The Coma Berenices star cluster and its moving group

We report on the analysis of astrometric and photometric data from Hipparcos, Tycho and the ACT catalogue in a 1200 sq. degree field around the nearby open star cluster in Coma Berenices and infer the characteristic features of this cluster. From a sample of 51 kinematic members we derive the cluster's distance, size and spatial structure as well as its stellar content, mass and age. We find that the cluster consists of an elliptical core-halo system with major axis parallel to the direction of galactic motion, and of a moving group of extratidal stars. The latter have tangential distances ≥10 pc from the cluster center, but have the same distance from the Sun, the same motion and the same age as the stars in the core and halo. The luminosity function of the core-halo system steeply declines beyond absolute magnitude M_V=4.5, but that of the moving group rises towards fainter magnitudes and suggests the existence of further low-mass members below the current magnitude limit. The cloud of extratidal stars witnesses the process of dissolution of the cluster.