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Hydrodynamic simulations of the merger of stellar mass black hole-neutron star binaries are compared with mergers of binary neutron stars. The simulations are Newtonian but take into account the emission and backreaction of gravitational waves. The use of a physical nuclear equation of state allows us to include the effects of neutrino emission. For low neutron star-to-black hole mass ratios, the neutron star transfers mass to the black hole during a few cycles of orbital decay and subsequent widening before finally being disrupted, whereas for ratios near unity the neutron star is destroyed during its first approach. A gas mass between ∼0.3 and ∼0.7 M , is left in an accretion torus around the black hole and radiates neutrinos at a luminosity of several times 10 53 ergs s Ϫ1 during an estimated accretion timescale of about 0.1 s. The emitted neutrinos and antineutrinos annihilate into e ‫ע‬ pairs with efficiencies of 1%-3% and rates of up to ∼ ergs s Ϫ1 , thus depositing an energy 52 2 # 10 ergs above the poles of the black hole in a region that contains less than 10 Ϫ5 M , of baryonic matter.

We consider hot accretion disk outflows from black hole-neutron star mergers in the context of the nucleosynthesis they produce. We begin with a three-dimensional numerical model of a black hole-neutron star merger and calculate the neutrino and antineutrino fluxes emitted from the resulting accretion disk. We then follow the element synthesis in material outflowing the disk along parameterized trajectories. We find that at least a weak r-process is produced, and in some cases a main r-process as well. The neutron-rich conditions required for this production of r-process nuclei stem directly from the interactions of the neutrinos emitted by the disk with the free neutrons and protons in the outflow.

Abstract. In this paper we present a compilation of results from our most advanced neutron star merger simulations. Special aspects of these models were refered to in earlier publications Janka et al. 1999), but a description of the employed numerical procedures and a more complete overview over a large number of computed models are given here. The three-dimensional hydrodynamic simulations were done with a code based on the Piecewise Parabolic Method (PPM), which solves the discretized conservation laws for mass, momentum, energy and, in addition, for the electron lepton number in an Eulerian frame of reference. Up to five levels of nested cartesian grids ensure higher numerical resolution (about 0.6 km) around the center of mass while the evolution is followed in a large computational volume (side length between 300 and 400 km). The simulations are basically Newtonian, but gravitational-wave emission and the corresponding back-reaction on the hydrodynamic flow are taken into account. The use of a physical nuclear equation of state allows us to follow the thermodynamic history of the stellar medium and to compute the energy and lepton number loss due to the emission of neutrinos. The computed models differ concerning the neutron star masses and mass ratios, the neutron star spins, the numerical resolution expressed by the cell size of the finest grid and the number of grid levels, and the calculation of the temperature from the solution of the entropy equation instead of the energy equation. The models were evaluated for the corresponding gravitational-wave and neutrino emission and the mass loss which occurs during the dynamical phase of the merging. The results can serve for comparison with smoothed particle hydrodynamics (SPH) simulations. In addition, they define a reference point for future models with a better treatment of general relativity and with improvements of the complex input physics. Our simulations show that the details of the gravitational-wave emission are still sensitive to the numerical resolution, even in our highest-quality calculations. The amount of mass which can be ejected from neutron star mergers depends strongly on the angular momentum of the system. Our results do not support the initial conditions of temperature and proton-to-nucleon ratio needed according to recent work for producing a solar r-process pattern for nuclei around and above the A ≈ 130 peak. The improved models confirm our previous conclusion that gamma-ray bursts are not powered by neutrino emission during the dynamical phase of the merging of two neutron stars.

Abstract. The instability of Bondi-Hoyle-Lyttleton accretion, observed in numerical simulations, is analyzed through known physical mechanisms and possible numerical artefacts. The mechanisms of the longitudinal and transverse instabilities, established within the accretion line model, are clarified. They cannot account for the instability of BHL accretion at moderate Mach number when the pressure forces within the shock cone are taken into account. The advective-acoustic instability is considered in the context of BHL accretion when the shock is detached from the accretor. This mechanism naturally explains the stability of the flow when the shock is weak, and the instability when the accretor is small. In particular, it is a robust proof of the instability of 3D accretion when γ = 5/3 if the accretor is small enough, even for moderate shock strength (M ∼ 3). The numerical artefacts that may be present in existing numerical simulations are reviewed, with particular attention paid to the advection of entropy/vorticity perturbations and the artificial acoustic feedback from the accretor boundary condition. Several numerical tests are proposed to test these mechanisms.

We present three-dimensional hydrodynamic simulations of the evolution of self-gravitating, thick accretion discs around hyperaccreting stellar-mass black holes. The black hole-torus systems are considered to be remnants of compact object mergers, in which case the disc is not fed by an external mass reservoir and the accretion is non-stationary. Our models take into account viscous dissipation, described by an α-law, a detailed equation of state for the disc gas, and an approximate treatment of general relativistic effects on the disc structure by using a pseudo-Newtonian potential for the black hole including its possible rotation and spin-up during accretion. Magnetic fields are ignored. The neutrino emission of the hot disc is treated by a neutrino-trapping scheme, and the νν-annihilation near the disc is evaluated in a postprocessing step. Our simulations show that the neutrino emission and energy deposition by νν-annihilation increase sensitively with the disc mass, with the black hole spin in case of a disc in corotation, and in particular with the α-viscosity. We find that for sufficiently large α-viscosity, νν-annihilation can be a viable energy source for gamma-ray bursts.

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