The fate of the pre-main sequence-rich clusters Collinder197 and vdB92: dissolution?

TL;DR

This study examines the properties and potential dissolution of two young Galactic star clusters, Cr 197 and vdB 92, revealing they are super-virial and may dissolve into the field, providing insights into early cluster evolution.

Contribution

It provides detailed analysis of the structure, dynamics, and evolutionary state of Cr 197 and vdB 92, highlighting their potential to dissolve and their role as links between embedded clusters and stellar associations.

Findings

Both clusters are in a super-virial state with high velocity dispersions.

They exhibit irregular stellar density distributions with excess in the core.

Clusters may dissolve into the field due to dynamical disequilibrium.

Abstract

We investigate the nature and possible evolution of the young Galactic star clusters Collinder 197 (Cr 197) and vdB 92. The colour-magnitude diagrams (CMDs) are basically characterised by a poorly-populated MS and a dominant fraction ($\ga75%$) of PMS stars, and the combined MS and PMS CMD morphology in both clusters consistently constrains the age to within $5\pm4$ Myr, with a $\sim10$ Myr spread in the star formation process. The MS$ + $PMS stellar masses are $\approx660^{+102}_{-59} \ms$ (Cr 197) and $\approx750^{+101}_{-51} \ms$ (vdB 92). 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 $\sim15 - 22 \kms$. 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 $10^2 \ms$. Cr 197 and vdB 92 may be the link between embedded clusters and young stellar associations.