Black Hole - Neutron Star Binary Mergers: the Impact of Stellar Compactness

Recent gravitational wave observations include possible detections of blackhole - neutron star binary mergers. As with binary black hole mergers,numerical simulations help characterize the sources. For binary systems withneutron star components, the simulations help to predict the imprint of tidaldeformations and disruptions on the gravitational wave signals. In a previousstudy, we investigated how the mass of the black hole has an impact on thedisruption of the neutron star and, as a consequence, on the shape of thegravitational waves emitted. We extend these results to study the effects ofvarying the compactness of the neutron star. We consider neutron starcompactness in the 0.123 to 0.2 range for binaries with mass ratios of 3 and 5.As the compactness and the mass ratio increase, the binary system behavesduring the late inspiral and merger more like a black hole binary. For the casewith the highest mass ratio and most compact neutron star, the gravitationalwaves emitted, in terms of mismatches, are almost indistinguishable from thoseby a binary black hole. The disruption of the star significantly suppresses thekicks on the final black hole. The disruption also affects, although notdramatically, the spin of the final black hole. Lastly, for neutron stars withlow compactness, the quasi-normal ringing of the black hole after the mergerdoes not show a clean quasi-normal ringing because of the late accretion ofdebris from the neutron star.