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Abstract

We study the merger of black hole-neutron star (BH-NS) binaries in numerical
relativity, 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. We
find that within the precision of our numerical simulations, the remnant disk
mass and ejecta mass normalized by the NS baryon mass ($\hat{M}_{\rm{rem}}$ and
$\hat{M}_{\rm{eje}}$, respectively), and the cutoff frequency $f_{\rm{cut}}$
normalized by the initial total gravitational mass of the system at infinite
separation approximately agree among the models with the same NS compactness
$C_{\rm{NS}}=M_{\rm{NS}}/R_{\rm{NS}}$, mass ratio $Q=M_{\rm{BH}}/M_{\rm{NS}}$,
and dimensionless BH spin $\chi_{\rm{BH}}$ irrespective of the NS mass
$M_{\rm{NS}}$ in the range of $1.092$--$1.691\,M_\odot$. This result shows that
the merger outcome depends sensitively on $Q$, $\chi_{\rm BH}$, and
$C_{\rm{NS}}$ but only weekly on $M_{\rm{NS}}$. This justifies the approach of
studying the dependence of NS tidal disruptions on the NS compactness by fixing
the NS mass but changing the EOS. We further perform simulations with massive
NSs of $M_{\rm{NS}}=1.8M_{\odot}$, and compare our results of
$\hat{M}_{\rm{rem}}$ and $\hat{M}_{\rm{eje}}$ with those given by existing
fitting formulas to test their robustness for more compact NSs. We find that
the fitting formulas obtained in the previous studies are accurate within the
numerical errors assumed, while our results also suggest that further
improvement is possible by systematically performing more precise numerical
simulations.