  We present a numerical study of the hydrodynamics in the final stages of
inspiral in a black hole-neutron star binary, when the separation becomes
comparable to the stellar radius. We 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 which accretes matter via an absorbing boundary at the
Schwarzschild radius. Our initial conditions correspond to tidally locked
binaries in equilibirium. The dynamical evolution is followed for approximately
23 ms, and in every case for Gamma=5/3 we find that the neutron star is tidally
disrupted on a dynamical timescale, forming a dense torus around the black hole
that contains a few tenths of a solar mass. A nearly baryon-free axis is
present in the system throughout the coalescence, a fireball expanding along
that axis would avoid excessive baryon contamination and could give rise to a
modestly beamed gamma-ray burst.

Astrophysics