Tw. Baumgarte et al., GENERAL-RELATIVISTIC MODELS OF BINARY NEUTRON-STARS IN QUASI-EQUILIBRIUM, Physical review. D. Particles and fields, 57(12), 1998, pp. 7299-7311
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.