The Evolution of Highly Compact Binaries in Globular Clusters

Abstract

We report on detailed calculations of the secular evolution, observational appearance, and numbers of highly compact binaries in globular clusters. Such binaries are subject to the combined influences of gravitational radiation, thermal evolution of the secondary star, gradual mass transfer from the secondary to the collapsed object primary when the system is in contact, and occasional gravitational encounters with field stars, some of which cause rapid mass transfer from the secondary to the primary. Some gravitational encounters with field stars end in direct physical collisions which can produce massive disks circling the collapsed primary. We present examples of the sorts of histories that are possible for highly compact binaries in globular clusters, and calculate the rate at which mass transfer is induced by gravitational encounters. We then describe the observational appearance of these systems, and report on calculations of their expected numbers. We also briefly discuss the relationship of highly compact binaries in globular clusters to the bright X-ray sources of the Galactic bulge.