A catalogue of open cluster radii determined from Gaia proper motions

Abstract: In this work we improve a previously published method to calculate in a reliable way the radius of an open cluster. The method is based on the behaviour of stars in the proper motion space as the sampling changes in the position space. Here we describe the new version of the method and show its performance and robustness. Additionally, we apply it to a large number of open clusters using data from Gaia DR2 to generate a catalogue of 401 clusters with reliable radius estimations. The range of obtained apparent radii goes from Rc=1.4+-0.1 arcmin (for the cluster FSR 1651) to Rc=25.5+-3.5 arcmin (for NGC~2437). Cluster linear sizes follow very closely a lognormal distribution with a mean characteristic radius of Rc=3.7 pc, and its high radius tail can be fitted by a power law as N \propto Rc^-3.11+-0.35. Additionally, we find that number of members, cluster radius and age follow the relationship Nc \propto Rc^1.2+-0.1 Tc^-1.9+-0.4 where the younger and more extensive the cluster, the more members it presents. The proposed method is not sensitive to low density or irregular spatial distributions of stars and, therefore, is a good alternative or complementary procedure to calculate open cluster radii not having previous information on star memberships.