POST-NEWTONIAN MODELS OF BINARY NEUTRON-STARS

Using an energy variational method, we calculate quasiequilibrium conf igurations of binary neutron stars modeled as compressible triaxial el lipsoids obeying a polytropic equation of state. Our energy functional includes terms both for the internal hydrodynamics of the stars and f or the external orbital motion. We add the leading post-Newtonian (PN) corrections to the internal and gravitational energies of the stars, and adopt hybrid orbital terms which are fully relativistic in the tes t-mass limit and always accurate to PN order. The total energy functio nal is varied to find quasiequilibrium sequences for both corotating a nd irrotational binaries in circular orbits. We examine how the orbita l frequency at the innermost stable circular orbit depends on the poly tropic index n and the compactness parameter GM/Rc(2). We find that, f or a given GM/Rc(2), the innermost stable circular orbit along an irro tational sequence is about 17% larger than the innermost secularly sta ble circular orbit along the corotating sequence when n=0.5, and 20% l arger when n=1. We also examine the dependence of the maximum neutron star mass on the orbital frequency and find that, if PN tidal effects can be neglected, the maximum equilibrium mass increases as the orbita l separation decreases.