Black Hole-Neutron Star Mergers: Disk Mass Predictions. (arXiv:1207.6304v1 [astro-ph.HE])

Black Hole-Neutron Star Mergers: Disk Mass Predictions. (arXiv:1207.6304v1 [astro-ph.HE]):
Determining the final result of black hole-neutron star mergers, and in
particular the amount of matter remaining outside the black hole at late times,
has been one of the main motivations behind the numerical simulation of these
systems. Black hole-neutron star binaries are amongst the most likely
progenitors of short gamma-ray bursts --- as long as they result in the
formation of massive (at least ~0.1 solar mass) accretion disks around the
black hole. Whether this actually happens strongly depends on the physical
characteristics of the system, and in particular on the mass ratio, the spin of
the black hole, and the radius of the neutron star. We present here a simple
two-parameter model, fitted to existing numerical results, for the
determination of the mass remaining outside the black hole a few milliseconds
after a black hole-neutron star merger. This model predicts the remnant mass
within a few percents of the mass of the neutron star, at least for remnant
masses up to 20% of the neutron star mass. Results across the range of
parameters deemed to be the most likely astrophysically are presented here. We
find that, for 10 solar mass black holes, massive disks are only possible for
fairly large neutron stars (R>12km), or quasi-extremal black hole spins
(a/M>0.9). We also use our model to discuss how the equation of state of the
neutron star affects the final remnant, and the strong influence that this can
have on the rate of short gamma-ray bursts produced by black hole-neutron star
mergers.