Mergers of Black Hole–Neutron Star Binaries. I. Methods and First Results

We use a 3-D relativistic SPH (Smoothed Particle Hydrodynamics) code to studymergers of black hole -- neutron star (BH--NS) binary systems with low massratios, adopting $M_{NS}/M_{BH} \simeq 0.1$ as a representative case. Theoutcome of such mergers depends sensitively on both the magnitude of the BHspin and its obliquity (i.e., the inclination of the binary orbit with respectto the equatorial plane of the BH). In particular, only systems withsufficiently high BH spin parameter $a$ and sufficiently low orbitalinclinations allow any NS matter to escape or to form a long-lived disk outsidethe BH horizon after disruption. Mergers of binaries with orbital inclinationsabove $\sim60^o$ lead to complete prompt accretion of the entire NS by the BH,even for the case of an extreme Kerr BH. We find that the formation of asignificant disk or torus of NS material around the BH always requires anear-maximal BH spin and a low initial inclination of the NS orbit just priorto merger.Comment: to appear in ApJ, 54 pages, 19 figure