Properties of general relativistic, irrotational binary neutron stars in close quasiequilibrium orbits: Polytropic equations of state - art. no. 104015

We investigate close binary neutron stars in quasiequilibrium states in a g eneral relativistic framework. The configurations are numerically computed assuming (1) the existence of a helicoidal Killing vector, (2) conformal fl atness for spatial components of the metric, (3) irrotational velocity fiel d for the neutron stars, and (4) masses of neutron stars to be identical. W e adopt the polytropic equation of state and the computation is performed f or a wide range of the polytropic index n( = 0.5,0.66667,0.8,1,1.25), and c ompactness of neutron stars (M/R)(infinity)(= 0.03 - 0.3), when M and R den ote the mass and radius of neutron stars in isolation. Because of the assum ption of the irrotational velocity field, a sequence of fixed rest mass can be identified as an evolutionary track as a result of the radiation reacti on of gravitational waves. Such solution sequences are computed from distan t detached to innermost orbits where a cusp (inner Lagrange point) appears at the inner edges of the stellar surface. The stability of the orbital mot ions and the gravitational wave frequency at the innermost orbits are inves tigated. It is found that the innermost stable circular orbits (ISCO) appea r for the case of a stiff equation of state with n less than or similar to2 /3. We carefully analyze the ISCO for n = 0.5 and show that the ISCO are ma inly determined by a hydrodynamic instability for (M/R)(infinity)less than or similar to0.2. We also investigate the total angular momentum and the sp ecific angular momentum distribution of the binary configuration at the inn ermost orbits, where the final merger process starts. From these quantities , we expect the final outcomes of the binary neutron star coalescence.