NGC1605a and b: an old binary open cluster in the Galaxy
  This work communicates the discovery of a binary open cluster within the
Galaxy. NGC 1605 presents an unusual morphology with a sparse stellar
distribution and a double core in close angular proximity. The 2MASS and
Gaia-EDR3 field-star decontaminated colour-magnitude diagrams (CMDs) show two
distinct stellar populations located at the same heliocentric distance of
$\sim2.6$ kpc suggesting that there are two clusters in the region, NGC 1605a
and NGC 1605b, with ages of $2$ Gyr and $600$ Myr, respectively. Both Gaia
parallax and PM distributions are compact and very similar indicating that they
are open clusters (OCs) and share the same kinematics. The large age
difference, 1.4 Gyr, points to a formation by tidal capture during a close
encounter and the close spatial proximity and similar kinematics suggest an
ongoing merger event. There are some prominent tidal debris that appear to
trace the clusters' orbits during the close encounter and, unexpectedly, some
of them appear to be bound structures, which may suggest that additionaly to
the evaporation the merging clusters are being broken apart into smaller
structures by the combination of Galactic disk, Perseus arm, and mutual tidal
interactions. In this sense, the newly found binary cluster may be a key object
on the observational validation of theoretical studies on binary cluster pairs
formation by tidal capture as well as in the formation of massive clusters by
merging, and tidal disruption of stellar systems.
