Black hole-neutron star binaries with high spins and large mass asymmetries: I. Properties of quasiequilibrium sequences

Black hole-neutron star (BHNS) mergers are a promising target of current gravitational-wave (GW) and electromagnetic (EM) searches, being the putative origin of ultrarelativistic jets, gamma-ray emission, and r-process nucleosynthesis. However, the possibility of any EM emission accompanying a GW detection crucially depends on the amount of baryonic mass left after the coalescence, i.e., whether the neutron star (NS) undergoes a "tidal disruption" or "plunges" into the black hole (BH) while remaining essentially intact. As the first of a series of two papers, we here report the most systematic investigation to date of quasiequilibrium sequences of nonprecessing initial data across a range of stellar compactnesses C, mass ratios q, BH spins chi BH, and equations of state satisfying all present observational constraints. Using an improved version of the elliptic initial-data solver FUKA, we have computed more than 1000 individual configurations and estimated the onset of mass-shedding or the crossing of the innermost stable circular orbit in terms of the corresponding characteristic orbital angular velocities Omega MS and Omega ISCO as a function of C, q, and chi BH. To the best of our knowledge, this is the first time that the dependence of these frequencies on the BH spin is investigated. In turn, by setting Omega MS 1/4 Omega ISCO it is possible to determine the separatrix between the tidal disruption or plunge scenarios as a function of the fundamental parameters of these systems, namely, q, C, and chi BH. Finally, we present a novel analysis of quantities related to the tidal forces in the initial data and discuss their dependence on spin and separation.