Astron.Astrophys.319:122-153,1997 Three-dimensional hydrodynamical, Newtonian calculations of the coalescence
of equal-mass binary neutron stars are performed, including a physical
high-density ...equation of state and a treatment of the neutrino emission of the
heated matter. The total neutrino luminosity climbs to a maximum value of
1--$1.5\cdot 10^{53}$~erg/s of which 90--95\% originate from the toroidal gas
cloud surrounding the very dense core formed after the merging. When the
neutrino luminosities are highest, $\nu\bar\nu$-annihilation deposits about
0.2--0.3\% of the emitted neutrino energy in the immediate neighborhood of the
merger, and the maximum integral energy deposition rate is 3--$4\cdot
10^{50}$~erg/s. Since the $3\,M_{\odot}$ core of the merged object will most
likely collapse into a black hole within milliseconds, the energy that can be
pumped into a pair-photon fireball is insufficient by a factor of about 1000 to
explain $\gamma$-ray bursts at cosmological distances with an energy of the
order of $10^{51}/(4\pi)$~erg/steradian. Analytical estimates show that the
additional energy provided by the annihilation of $\nu\bar\nu$ pairs emitted
from a possible accretion torus of $\sim 0.1\,M_{\odot}$ around the central
black hole is still more than a factor of 10 too small, unless focussing of the
fireball into a jet-like expansion plays an important role. About
$10^{-4}$--$10^{-3}$~$M_\odot$ of material lost during the neutron star merging
and swept out from the system in a neutrino-driven wind might be a site for
nucleosythesis. Aspects of a possible r-processing in these ejecta are
discussed.
Recent progress in modeling core-collapse supernovae is summarized and set in perspective. Two-dimensional simulations with state-of-the-art treatment of neutrino transport still fail to produce ...powerful explosions, but evidence is presented that they are very close to a success.
New two-dimensional, high-resolution calculations of a core collapse supernova in a 15 Msol blue supergiant are presented, which cover the entire evolution from shock revival until the first few ...hours of the explosion. Explosive nucleosynthesis, its dependence upon convective mixing during the first second of the evolution and the growth of Rayleigh-Taylor instabilities at the composition interfaces of the progenitor star are all modeled consistently and allow for a comparison with observational data. We confirm our earlier findings, that the perturbations induced by neutrino driven convection are sufficiently strong to seed large-scale Rayleigh-Taylor mixing and to destroy the onion-shell structure of the stellar He-core. As in our earlier calculations, the strong deceleration of the nickel clumps in the layers adjacent to the He/H interface suggests that the high velocities of iron-group elements observed in SN 1987A cannot be explained on the basis of currently favored progenitor models. Possible solutions to this dilemma and the implications of the mixing for type Ib explosions are briefly discussed.
The identification of the astrophysical site and the specific conditions in which r-process nucleosynthesis takes place remain unsolved mysteries of astrophysics. The present paper emphasizes some ...important future challenges faced by nuclear physics in this problem, particularly in the determination of the radiative neutron capture rates by exotic nuclei close to the neutron drip line and the fission probabilities of heavy neutron-rich nuclei. These quantities are particularly relevant to determine the composition of the matter resulting from the decompression of initially cold neutron star matter. New detailed r-process calculations are performed and the final composition of ejected inner and outer neutron star crust material is estimated. We discuss the impact of the many uncertainties in the astrophysics and nuclear physics on the final composition of the ejected matter. The similarity between the predicted and the solar abundance pattern for A > 140 nuclei as well as the robustness of the prediction with varied input parameters makes this scenario one of the most promising that deserves further exploration.
Two- and three-dimensional simulations demonstrate that hydrodynamic instabilities can lead to low-mode (l=1,2) asymmetries of the fluid flow in the neutrino-heated layer behind the supernova shock. ...This provides a natural explanation for aspherical mass ejection and for pulsar recoil velocities even in excess of 1000 km/s. We propose that the bimodality of the pulsar velocity distribution might be a consequence of a dominant l=1 mode in case of the fast component, while higher-mode anisotropy characterizes the postshock flow and SN ejecta during the birth of the slow neutron stars. We argue that the observed large asymmetries of supernovae and the measured high velocities of young pulsars therefore do not imply rapid rotation of the iron core of the progenitor star, nor do they require strong magnetic fields to play a crucial role in the explosion. Anisotropic neutrino emission from accretion contributes to the neutron star acceleration on a minor level, and pulsar kicks do not make a good case for non-standard neutrino physics in the nascent neutron star.
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.
We present results of high-resolution two-dimensional simulations which follow the first five minutes of a core collapse supernova explosion in a 15 solar mass blue supergiant progenitor. The ...computations start shortly after core 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 a significant fraction of the stellar helium core by the concerted action of convective and Rayleigh-Taylor instabilities. Fast moving nickel mushrooms with velocities up to 4000 km/s are observed. A continuation of the calculations to later times, however, indicates, that the iron velocities observed in SN 1987 A cannot be reproduced due to a strong deceleration of the clumps during their interaction with the dense shell left behind by the shock at the He/H interface. Therefore, we cannot confirm the claim that convective "premixing" of the nickel in the early phases of the explosion solves the problem of the high iron velocities.
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.
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.
We investigate the dynamics and evolution of coalescing neutron stars. The
three-dimensional Newtonian equations of hydrodynamics are integrated by the
`Piecewise Parabolic Method' However, we do ...include the effects of the emission
of gravitational waves on the hydrodynamics. The properties of neutron star
matter are described by the equation of state of Lattimer & Swesty. In addition
to the fundamental hydrodynamic quantities, density, momentum, and energy, we
follow the time evolution of the electron density in the stellar gas. Energy
losses and changes of the electron abundance due to the emission of neutrinos
are taken into account by an elaborate ``neutrino leakage scheme'', which
employs a careful calculation of the lepton number and energy source terms of
all neutrino types. The grid is Cartesian and equidistant with a resolution of
64**3 or 128**3, which allows us to calculate the self-gravity via fast Fourier
transforms.
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