Cluster Disruption: From infant mortality to long term survival

Abstract

How stellar clusters disrupt, and over what timescales, is intimately linked with how they form. Here, we review the theory and observations of cluster disruption, both the suggested initial rapid dissolution phase (infant mortality) and the longer timescale processes that affect clusters after they emerge from their progenitor GMCs. Over the past decade, the standard paradigm that has developed is that all/most stars are formed in clusters and that the vast majority of these groups are disrupted over short timescales (< 10 Myr). This is thought to be due to the removal of the left over gas from the star-formation process, known as infant mortality. However, recent results have suggested that the fraction of stars that form in clusters has been overestimated, with the majority being formed in unbound groups (i.e. associations) which expand and disrupt without the need of invoking gas removal. Dynamical measurements of young massive clusters in the Galaxy suggest that clusters reach a stable equilibrium at very young (<3 Myr) ages, suggesting that gas expulsion has little effect on the cluster. After the early dynamical phase, clusters appear to be long lived and stable objects. We use the recent WFC3 image of the cluster population in M83 to test empirical disruption laws and find that the lifetime of clusters strongly depends on their ambient environment. While the role of cluster mass is less well constrained (due to the added parameter of the form of the cluster mass function), we find evidence suggesting that higher mass clusters survive longer, and that the cluster mass function (at least in M83, outside the nuclear region) is truncated above ~10^5Msun.