Expansion kinematics of young clusters III. The kiloparsec sample

Journal article, 2026

Context. Most stars form in clusters, but only a small fraction are thought to remain gravitationally bound for longer than similar to 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 (<60 Myr) clusters.
Results. We characterised the plane-of-sky structure of the clusters using the Q-parameter and angular dispersion parameter methods. We measured the plane-of-sky expansion using several methods. We determined the plane-of-sky orientations along which the expansion was maximised. We estimated expansion timescales and trace-back ages and compared them to the isochronal ages. We searched for correlations between the cluster properties and discuss sample-wide trends.
Conclusions. Most young clusters are more smoothly structured in their centres where the dynamical interaction rate is highest, while hierarchical structure can survive in the sparse outskirts for >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 <10 Myr old. However, many clusters with older isochronal ages appear to have significantly younger kinematic ages. We discuss potential reasons for this discrepancy, including a prolonged embedded and/or gravitationally bound phase in the early stages of the clusters.