Both stars obey a n = 1 polytropic equation of state, P=Kρ0Γ, at t = 0. They are evolved according to the
adiabatic law, P=(Γ-1)ρ0ε. Here P is the pressure, K the polytropic gas constant, Γ is the adiabatic index, &epsilon is the specific internal energy, and ρ0 is the rest-mass density. We chose Γ = 2 to mimick stiff nuclear matter. Both stars are irrotational
(nonspinning) and are in a quasiequilibrium circular orbit.
We consider three different NSNS binaries: