Vertical distribution of Galactic disk stars : ⋆ II . The surface mass density in the Galactic plane

High resolution spectra data of red clump stars towards the NGP have been obtained with the high resolution spectrograph Elodie at OHP for Tycho-2 selected stars. Combined with Hipparcos local analogues, we determine both the gravitational force law perpendicular to the Galactic plane, and the total surface mass density and thickness of the Galactic disk. The surface mass density of the Galactic disk within 800 pc derived from this analysis is Σ (|z| <800 pc) = 76 M⊙pc −2 and, removing the dark halo contribution, the total disk mass density is Σ0 = 67 M⊙pc −2 at solar radius. The thickness of the total disk mass distribution is dynamically measured for the first time and is found to be 390 +330 −120 pc in relative agreement with the old stellar disk scaleheight. All dynamical evidences concerning the structure of the disk (its local volume density –i.e. the Oort limit–, its surface density and its thickness) are compatible with our current knowledge of the corresponding stellar disk properties.