Stellar Systems Group

In this work, we improve a previously published method to calculate in a reliable way the radius of an open cluster (OC). 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 OCs using data from Gaia second data release 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∝R−3.11±0.35c. Additionally, we find that number of members, cluster radius, and age follow the relationship Nc∝R1.2±0.1c⋅T−1.9±0.4c 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 OC radii not having previous information on star memberships.