Structural properties of subgroups of stars associated with open clusters

Abstract

ABSTRACT Recent studies have identified star clusters with multiple components based on accurate spatial distributions and/or proper motions from Gaia Data Release 3 (DR3), utilizing diverse diagnostics to gain an understanding of subgroup evolution. These findings motivated us to search for subgroups among the objects examined in our previous work, which employed fractal statistics. The present study considers seven open clusters that exhibit significant dispersion in age and/or proper motion distributions, suggesting that they are not single clusters. In order to characterize the stellar groups, we calculate the membership probability using Bayesian multidimensional analysis by fitting the observed proper motion distribution of the candidates. A probability distribution is also used to determine the distance of the cluster, which is obtained from the mean value of the distance modes. The photometry from Gaia DR3 is compared with evolutionary models to estimate the cluster age and total mass. In our sample, double components are found only for Markarian 38 and NGC 2659. The other five clusters are confirmed as being single. The structural parameters, such as $\mathcal {Q}$, $\Lambda _{\rm MSR}$, and $\Sigma _{\rm LDR}$, are compared with results from N-body simulations to investigate how the morphology of the stellar clustering evolves. The new results, for a more complete sample of cluster members, provide a better definition of the distribution type (central concentration or substructured region) inferred from the $\overline{m} - \overline{s}$ plot.