2024MNRAS.527.1784G

We investigate the spatial structure and dynamical properties of the young (∼60 Myr), richly populated open cluster NGC 6649 based on reliable cluster members from Gaia-DR3 data. The density-based spatial clustering of applications with noise is used to estimate membership probabilities for 25045 sample stars in the field of the cluster. More than 1000 likely cluster members down to G ∼ 20 mag can be identified based on the membership probabilities. The cluster members of NGC 6649 extend up to a radial distance of ${\sim} 21\,\mathrm{ arcmin}$ (12 pc), which is much larger than previously thought. We estimate a core radius of $R_{\mathrm{ c}}=2\overset{^{\prime }}{.}38\pm 0\overset{^{\prime }}{.}06$ (1.4 pc) and a tidal radius of $R_{\mathrm{ t}}=35\overset{^{\prime }}{.}85\pm 6\overset{^{\prime }}{.}64$ (20.9 pc) for the cluster, indicating that the cluster has formed a dense core and an extended halo. We find that mass segregation has taken place within the cluster. We also detect two small and sparse clusters (C1 and C2) around NGC 6649. C1 has recently been reported, but C2 is detected for the first time. The Gaussian mixture model clustering method is used to identify 184 and 60 members for C1 and C2, respectively. We find that NGC 6649, C1, and C2 are located at similar distances. Moreover, we estimate a small spatial separation of ∼11.6 pc between C1 and NGC 6649, which is close to or slightly smaller than the detectable radius of NGC 6649. This indicates that NGC 6649 and C1 are physically connected, and there may exist strong tidal interactions and mass transfer between the two clusters. C2 shows a significant lack of faint members (G > 17 mag), probably due to star evaporation and tidal stripping. Our results show that NGC 6649 may be the dominant cluster in a triple-cluster system (NGC 6649+C1+C2).