2006A&A...457..537C

A large sample with the full space motions drawn from the Hipparcos Catalogue allows us to select some nested subsamples containing an increasing number of thick disk stars. Segregated thick disk populations show a progressive decreasing vertex deviation according to a trend of axial symmetry, as well as an increasing velocity dispersion and mean age, which are consistent with a trend of steady state. Thus, we propose to extrapolate the oldest thick disk by using a quite general kinematic model, that is time dependent and with point-to-point axial symmetry, until reaching a hypothetical stellar population with axial symmetry, vanishing radial differential movement, and steady state. In the Galactic plane, the extrapolated population determines the point of the velocity space with differential rotation alone, that is with no net radial motion. The present work improves the estimation of the local radial mean velocity that we obtained in a previous work, where a raw linear extrapolation was performed. Now we complete the analysis by using a more precise estimation based on the kinematic model. The resulting no net radial motion point has radial heliocentric velocity U=-20±1km/s and rotation heliocentric velocity V=-82±2km/s. During the disk heating process, the disk populations lose angular momentum and radial motion while increasing the velocity dispersion. And, in particular, the thick disk loses its total radial motion before reaching the steady state, in an attempt to recover axiality. The new estimation stresses previous conclusions about the vanishing radial mean velocity of the stars associated with the Hyades-Pleiades supercluster, originated by large molecular clouds.