Evolution of the Galactic globular cluster system

Abstract

We study the dynamical evolution of disc and halo globular clusters in the Milky Way using a series of Fokker-Planck calculations combined with parametric statistical models. Our sample of 111 clusters with velocity data is predicted to descend from an initial population of 250 clusters, implying more than a factor of two decrease in population size due to evolution. Approximately 200 of these clusters are in a halo component and 50 in a disc component. The estimated initial halo population follows a coreless R-3.38 density profile in good agreement with current estimates for the distribution of halo field stars. The observed core in the present-day distribution of halo clusters results from the rapid evaporation of clusters in the inner regions of the Galaxy. The initial halo population is also predicted to have a radially biased orbit distribution in rough agreement with the observed kinematics of halo field stars. The isotropy of the present-day halo cluster distribution results from the evaporation of clusters on elongated orbits. Similarly, the initial disc component has a nearly isotropic initial distribution that becomes more tangentially biased with time. However, the inferred initial characteristics of the disc component do not match the kinematics of the rapidly rotating thin or thick disc stellar populations. These characteristics may be more indicative of the flattened halo component discussed by Zinno Detailed examination of cluster evolution confirms the importance of disc heating. Clusters on low-inclination orbits experience the strongest disc heating because of optimal matches in resonant frequencies. Disc heating on high­ inclination orbits is weaker but still dominates over spheroidal heating. Evaporation times depend weakly on initial concentration, density and height of oscillation above the disc.