Complex stellar populations in massive clusters: trapping stars of a dwarf-disc galaxy in a newborn stellar super-cluster

Abstract

Some of the most massive globular clusters of our Milky Way, such as for example omega-Centauri, show a mixture of stellar populations spanning a few Gyr in age and 1.5 dex in metallicities. In contrast, standard formation scenarios predict that globular and open clusters form in one single star-burst event of duration less than about 10 Myr and therefore should exhibit only one age and one metallicity in its stars. Here, we investigate the possibility that a massive stellar super-cluster may trap older galactic field stars during its formation process that are later detectable in the cluster as an apparent population of stars with a very different age and metallicity. With a set of numerical N-body simulations, we are able to show that, if the mass of the stellar super-cluster is high enough and the stellar velocity dispersion in the cluster is comparable to the dispersion of the surrounding disc stars in the host galaxy, then up to about 40 per cent of its initial mass can be additionally gained from trapped disc stars. We also suggest that a super-cluster may capture in excess of its own mass under certain conditions.