Stationary states of irrotational binary neutron star systems and their evolution due to gravitational wave emission

We have succeeded in obtaining exact configurations of irrotational binary systems for compressible (polytropic) equations of state. Our models correspond to binaries of equal mass neutron star systems in the viscosity free limit. By using the obtained sequences of stationary states, the evolution of binary systems of irrotational neutron stars due to gravitational wave emission has been examined. For inviscid binary systems the spin angular velocity of each component in a detached phase is smaller than the orbital angular velocity at a contact phase. Thus the irrotational approximation during evolution of binary neutron stars due to gravitational wave emission can be justified. Our computational results show that the binary will never reach a dynamically unstable state before a contact phase even for rather stiff polytropes with the index N > 0.7, as the separation of two components decreases due to gravitational wave emission. This conclusion is quantitatively different from that of Lai, Rasio & Shapiro who employed approximate solutions for polytropic binary systems.