The rapid dispersal of low-mass virialised clusters

TL;DR

This study uses N-body simulations to show that low-mass star clusters can rapidly expand due to internal dynamics, challenging the idea that gas expulsion is the main cause of cluster dispersal at this scale.

Contribution

It demonstrates that low-mass clusters can undergo significant dynamical expansion within 10 Myr without gas expulsion, highlighting a different evolutionary pathway.

Findings

Low-mass clusters expand significantly over 10 Myr due to internal dynamics.

Stellar densities at 2 Myr match observed densities in the Solar neighborhood.

Deficit of older clusters does not necessarily indicate gas expulsion disruption.

Abstract

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 order $10^4$. Here we argue that the evolutionary timescales associated with the compact low-mass clusters typical of the median cluster in the Solar neighborhood are short enough that significant dynamical evolution can take place over the ages usually associated with gas expulsion. To test this we perform {\it 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 virialised 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 YSOs in the Solar neighborhood. We argue that at the low-mass end of the cluster mass spectrum, a deficit of clusters at 10s of Myr does not necessarily imply gas expulsion as a disruption mechanism.