Black Hole-Neutron Star Mergers with Massive Neutron Stars in Numerical Relativity

We study the merger of black hole-neutron star (BH-NS) binaries in numericalrelativity, focusing on the properties of the remnant disk and the ejecta,varying the mass of compactness of the NS and the mass and spin of the BH. Wefind that within the precision of our numerical simulations, the remnant diskmass and ejecta mass normalized by the NS baryon mass (M̂_rem andM̂_eje, respectively), and the cutoff frequency f_cutnormalized by the initial total gravitational mass of the system at infiniteseparation approximately agree among the models with the same NS compactnessC_NS=M_NS/R_NS, mass ratio Q=M_BH/M_NS,and dimensionless BH spin χ_BH irrespective of the NS massM_NS in the range of 1.092–1.691 M_⊙. This result shows thatthe merger outcome depends sensitively on Q, χ_ BH, andC_NS but only weekly on M_NS. This justifies the approach ofstudying the dependence of NS tidal disruptions on the NS compactness by fixingthe NS mass but changing the EOS. We further perform simulations with massiveNSs of M_NS=1.8M_⊙, and compare our results ofM̂_rem and M̂_eje with those given by existingfitting formulas to test their robustness for more compact NSs. We find thatthe fitting formulas obtained in the previous studies are accurate within thenumerical errors assumed, while our results also suggest that furtherimprovement is possible by systematically performing more precise numericalsimulations.