2001A&A...371..115C

The structure of the central part of the UMa stellar kinematic group was considered according to the data from Hipparcos, Tycho and ACT catalogues. We have shown that in the space occupied by the nucleus there are several populations of stars. The first includes ten stars which are very compactly located in the proper motion diagram and have a normal proper motion distribution. The second group includes six stars with large kinematic difference and there are six field stars placed here too. The question about their coexistence in a common region of space remains open. The well-known stars Mizar and Alcor appear to belong to the different subsystems and do not belong to a unique stellar system. The published radial velocities, as a rule, are determined with insufficient accuracy compared to the errors of the currently available proper motions. A high precision in the radial velocities measurements is necessary for a thorough study of the kinematics of stars inside UMa star system.