Praesepe - two merging clusters?

Abstract

A membership catalogue for Praesepe was compiled and split into four mass bins. A contour plot indicates the presence of a subcluster some 3 pc from the centre of the cluster, of approximately 30 M-circle dot. A tidally truncated King profile was fitted to the remainder of the cluster and the tidal radius is found to be 12.1 pc; the mass of the cluster (excluding the subcluster) is 630 M-circle dot. From the calculated velocity dispersions we find that the cluster appears to have too much kinetic energy and should be rapidly disintegrating. X-ray data suggest that there may be an age spread between the main core stars and the subcluster stars. This leads us to the conclusion that Praesepe is two merging clusters.