We construct new models of black hole-neutron star binaries inquasiequilibrium circular orbits by solving Einstein's constraint equations inthe conformal thin-sandwich decomposition together with the relativisticequations of hydrostationary equilibrium. We adopt maximal slicing, assumespatial conformal flatness, and impose equilibrium boundary conditions on anexcision surface (i.e., the apparent horizon) to model the black hole. In ourprevious treatment we adopted a "leading-order" approximation for a parameterrelated to the black-hole spin in these boundary conditions to constructapproximately nonspinning black holes. Here we improve on the models bycomputing the black hole's quasilocal spin angular momentum and setting it tozero. As before, we adopt a polytropic equation of state with adiabatic indexGamma=2 and assume the neutron star to be irrotational. In addition torecomputing several sequences for comparison with our earlier results, we studya wider range of neutron star masses and binary mass ratios. To locate theinnermost stable circular orbit we search for turning points along both thebinding energy and total angular momentum curves for these sequences. Unlikefor our previous approximate boundary condition, these two minima now coincide.We also identify the formation of cusps on the neutron star surface, indicatingthe onset of tidal disruption. Comparing these two critical binary separationsfor different mass ratios and neutron star compactions we distinguish thoseregions that will lead to a tidal disruption of the neutron star from thosethat will result in the plunge into the black hole of a neutron star more orless intact, albeit distorted by tidal forces.
展开▼
