We construct new models of black hole-neutron star binaries in quasiequilibrium circular orbits by solving Einstein ’ s constraint equations in the conformal thin-sandwich decomposition together with the relativistic equations of hydrostationary equilibrium .
We adopt maximal slicing , assume spatial conformal flatness , and impose equilibrium boundary conditions on an excision surface ( i.e. , the apparent horizon ) to model the black hole .
In our previous treatment we adopted a “ leading-order ” approximation for a parameter related to the black-hole spin in these boundary conditions to construct approximately nonspinning black holes .
Here we improve on the models by computing the black hole ’ s quasilocal spin angular momentum and setting it to zero .
As before , we adopt a polytropic equation of state with adiabatic index \Gamma = 2 and assume the neutron star to be irrotational .
In addition to recomputing several sequences for comparison with our earlier results , we study a wider range of neutron star masses and binary mass ratios .
To locate the innermost stable circular orbit we search for turning points along both the binding energy and total angular momentum curves for these sequences .
Unlike for our previous approximate boundary condition , these two minima now coincide .
We also identify the formation of cusps on the neutron star surface , indicating the onset of tidal disruption .
Comparing these two critical binary separations for different mass ratios and neutron star compactions we distinguish those regions that will lead to a tidal disruption of the neutron star from those that will result in the plunge into the black hole of a neutron star more or less intact , albeit distorted by tidal forces .