Incompressible fluid binary systems with internal flows – models of close binary neutron star systems with spin

In this paper we have examined numerically exact configurations of close binary systems composed of incompressible fluids with internal flows. Component stars of binary systems are assumed to be circularly orbiting each other but rotating nonsynchronously with the orbital motion, i.e. stars in binary systems have steady motions seen from a rotational frame of reference. We have computed several equilibrium sequences by taking fully into account the tidal effect of Newtonian gravity without approximation. We consider two binary systems consisting of either 1) a point mass and a fluid star or 2) a fluid star and a fluid star. Each of them corresponds to generalization of the Roche–Riemann or the Darwin–Riemann problem, respectively. Our code can be applied to various types of incompressible binary systems with various mass ratios and spin as long as the vorticity is constant. We compare these equilibrium sequences of binaries with approximate solutions which were studied extensively by Lai, Rasio and Shapiro (LRS) as models for neutron star–neutron star (NS–NS) binary systems or black hole–neutron star (BH–NS) binary systems. Our results coincide qualitatively with those of LRS but are different from theirs for configurations with small separations. For these binary systems, our sequences show that dynamical or secular instability sets in as the separation decreases. The quantitative errors of the ellipsoidal approximation amount to 2 ∼ 10% for configurations near the instability point. Compared to the results of LRS, the separation of the stars at the point where the instability sets in is larger and correspondingly the orbital frequency at the critical state is smaller for our models. Since these sequences can be considered as evolutionary models of binary NS systems where component stars are approaching due to the effect of gravitational wave emission, we can expect that the final fate of such binary systems will be dynamical coalescence due to dynamical instability.