Rates of collapse and evaporation of globular clusters

Abstract

Of the known galactic globular clusters, ~ 140 in number, about one-fifth have undergone core collapse, and an unknown number have evaporated altogether. From observational estimates of the dynamical relaxation times of globular clusters, measured at their cores and at their half-light radii, we estimate here the present rates at which core collapse and evaporation are occurring. We find a core collapse rate of 2 ± 1 per Gyr, which for a galactic age of ~ 12 Gyr agrees well with the fact that 27 clusters have surface brightness profiles with the morphology expected for the post-collapse phase. We find a destruction and evaporation rate of 5±3 per Gyr, suggesting that a significant fraction of the Galaxy's original complement of globular clusters have perished, through the combined effects of mechanisms such as relaxation-driven evaporation and shocking due to interaction with the galactic disk and bulge.