Discovery of Tidal Tails in Disrupting Open Clusters: Coma Berenices and
  a Neighbor Stellar Group
  We report the discovery of tidal structures around the intermediate-aged
($\sim$ 700--800~Myr), nearby ($\sim85$~pc) star cluster Coma Berenices. The
spatial and kinematic grouping of stars is determined with the {\it Gaia} DR2
parallax and proper motion data, by a clustering analysis tool,
\textsc{StarGO}, to map 5D parameters ($X, Y, Z$, $\mu_\alpha \cos\delta,
\mu_\delta$) onto a 2D neural network. A leading and a trailing tails, each
with an extension of $\sim50$~pc are revealed for the first time around this
disrupting star cluster. The cluster members, totaling to
$\sim115^{+5}_{-3}\,\rm {M_\odot}$, are clearly mass segregated, and exhibit a
flat mass function with $\alpha \sim 0.79\pm0.16$, in the sense of $dN/dm
\propto m^{-\alpha}$, where $N$ is the number of member stars and $m$ is
stellar mass, in the mass range of $m=0.25$--$2.51~{\rm M_\odot}$. Within the
tidal radius of $\sim$6.9~pc, there are 77 member candidates with an average
position, i.e., as the cluster center, of R.A.= 186.8110~deg, and decl.=
25.8112~deg, and an average distance of 85.8~pc. Additional 120 member
candidates reside in the tidal structures, i.e., outnumbering those in the
cluster core. The expansion of escaping members lead to an anisotropy in the
velocity field of the tidal tails. Our analysis also serendipitously uncovers
an adjacent stellar group, part of which has been cataloged in the literature.
We identify 218 member candidates, 10 times more than previously known. This
star group is some 65~pc away from, and $\sim400$~Myr younger than, Coma Ber,
but is already at the final stage of disruption.
