GRAVITATIONAL-RADIATION FROM COALESCING BINARY NEUTRON-STARS .4. TIDAL DISRUPTION

We have performed three numerical simulations of coalescence of binary neutron stars using a Newtonian hydrodynamics code including the effect of radiation reaction due to emission of gravitational waves. We use the Cartesian coordinate system (x, y, z) with a 141 X 141 X 131 grid and assume the reflection symmetry with respect to z = 0 plane. In the first simulation, we start the calculation from a rotational equilibrium of two neutron stars of mass m1 = m2 = 0.85M. which touches with each other. This corresponds to a fission model of collapse of a rapidly rotating core. The radiated energy is 0.8% of the rest mass and the maximum amplitude of the gravitational waves is 2 x 10(-21) if it occurs at a distance of 10 Mpc. The ratio of the rotational energy to the gravitational energy T/W is approximately 0.15 in the final stage of the numerical simulation. Since this value is greater than the secular instability limit, non-axisymmetric instability with the emission of gravitational waves occurs. This is quite different from the case with m1 = m2 = 1.49M. in a previous paper. In the other two simulations, two neutron stars have different masses; m1 = 1.70 M., m2 = 1.28 M. and m1 = 1.83 M., m2 = 0.97 M., respectively. We assume that the system is in a rotational equilibrium at first. Calculations are started when separation between neutron stars are 19 km and 21 km, respectively. In both cases, tidal disruption of neutron star of smaller mass occurs. The radiated energy amounts to 2.9% and 2.3% of the total rest mass, respectively. The maximum amplitude of the gravitational waves at a distance of 10 Mpc is 4.4 X 10(-21) and 3.4 X 10(-21).