Young, wild, and free: The early expansion of star clusters

Early expansion plays a fundamental role in the dynamical evolution of young star clusters. However, until very recently most of our understanding of cluster expansion was based only on indirect evidence or on statistically limited samples of clusters. Here we present a comprehensive kinematic analysis of virtually all known young Galactic clusters (t < 300 Myr) based on the improved astrometric quality of the Gaia DR3 data. Such a large sample provides an unprecedented opportunity to robustly constrain the fraction of clusters and the timescale during which expansion has a prominent impact on the overall kinematics. We find that a remarkable fraction (up to 80%) of clusters younger than similar to 30 Myr is currently experiencing significant expansion, whereas older systems are mostly compatible with equilibrium configurations. We observe a trend in which the expansion speed increases with the cluster-centric distance, suggesting that clusters undergoing expansion will likely lose a fraction of their present-day mass. Also, most young expanding clusters are extended, possibly due to the expansion itself. A comparison with a set of N-body simulations of young star clusters shows that the observed expansion pattern is in general qualitative agreement with that found for systems undergoing violent relaxation and evolving toward a final virial equilibrium state. However, we also note that additional processes likely associated with residual gas expulsion and mass loss due to stellar evolution likely also play a key role in driving the observed expansion.