Double or binary: on the multiplicity of open star clusters
  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.
