Expansion kinematics of young clusters

Context. Most stars form in clusters or associations, but only a small number of these groups are expected to remain bound for longer than a few megayears. Once star formation has ended and the molecular gas around young stellar objects has been expelled via feedback processes, most initially bound young clusters lose the majority of their binding mass and begin to disperse into the Galactic field. Aims. This process can be investigated by analysing the structure and kinematic trends in nearby young clusters, particularly by analyzing the trend of expansion, which is a tell-tale sign that a cluster is no longer gravitationally bound and dispersing into the field. Methods. We combined Gaia DR3 five-parameter astrometry with calibrated RVs for members of the nearby young cluster λ Ori (Collinder 69). Results. We characterised the plane-of-sky substructure of the cluster using the Q-parameter and the angular dispersion parameter. We find evidence that the cluster contains a significant substructure but that this is preferentially located away from the central cluster core, which is smooth and likely remains bound. We found strong evidence for expansion in λ Ori in the plane of sky by using a number of metrics, but we also found that the trends are asymmetric at the 5σ significance level, with the maximum rate of expansion being directed nearly parallel to the Galactic plane. We subsequently inverted the maximum rate of expansion of 0.144−0.003+0.003 kms−1 pc−1 to give an expansion timescale of 6.944−0.142+0.148 Myr, which is slightly larger than the typical literature age estimates for the cluster. We also found asymmetry in the velocity dispersion as well as signatures of cluster rotation, and we calculated the kinematic ages for individual cluster members by tracing their motion back in time to their closest approach to the cluster centre.