Evolution of Neutron-Star, Carbon-Oxygen White-Dwarf Binaries

Abstract

We consider the evolution of neutron-star (ns), carbon-oxygen white-dwarf (co) binaries, using both the Bethe & Brown (1998) schematic analytic evolutions and the Portegies Zwart & Yungelson (1998) numerical population syntheses. In the earlier literature, the five observed (ns,co)_c binaries were evolved through common envelope, but now a concensus is arising that common envelope evolution is avoided, as we shall discuss. Accepting this, we see that the present discrepancy between observed (ns,co)_c binaries (none) and the predicted $\sim 50$ is great. We show that the introduction of hypercritical accretion, which sends the neutron star into a black hole, is helpful in explaining this discrepancy. One of our main purposes in these evolutions is to compare the schematic, analytic and numerical population syntheses. We show there to be excellent agreement between these two approaches and we outline why this results.