Kinematics of Disk Stars in the Solar Neighbourhood

Using Hipparcos parallaxes and proper motions together with radial velocity complementary data (Coravel for late type stars and new ground-based data for early type stars) for several thousand B-F type stars, the velocity ellipsoid has been determined as a function of age. The variations with age of the ratio of the velocity dispersions, of the vertex deviation and the age-velocity dispersion relation (AVR) have been estimated. Our results connrm that mixing is not complete at about 0:8 ? 1 Gyr. The shape of the velocity ellipsoid changes with time, getting rounder from a maximum, thereafter remaining roughly constant; there is no dynamically signiicant evolution of the disk after about 4 ? 5 Gyr. The velocity dispersion in the direction perpendicular to the galactic plane saturates at about 15?17 km s ?1 for thin disk stars. The vertex deviation declines with age and remains near zero after 5 Gyr.