GENERAL-RELATIVISTIC MODELS OF BINARY NEUTRON-STARS IN QUASI-EQUILIBRIUM

We perform fully relativistic calculations of binary neutron stars in corotating, circular orbit. While Newtonian gravity allows for a stric t equilibrium, a relativistic binary system emits gravitational radiat ion, causing the system to lose energy and slowly spiral inwards. Howe ver, since inspiral occurs on a time scale much longer than the orbita l period, we can treat the binary to be in quasiequilibrium. In this a pproximation, we integrate a subset of the Einstein equations coupled to the relativistic equation of hydrostatic equilibrium to solve the i nitial value problem for binaries of arbitrary separation. We adopt a polytropic equation of state to determine the structure and maximum ma ss of neutron stars in close binaries for polytropic indices n = 1, 1. 5 and 2. We construct sequences of constant rest-mass and locate turni ng points along energy equilibrium curves to identify the onset of orb ital instability. In particular, we locate the innermost stable circul ar orbit and its angular velocity. We construct the first contact bina ry systems in full general relativity. These arise whenever the equati on of state is sufficiently soft (n greater than or similar to 1.5). A radial stability analysis reveals no tendency for neutron stars in cl ose binaries to collapse to black holes prior to merger.