Expansion kinematics of young clusters

Context . Most stars form in clusters, but only a small fraction are thought to remain gravitationally bound for longer than ∼10 Myr. Aims . Cluster formation and dispersal can be investigated by analysing the spatial and kinematic structure in nearby young systems, particularly, by analysing expansion. Methods . We combined Gaia DR3 five-parameter astrometry with calibrated radial velocities for 23 nearby (< 1 kpc) young (10 Myr. The majority of nearby young clusters exhibit clear signatures of expansion in the plane of sky, which in many cases is significantly anisotropic, even at ages >30Myr. We find evidence that the directions of maximum expansion of older clusters tend to be oriented closer to parallel with the Galactic plane. The high degree of spatial structure and significant expansion anisotropy imply that the majority of these young clusters formed with significant spatial and kinematic substructure and not as dense, monolithic clusters. The kinematic ages estimated from expansion timescales and on-sky trace-back generally agree well with estimates inferred from stellar evolution models for clusters