Metallicity effects on open cluster dynamics

Abstract

We take advantage of an N-body code that treats a range of metallicities to investigate the effect of metallicity on the internal dynamics of star clusters. Simulations of a set of large open clusters without primordial binaries are performed. We find that core collapse is postponed in low-metallicity star clusters, compared with Population I clusters, owing to the increased rate of mass loss through stellar evolution suffered at early times. However, earlier core collapse in high-metallicity clusters leads to an increase in the escape of stars from the cluster. We find that by remarkable cancellation of these two effects cluster dissolution times are little affected by changes in metallicity: high-metallicity clusters dissolve first but the difference is less than 10 per cent of the lifetime. We illustrate the behaviour of key structural properties of star clusters for models of different metallicity. We also look at the effect this has on the stellar populations of the clusters and the incidence of binary formation and exchange interactions. While the effect of metallicity on certain gross characteristics of a star cluster is found to be weak, we also find that in many respects metallicity is an important parameter in the evolution of a star cluster and should not be ignored.