Newtonian hydrodynamics of the coalescence of black holes with neutron stars - II. Tidally locked binaries with a soft equation of state

We present a numerical study of the hydrodynamics in the final stages of in spiral of a black hole-neutron star binary, when the binary separation beco mes comparable to the stellar radius. Mie use a Newtonian three-dimensional smooth particle hydrodynamics (SPH) code, and model the neutron star with a soft (adiabatic index Gamma = 5/3) polytropic equation of state, and the black hole as a Newtonian point mass that accretes matter via an absorbing boundary at the Schwarzschild radius. Our initial conditions correspond to tidally locked binaries in equilibrium, and we have explored configurations with different values of the mass ratio q = M-NS/M-BH, ranging from q = 1 to 0.1. The dynamical evolution is followed for approximately 23 ms, and in every case studied here we find that the neutron star is tidally disrupted on a dynamical time-scale, forming a dense torus around the black hole tha t contains a few tenths of a solar mass. A nearly baryon-free axis is prese nt in the system throughout, the coalescence, and only modest beaming of a fireball that could give rise to a gamma-ray burst would be sufficient to a void excessive baryon contamination. We find that some mass (of the order o f 10(-3)-10(-2) M.) may be dynamically ejected from the system, and could t hus contribute substantially to the amount of observed r-process material i n the galaxy. We calculate the gravitational radiation waveforms and lumino sity emitted during the coalescence in the quadrupole approximation.