Abstract:
   By utilizing Gaia DR 2 data, we search for tidal substructures around the nearby star cluster Coma Berenices (Coma Ber), via an artificial neural network based clustering method, StarGO, which maps the 5-D parameter data-set (X, Y, Z, $\mu_\alpha \cos\delta, \mu_\delta$) to a 2-D neural network to identify kinematically related structures. A leading and a trailing tails, each with an extension of $\sim$ 50~pc are discovered around Coma Ber. There are 83 member candidates inside central Coma Ber ($\sim7$~pc) with additional 114 in the tails. Serendipitously, we re-discover a nearby stellar group, group-X, which is 65~pc away from Coma Ber and $\sim400$~Myr younger than Coma Ber ($\sim 800$~Myr). Group-X with 218 members was identified and is about 10 times more than previous study.} The dynamical mass of both Coma Ber and group-X is significantly bigger than their photometric mass, implying dissolution of each group. Evident expansion is observed in both groups, again confirming their disrupting state.