Detection of moving groups among early type stars

Our procedure to detect moving groups in the solar neighbourhood (Chen et a l., 1997) in the four-dimensional space of the stellar velocity components and age has been improved. The method, which takes advantadge of nan-parame tric estimators of density distribution to avoid any a priori knowledge of the kinematic properties of these stellar groups, now includes the effect o f observational errors on the process to select moving group stars, uses a better estimation of the density distribution of the total sample and field stars, and classifies moving group stars using all the available informati on. It is applied here to an accurately selected sample of early-type stars wit h known radial velocities and Stromgren photometry. Astrometric data are ta ken from the HIPPARCOS catalogue (ESA, 1997), which results in an important decrease in the observational errors with respect to ground-based data, an d ensures the uniformity of the observed data. Both the improvement of our method and the use of precise astrometric data have allowed us not only to confirm the existence of classical moving groups, but also to detect finer structures that in several cases can be related to kinematic properties of nearby open clusters or associations.