The fate of the pre-main sequence-rich clusters collinder 197 and vdB92 : dissolution?

Abstract

Context. Since most of the star clusters do not survive the embedded phase, their early dissolution appears to be a major source of field stars. However, catching a cluster in the act of dissolving is somewhat elusive. Aims. We investigate the nature and possible evolution of the young Galactic star clusters Collinder 197 (Cr 197) and vdB 92. Methods. Photometric and structural properties were derived with near-infrared photometry and field-star decontamination. Kinematical properties were inferred from proper motions of the main sequence (MS) and pre-MS (PMS) member stars. Results. The colour–magnitude diagrams (CMDs) are basically characterised by a poorly-populated MS and a dominant fraction (>∼75%) of PMS stars, and the combined MS and PMS CMD morphology in both clusters consistently constrains the age to within 5 ± 4 Myr, with a ∼10 Myr spread in the star formation process. The MS+PMS stellar masses are ≈660+102 −59 Mʘ (Cr 197) and ≈750+101 −51 Mʘ (vdB 92). Both Cr 197 and vdB 92 appear to be abnormally large when compared to clusters within the same age range. They have irregular stellar radial density distributions (RDPs) with a marked excess in the innermost region, a feature that, at less than 10 Myr, is more likely related to the star formation and/or molecular cloud fragmentation than to age-dependent dynamical effects. The velocity dispersion of both clusters, derived from proper motions, is in the range ∼15−22 km s-ˡ. Conclusions. Both clusters appear to be in a super-virial state, with velocity dispersions higher than those expected of nearly-virialised clusters of similar mass and size. A possible interpretation is that Cr 197 and vdB 92 deviate critically from dynamical equilibrium, and may dissolve into the field. We also conclude that early cluster dissolution leaves detectable imprints on RDPs of clusters as massive as several 102 Mʘ. Cr 197 and vdB 92 may be the link between embedded clusters and young stellar associations.