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

TL;DR

This study uses MMT/Hectospec spectroscopy and Gaia DR3 astrometry to identify >600 PMS candidates across NGC 2264 N&S and Collinder 95 (plus Collinder 359) and to map their kinematic structure. By computing traceback (kinematic) ages and comparing them to isochronal ages from Baraffe, PARSEC, and SPOTS models, the work tests cluster formation histories and halo formation around cluster cores. The results indicate NGC 2264 N&S formed as bound, compact clusters that have gradually evaporated into expanding halos, whereas Collinder 95 formed sparsely and is dispersing; systematic differences between kinematic and isochronal ages depend on model and photometric color, highlighting the need to account for embedded phases and substructure in young clusters. The findings have implications for understanding star cluster evolution, halo populations, and the interpretation of ages in nearby star-forming regions.

Abstract

Aims: Studying the dynamical evolution of young clusters is crucial for 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 Hα 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. In particular, we searched for halos of YSOs around our targets 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 calculate kinematic "traceback ages" for YSOs in these clusters, which we compare to isochronal ages estimated using several sets of stellar evolution models. We find for NGC 2264 N & S that kinematic ages are generally smaller than their isochronal ages, which may indicate these systems remained bound for a few Myr before their current state of expansion. On the other hand, kinematic ages for Collinder 95 are often significantly larger than 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, forming halos of unbound YSOs surrounding the cluster cores. We conclude that Collinder 95 likely formed initially sparse and substructured and has been dispersing since gas expulsion.

Figures (15)

• Figure 1: (a) Left: Example H$\alpha$ spectrum of YSO candidate (Gaia Source ID: 3326702657541965952) from NGC 2264, with the source selected as a YSO due to its strong H$\alpha$ line, i.e., with an EW(H$\alpha$) $= -48.97$Å. (b) Right: Example $6708$Å Li spectrum of YSO candidate (Gaia Source ID: 3326714782234731520) from NGC 2264, with this source selected as a YSO due to its strong Li absorption line, i.e., with EW(Li) $= 0.32$Å.
• Figure 2: Scatter plots of H$\alpha$ equivalent width vs lithium equivalent width for all the stellar spectra in our samples of clusters NGC 2264 (N & S), Collinder 95, and Collinder 359. If a star has an EW(H$\alpha$)$-\sigma< -10$Å (above the horizontal blue dashed line), or an EW(Li)$-\sigma>0.15$Å (right of the vertical green dot-dashed line), it is flagged as a YSO. Gaia variability catalog stars are additionally marked with a closed circle.
• Figure 3: Proper motion maps of YSOs in NGC 2264 N, NGC 2264 S, and Collinder 95. Stars flagged as YSOs via our spectroscopic analysis are marked with an open circle, YSOs flagged through the Gaia variability catalog are marked with smaller black point. (Stars flagged by both indicators are denoted through a filled in circle / larger black point.) The proper motion of each YSO is shown with an arrow which is color-coded according to the direction of motion. The average motion of each cluster or subcluster has been subtracted out (see these values in Table \ref{['tab:cluster_info']}), allowing us to observe the plane of sky motion of each YSO within the cluster frame. The center of the cluster is denoted by the intersection point of the two blue dotted lines. The dotted blue circle denotes the half-mass radius of the identified YSOs in each cluster.
• Figure 4: Expansion direction histograms of YSOs in NGC 2264 N, NGC 2264 S, and Collinder 95, where the measured angle, $\Delta\theta$, is the difference in a YSO's direction of motion from pure radial outward motion from the cluster center. We observe that there is a large proportion of stars near zero ($\pm 50$°) for NGC 2264 N and Collinder 95, indicating that there is significant outward expansion of YSOs from their cluster centers. We do not observe this same trend for NGC 2264 S. The shaded region shows a range $\pm 50^\circ$, defining a subset of YSOs with strongest radial expansion.
• Figure 5: Radial profiles of areal stellar number density profiles of NGC 2264 N (Top), NGC 2264 S (Middle) and Collinder 95 (Bottom). Vertical dashed lines indicate the core radii ($r_c$), as defined in Section \ref{['DensityProfiles']}, and half-mass radii ($r_{50}$).