Double or binary: on the multiplicity of open star clusters

Abstract

<i>Context. <i/>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.<i>Aims. <i/>Here we investigate the multiplicity of clusters in the Milky Way disk to either confirm or disprove this dearth of binaries.<i>Methods. <i/>We quantify the open cluster multiplicity using complete, volume-limited samples from WEBDA and NCOVOCC.<i>Results. <i/>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 at 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.<i>Conclusions. <i/>Our results show that the fraction of possible binary clusters in the Galactic disk is comparable to that in the Magellanic Clouds.