Coalescing neutron stars - A step towards physical models - III. Improved numerics and different neutron star masses and spins

In this paper we present a compilation of results from our most advanced ne utron star merger simulations. Special aspects of these models were refered to in earlier publications (Ruffert & Janka 1999; Janka et al. 1999), but a description of the employed numerical procedures and a more complete over view over a large number of computed models are given here. The three-dimen sional hydrodynamic simulations were done with a code based on the Piecewis e Parabolic Method (PPM), which solves the discretized conservation laws fo r mass, momentum, energy and, in addition, for the electron lepton number i n an Eulerian frame of reference. Up to five levels of nested cartesian gri ds ensure higher numerical resolution (about 0.6 km) around the center of m ass while the evolution is followed in a large computational volume (side l ength between 300 and 400 km). The simulations are basically Newtonian, but gravitational-wave emission and the corresponding back-reaction on the hyd rodynamic ow are taken into account. The use of a physical nuclear equation of state allows us to follow the thermodynamic history of the stellar medi um and to compute the energy and lepton number loss due to the emission of neutrinos. The computed models differ concerning the neutron star masses an d mass ratios, the neutron star spins, the numerical resolution expressed b y the cell size of the finest grid and the number of grid levels, and the c alculation of the temperature from the solution of the entropy equation ins tead of the energy equation. The models were evaluated for the correspondin g gravitational-wave and neutrino emission and the mass loss which occurs d uring the dynamical phase of the merging. The results can serve for compari son with smoothed particle hydrodynamics (SPH) simulations. In addition, th ey define a reference point for future models with a better treatment of ge neral relativity and with improvements of the complex input physics. Our si mulations show that the details of the gravitational-wave emission are stil l sensitive to the numerical resolution, even in our highest-quality calcul ations. The amount of mass which can be ejected from neutron star mergers d epends strongly on the angular momentum of the system. Our results do not s upport the initial conditions of temperature and proton-to-nucleon ratio ne eded according to recent work for producing a solar r-process pattern for n uclei around and above the A approximate to 130 peak. The improved models c onfirm our previous conclusion that gamma-ray bursts are not powered by neu trino emission during the dynamical phase of the merging of two neutron sta rs.