We propose a method for calculating the radius of an open cluster in an objective way from an astrometric catalogue containing, at least, positions and proper motions. It uses the minimum spanning tree in the proper motion space to discriminate cluster stars from field stars and it quantifies the strength of the cluster-field separation by means of a statistical parameter defined for the first time in this paper. This is done for a range of different sampling radii from where the cluster radius is obtained as the size at which the best cluster-field separation is achieved.
Applied to Gaia data here: A catalogue of open cluster radii determined from Gaia proper motions (2020)
Applied to Gaia data here: A catalogue of open cluster radii determined from Gaia proper motions (2020)