Double or binary: on the multiplicity of open star clusters

TL;DR

This study investigates the frequency and characteristics of binary star clusters in the Milky Way disk, finding a comparable fraction to that in the Magellanic Clouds and providing insights into their formation and evolution.

Contribution

It provides the first comprehensive analysis of open cluster multiplicity in the Milky Way, confirming the presence of a significant binary cluster population.

Findings

At least 12% of open clusters near the Solar Circle are interacting or binary.

The pair separation histogram suggests a bimodal distribution.

Approximately 40% of pairs are likely primordial.

Abstract

Observations indicate that the fraction of potential binary star clusters in the Magellanic Clouds is about 10%. In contrast, it is widely accepted that the binary cluster frequency in the Galaxy disk is much lower. Here we investigate the multiplicity of clusters in the Milky Way disk to either confirm or disprove this dearth of binaries. We quantify the open cluster multiplicity using complete, volume-limited samples from WEBDA and NCOVOCC. At the Solar Circle, at least 12% of all open clusters appear to be experiencing some type of interaction with another cluster; i.e., are possible binaries. As in the Magellanic Clouds, the pair separation histogram hints of a bimodal distribution. Nearly 40% of identified pairs are probably primordial. Most of the remaining pairs could be undergoing some type of close encounter, perhaps as a result of orbital resonances. Confirming early theoretical predictions, the characteristic time scale for destruction of bound pairs in the disk is 200 Myr or one galactic orbit. Our results show that the fraction of possible binary clusters in the Galactic disk is comparable to that in the Magellanic Clouds.