Mass Transfer in Eccentric Black Hole-Neutron Star Mergers

Black hole-neutron star binaries are of interest in many ways: they are intrinsically transient, radiate gravitational waves detectable by LIGO, and may produce gamma-ray bursts. Although it has long been assumed that their late-stage orbital evolution is driven entirely by gravitational wave emission, we show here that in certain circumstances, mass transfer from the neutron star onto the black hole can both alter the binary's orbital evolution and significantly reduce the neutron star's mass: when the fraction of its mass transferred per orbit is greater than or similar to 10(-2), the neutron star's mass diminishes by order unity, leading to mergers in which the neutron star mass is exceptionally small. The mass transfer creates a gas disk around the black hole before merger that can be comparable in mass to the debris remaining after merger, i.e., similar to 0.1 M-circle dot. These processes are most important when the initial neutron star-black hole mass ratio q is in the range approximate to 0.2-0.8, the orbital semimajor axis is 40 less than or similar to a(0)/r(g) less than or similar to 300 (r(g) equivalent to GM(BH)/c(2)), and the eccentricity is large at e(0) greater than or similar to 0.8. Systems of this sort may be generated through the dynamical evolution of a triple system, as well as by other means.