On the trapping of stars by a newborn stellar supercluster

Abstract

Numerical experiments conducted by Fellhauer et al. suggest that a supercluster may capture up to about 40 per cent of its mass from the galaxy where it belongs. Nevertheless, in those experiments the cluster was created making appear its mass out of nothing, rather than from mass already present in the galaxy. Here, we use a thought experiment, plus a few simple computations, to show that the difference between the dynamical effects of these two scenarios (i.e. mass creation versus mass concentration) is actually very important. We also present the results of new numerical experiments, simulating the formation of the cluster through mass concentration, that show that trapping depends critically on the process of cluster formation and that the amounts of gained mass are substantially smaller than those obtained from mass creation. Besides, the criterion used by Fellhauer et al. to decide the membership to the supercluster is not adequate, and the use of a more sensible criterion, based on Jacobi's integral, renders a number of captures at least an order of magnitude smaller. All things considered, the captures cannot exceed, at most, a few per cent of the mass of the cluster.