Extended stellar systems in the solar neighborhood - III. Like ships in
  the night: the Coma Berenices neighbor moving group
  We report the discovery of a kinematically cold group of stars, located in
the immediate neighborhood of the well-known star cluster Coma Berenices (Mel
111). The new group identified in tangential velocity space as measured by Gaia
contains at least 177 coeval members distributed in two subgroups, and appears
as a flattened structure parallel to the plane, stretching for about 50 pc.
More remarkably, the new group, which appears to have formed about 300 Myr
later than Mel 111 in a different part of the Galaxy, will share essentially
the same volume with the older cluster when the centers of both groups will be
at their closest in 13 Myr. This will result in the mixing of two unrelated
populations with different metallicities. The phase of cohabitation for these
two groups is about 20-30 Myr, after which the two populations will drift
apart. We estimate that temporal cohabitation of such populations is not a rare
event in the disk of the Milky Way, and of the order of once per Galactic
revolution. Our study also unveils the tidal tails of the Mel 111 cluster.
