Expansion Kinematics of Young Clusters. I. Lambda Ori

Abstract: 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 Myr. 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 expansion, the tell-tale sign that a cluster is no longer gravitationally bound but is dispersing into the field. Methods. We combine Gaia DR3 5-parameter astrometry with calibrated radial velocities for members of the nearby young cluster Lambda Ori (Collinder 69). Results. We characterise the plane-of-sky substructure of the cluster using the Q-parameter and Angular Dispersion parameter. We find evidence that the cluster contains significant substructure, but that this is preferentially located away from the central cluster core, which is smooth and likely remains bound. We find strong evidence for expansion in Lambda Ori in the plane-of-sky using a number of metrics, but also that the trends are asymmetric at the 5$\sigma$ significance level. with the maximum rate of expansion being directed nearly parallel to the Galactic plane. We then invert the maximum rate of expansion of $0.144^{+0.003}_{-0.003} kms^{-1}pc^{-1}$ to give an expansion timescale of $6.944^{+0.148}_{-0.142} Myr$, which is slightly larger than typical literature age estimates for the cluster. We also find asymmetry in the velocity dispersion, potential signatures of cluster rotation, and calculate kinematic ages for individual cluster members by tracing their motion back in time to their closest approach to the cluster center.