Galactic populations of ultracompact binaries

Recent RXTE and Chandra discoveries of low-mass X-ray binaries with ultrashort orbital periods have initiated theoretical work on the origins of these peculiar systems. Using the StarTrack population synthesis code, we analyze the formation and evolution of X-ray ultracompact binaries (UCBs) in the Galactic field. The relative number of UCBs with a neutron star or a black hole accretor populating our Galaxy is predicted. Our results demonstrate that standard evolutionary scenarios involving primordial binaries can be sufficient to produce the UCBs in the Galactic field without requiring additional processes associated with the dense stellar environments in the cores of globular clusters. In contrast to previous studies, we find that the majority of the immediate progenitors of these systems consist of a hydrogen-exhausted donor with an ONeMg white dwarf. The evolution of these systems leads to the accretion-induced collapse of the white dwarf to a neutron star, which can play an important role in the formation of a majority of Galactic UCBs. We predict that with an increase in the number of X-ray active UCBs hosting neutron stars by an order of magnitude, a system with a black hole accretor may be found.