The Effect of Age on the Grouping of Open Clusters: II—Are There Old Binary Clusters?

Abstract

In the present study, we continue testing the Primordial Group hypothesis (Casado 2022), which postulates that only sufficiently young open clusters can be binary or multiple, and old clusters are essentially single. To this end, we revisit all the remaining binary cluster candidates in the Galaxy having at least one cluster older than 100 Myr through Gaia data and careful revision of the literature. We found no convincing case for an old binary system among the 120 pairs/groups revised. Most of the pairs are optical pairs or flyby encounters. However, we found three dubious pairs that could falsify the title hypothesis upon further research. We also found two possible primordial pairs older than expected. Our results confirm that the vast majority of binary/multiple OCs in the Galaxy, if not all, are of primordial origin and are not stable for a long time. This finding is in line with similar studies of the Magellanic Clouds and theoretical N-body simulations in the Galaxy. The pairs of OCs in these groups are generally not binary systems since they are not gravitationally bound. We also point out some inconsistencies in previous works and databases, such as false open clusters and duplicities.

Star-by-star dynamical evolution of the physical pair of the Collinder 135 and UBC 7 open clusters

“…Vázquez et al 2010;de La Fuente Marcos & de La Fuente Marcos 2009), recent investigations have provided a number of examples of pairs of OCs in our Galaxy that suggest a common origin (see, e.g. Qin et al 2023;Angelo et al 2022;Ye et al 2022;Casado 2022a;Bisht et al 2021;Zhong et al 2019). Some clusters that were previously considered single have shown a binary signature upon detailed examination (Alejo et al 2020;Dias et al 2018), including clusters with signs of merging by tidal capture (Camargo 2021), according to Gaia data.…”