenomena observed in supernovae
like SN 1987A.Comment: 11 pages, 6 figures; review proceeding for "Supernova 1987A: 20 Years
After: Supernovae and Gamma-Ray Bursters" AIP, New York, eds. S. Immler, K.W.
Weiler, and R. McCra
Prompt merger collapse and the maximum mass of neutron stars
We perform hydrodynamical simulations of neutron-star mergers for a large
sample of temperature-dependent, nuclear equations of state, and determine the
threshold mass above which the merger remnant promptly collapses to form a
black hole. We find that, depending on the equation of state, the threshold
mass is larger than the maximum mass of a non-rotating star in isolation by
between 30 and 70 per cent. Our simulations also show that the ratio between
the threshold mass and maximum mass is tightly correlated with the compactness
of the non-rotating maximum-mass configuration. We speculate on how this
relation can be used to derive constraints on neutron-star properties from
future observations.Comment: 6 pages, 3 figures, accepted for publication in Phys. Rev. Let
Rotation-supported Neutrino-driven Supernova Explosions in Three Dimensions and the Critical Luminosity Condition
We present the first self-consistent, three-dimensional (3D) core-collapse
supernova simulations performed with the Prometheus-Vertex code for a rotating
progenitor star. Besides using the angular momentum of the 15 solar-mass model
as obtained in the stellar evolution calculation with an angular frequency of
about 0.001 rad/s (spin period of more than 6000 s) at the Si/Si-O interface,
we also computed 2D and 3D cases with no rotation and with a ~300 times shorter
rotation period and different angular resolutions. In 2D, only the nonrotating
and slowly rotating models explode, while rapid rotation prevents an explosion
within 500 ms after bounce because of lower radiated neutrino luminosities and
mean energies and thus reduced neutrino heating. In contrast, only the fast
rotating model develops an explosion in 3D when the Si/Si-O interface collapses
through the shock. The explosion becomes possible by the support of a powerful
SASI spiral mode, which compensates for the reduced neutrino heating and pushes
strong shock expansion in the equatorial plane. Fast rotation in 3D leads to a
"two-dimensionalization" of the turbulent energy spectrum (yielding roughly a
-3 instead of a -5/3 power-law slope at intermediate wavelengths) with enhanced
kinetic energy on the largest spatial scales. We also introduce a
generalization of the "universal critical luminosity condition" of Summa et al.
(2016) to account for the effects of rotation, and demonstrate its viability
for a set of more than 40 core-collapse simulations including 9 and 20
solar-mass progenitors as well as black-hole forming cases of 40 and 75
solar-mass stars to be discussed in forthcoming papers.Comment: 24 pages, 19 figures; refereed version with additional section on
resolution dependence; accepted by Ap
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