Simulations of coalescing neutron star and black hole binaries

We investigate the dynamics and evolution of merging neutron stars, of neut ron stars coalescing with stellar-mass black holes, and the formation and p roperties of accretion tori around the (remnant) black holes. The three-dimensional Newtonian hydrodynamics equations are integrated by a Eulerian PPM code on four nested Cartesian grids. The code includes the em ission and backreaction of gravitational waves as well as a physical nuclea r equation-of-state [Lattimer & Swesty 1991] and the neutrino emission from the hot matter. Lepton number and energy losses of the gas due to neutrino emission are treated by an elaborate neutrino leakage scheme which takes i nto account neutrinos and antineutrinos of all flavors. Neutrino-antineutri no annihilation in the vicinity of the merger is evaluated in a post-proces sing step. The gravity of the black hole is described with a Newtonian or, alternatively, with a Paczynski-Wiita potential. The hydrodynamic effect of the black hole is simulated by extracting all matter that flows into a sph ere with radius equal to the Schwarzschild radius of the black hole or, in accretion simulations, twice the Schwarzschild radius. The NS/NS and BH/NS merging models yield information about gravitational wa ve and neutrino emission. Also, they allow us to determine the physical pro perties (masses, densities, temperatures, estimated life times) of the remn ants of the mergers: a black hole surrounded by an accretion torus. In the NS/NS case, a thick disk with a mass around 0.1M(.) forms after the massive and very compact central object that contains most of the mass of the merg ed neutron stars, has collapsed to a black hole, presumably on a dynamical time scale. The dynamics of BH/NS mergers is very sensitive to the neutron star to black hole mass ratio. For low ratios the neutron star transfers ma ss to the black hole during a few cycles of orbital decay and subsequent wi dening before finally being disrupted, whereas for ratios near unity the ne utron star is already destroyed during its first approach. A gas mass of ab out 0.5M(.) is left in an accretion torus around the black hole. The accretion tori radiate neutrinos at luminosities of up to several 10(53 ) erg/s. Higher luminosities are found for larger disk masses and smaller b lack holes. The emitted neutrinos and antineutrinos annihilate into e(+/-) pairs with efficiencies of a few percent and rates of up to similar to 2 X 10(52) erg/s, releasing an estimated energy of up to similar to 10(51) erg in a pair-plasma fireball. The torus geometry favors relativistic expansion of the pair plasma along the baryon depleted system axis. The occurrence o f moderately beamed jets with opening angles of several ten degrees is ther efore likely. The jet energies and short accretion times of the tori of abo ut 0.1s are in the right range to account for the subclass of short and les s energetic gamma-ray bursts.