Expansion kinematics of young clusters II. NGC 2264 N & S and Collinder 95 with HectoSpec

Journal article, 2026

Aims. Studying the dynamical evolution of young clusters is crucial to gaining a more general understanding of the star formation process. Methods. We took spectra of >600 candidate pre-main sequence (PMS) stars in several nearby young clusters (NGC 2264 N & S, Collinder 95, and Collinder 359) using MMT/Hectospec. These spectra were analyzed for H alpha emission and lithium absorption, features indicative of low-mass young stellar objects (YSOs) still in their PMS evolution. We complemented these samples with YSOs identified via Gaia DR3 variability. In conjunction with Gaia astrometry, these data enable an analysis of cluster structure, kinematics, and ages. We searched for halos of YSOs around our targets in order to test models of young cluster dynamical evolution. Results. For the NGC 2264 N & S cluster pair, we identified 354 YSOs, while for Collinder 95 and 359 we identified 130 and 7 YSOs, respectively. We calculated kinematic "traceback ages" for YSOs in these clusters, which we compared to isochronal ages estimated using several sets of stellar evolution models. We find that for NGC 2264 N & S, the kinematic ages are generally less than their isochronal ages, which may indicate these systems remained bound for a few megayears before their current state of expansion. On the other hand, kinematic ages for Collinder 95 are often significantly greater than their isochronal ages, which implies many of these YSOs did not originate from a central dense region, leading to overestimated kinematic ages. Conclusions. We conclude that NGC 2264 N & S clusters likely formed as initially bound and compact systems but have been gradually evaporating as cluster members become unbound and form halos of unbound YSOs surrounding the cluster cores. Further, we conclude that Collinder 95 was initially sparse and substructured when formed and has been dispersing since gas expulsion.