The Effect of Age on the Grouping of Open Clusters: II. Are there old binary clusters?

TL;DR

This study investigates whether old open clusters can be binary systems, finding most are not gravitationally bound and supporting the idea that binary clusters are primarily primordial and short-lived.

Contribution

It provides a comprehensive revision of binary open cluster candidates using Gaia data, confirming the primordial origin of most binary clusters and identifying potential false positives.

Findings

No convincing old binary clusters found among 120 candidates

Most pairs are optical or flyby encounters, not gravitationally bound

Supports the hypothesis that binary clusters are mostly primordial and short-lived

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 have 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 have found three dubious pairs that could falsify the title hypothesis upon further research. We also have 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.