THE DISTRIBUTION OF NEARBY STARS IN PHASE SPACE MAPPED BY HIPPARCOS: THE POTENTIAL WELL AND LOCAL DYNAMICAL MASS M. Cr'ez'e

Hipparcos data provide the first, volume limited and absolute magnitude limited homogeneous tracer of stellar density and velocity distributions in the solar neighbourhood. The density of A-type stars more luminous than M v = 2:5 can be accurately mapped within a sphere of 125 pc radius, while proper motions in galactic latitude provide the vertical velocity distribution near the galactic plane. The potential well across the galactic plane is traced practically hypothesis-free and model-free. The local dynamical density comes out as ae 0 = 0:076 \Sigma 0:015M fi pc \Gamma3 a value well below all previous determinations leaving no room for any disk shaped component of dark matter. Key words: Galaxy: kinematics and dynamics, disc density. 1. INTRODUCTION The paramount contribution of Hipparcos to astrophysics is to provide direct geometric distances to stars. That is to tie firmly astrophysics to laboratory physics. Not only does it bring some individuals, some stars of such or such ki..