Hydrodynamics of binary coalescence. 1: Polytropes with stiff equations of state

We have performed a series of three-dimensional hydrodynamic calculations of binary coalescence using the smoothed particle hydrodynamics (SPH) method. The initial conditions are exact polytropic equilibrium configurations with > 5/3, on the verge of dynamical instability. We calculate the emission of gravitational radiation in the quadrupole approximation. The fully nonlinear development of the instability is followed until a new equilibrium configuration is reached. We find that the properties of this final configuration depend sensitively on both the compressibility and mass ratio. An { axisymmetric} merged configuration is always produced when 2.3. As a consequence, the emission of gravitational radiation shuts off abruptly right after the onset of dynamical instability. In contrast, { triaxial\/} merged configurations are obtained when 2.3, and the system continues to emit gravitational waves after the final coalescence. Systems with mass ratios q≠1 typically become dynamically unstable before the onset of mass transfer. Stable mass transfer from one neutron star to another in a close binary is therefore probably ruled out. The maximum amplitude hₘₐₓ and peak luminosity Lₘₐₓ of the gravitational waves emitted during the final coalescence are nearly independent of , but depend very sensitively on the mass ratio q.