The rapid dispersal of low-mass virialized clusters

Infant mortality brought about by the expulsion of a star cluster's natal gas is widely invoked to explain cluster statistics at different ages. While a well-studied problem, most recent studies of gas expulsion's effect on a cluster have focused on massive clusters, with stellar counts of the order of 104. Here we argue that the evolutionary time-scales associated with the compact low-mass clusters typical of the median cluster in the solar neighbourhood are short enough that significant dynamical evolution can take place over the ages usually associated with gas expulsion. To test this, we perform N-body simulations of the dynamics of a very young star-forming region, with initial conditions drawn from a large-scale hydrodynamic simulation of gravitational collapse and fragmentation. The subclusters we analyse, with populations of a few hundred stars, have high local star formation efficiencies and are roughly virialized even after the gas is removed. Over 10?Myr, they expand to a similar degree as would be expected from gas expulsion if they were initially gas rich, but the expansion is purely due to the internal stellar dynamics of the young clusters. The expansion is such that the stellar densities at 2?Myr match those of young stellar objects in the solar neighbourhood. We argue that at the low-mass end of the cluster mass spectrum, a deficit of clusters at tens of Myr does not necessarily imply gas expulsion as a disruption mechanism.