A multicolor survey of absolute proper motions: galactic structure and kinematics in the direction of galactic centre at intermediate latitude

Abstract

Bienayme et al. (1992) have published a magnitude-limited sample of proper motions for stars at the intermediate galactic latitude (l = 3°, b = 47° ; α₁₉₅₀ = 15^h18^m δ₁₉₅₀ = +02°16') for a 2 square degree field. In their study there was a lack of an accurate absolute astrometric reference due to very small number of galaxies or extragalactic objects in the field. However, since Bienayme et al. paper, more deep photographic plates and CCD standards have been obtained. Here we have derived a new photographic photometry and proper motions for ~20000 stars with completeness to V~18 in the same direction for a 15.5 square degree field. The combination of four glass copies of the Palomar Observatory Sky Survey (i.e. POSS 1402 E&O and POSS 1429 E&O) has been used as a first epoch for proper motion determination. The random error of the proper motions is approximately 0."3century^-1 to V~17. The photometric accuracy ranges between 0.07 to 0.10 in the V, B and U bands. We stress the importance of the magnitude and color effects in astrometric surveys of field stars. Using color-magnitude diagrams of a few cluster member stars, a new distance of 6.9±0.5kpc is derived for M5 and 20.3±0.8kpc for Pal 5 globular clusters. This is in good agreement with other determinations. We have analyzed the components of U+W and V galactic space motions resulting from the accurate proper motions survey. No dependence with z distance is found in the asymmetric drift of the thick disk population. New estimates of the parameters of the velocity ellipsoid have been derived for the thin disk, thick disk and halo populations of the Galaxy.