Contribution of High-Mass Black Holes to Mergers of Compact Binaries

We consider the merging of high-mass black-hole binaries of the Cyg X-1 type, in which the O-star companion becomes a neutron star following a supernova explosion. Our estimated rate of mergers, $\sim 2\times 10^{-5} yr^{-1}$ for the Galaxy, is relatively low because of the paucity of high-mass black holes. None the less, because of their high masses, these black-hole, neutron-star binaries could contribute importantly to the merging sought by LIGO. From stellar evolutionary calculations including mass loss, we estimate that a ZAMS mass of $\sim 80 \msun$ is necessary before a high-mass black hole can result in a massive binary. We estimate the detection rate for LIGO of high-mass black hole neutron-star mergers to be a factor $\sim 40$ greater than that for binary neutron stars. We suggest how our high cut-off mass can be reconciled with the requirements of nucleosynthesis, and show that a bimodal distribution with masses of black holes can account, at least qualitatively, for the many transient sources which contain high-mass black holes.