THE DISTRIBUTION OF NEARBY STARS IN PHASE-SPACE MAPPED BY HIPPARCOS I- THE POTENTIAL WELL AND LOCAL DYNAMICAL MASS

Hipparcos data provide the first, volume limited and absolute magnitud e limited homogeneous tracer of stellar density and velocity distribut ions in the solar neighbourhood. The density of A-type stars more lumi nous than M-v = 2.5 can be accurately mapped within a sphere of 125 pc radius, while proper motions in galactic latitude provide the vertica l velocity distribution near the galactic plane. The potential well ac ross the galactic plane is traced practically hypothesis-free and mode l-free. The local dynamical density comes out as rho(o) = 0.076 +/- 0. 015 M-. pc(-3) a value well below all previous determinations leaving no room for any disk shaped component of dark matter.