Vertical distribution of Galactic disk stars ⋆

Nearly 400 Tycho-2 stars have been observed in a 720 square degree field in the direction of the North Galactic Pole with the high resolution echelle spectrograph ELODIE. Absolute magnitudes, effective temperatures, gravities and metallicities have been estimated, as well as distances and 3D velocities. Most of these stars are clump giants and span typical distances from 200 pc to 800 pc to the galactic mid-plane. This new sample, free of any kinematical and metallicity bias, is used to investigate the vertical distribution of disk stars. The old thin disk and thick disk populations are deconvolved from the velocity-metallicity distribution of the sample and their parameters are determined. The thick disk is found to have a moderate rotational lag of −51 ± 5 km s −1 with respect to the Sun with velocity ellipsoid (σU , σV , σW) = (63 ± 6, 39 ± 4, 39 ± 4) km s −1 , mean metallicity of [Fe/H] =−0.48 ± 0.05 and a high local normalization of 15 ± 7%. Combining this NGP sample with a local sample of giant stars from the Hipparcos catalogue, the orientation of the velocity ellipsoid is investigated as a function of distance to the plane and metallicity. We find no vertex deviation for old stars, consistent with an axisymmetric Galaxy. Paper II is devoted to the dynamical analysis of the sample, puting new constraints on the vertical force perpendicular to the galactic plane and on the total mass density in the galactic plane.