Structure and colour-magnitude diagrams of Scorpius OB2 based on kinematic modelling of Hipparcos data

All stars in a `moving group' share the same space motion. The presence of such a coherent structure in velocity space can be used to derive (improved) estimates of the stellar parallaxes from the observed positions and proper motions. A Lund Observatory group (Dravins et al.) describes an elegant and sophisticated maximum likelihood procedure to do so using Hipparcos data. Their method determines the cluster centroid space motion, the internal velocity dispersion, which is assumed to be isotropic, and the individual model parallaxes for all member stars, implicitly using the assumption of a `compact' cluster with a `small' velocity dispersion.