Double or binary: on the multiplicity of open star clusters

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