The kinematics of late-type stars in the solar cylinder studied with SDSS data

Abstract

We study the velocity distribution of Milky Way disk stars in a kiloparsec-sized region around the Sun, based on ∼2 million M-type stars from DR7 of SDSS, which have newly re-calibrated absolute proper motions from combining SDSS positions with the USNO-B catalogue. We estimate photometric distances to all stars, accurate to ∼20%, and combine them with the proper motions to derive tangential velocities for this kinematically unbiased sample of stars. Based on a statistical deprojection method we then derive the vertical profiles (to heights of Z = 800 pc above the disk plane) for the first and second moments of the three-dimensional stellar velocity distribution. We find that 〈W〉 = -7 ± 1 km s-1 and 〈U〉 = -9 ± 1 km s-1, independent of height above the mid-plane, reflecting the Sun's motion with respect to the local standard of rest. In contrast, 〈V〉 changes distinctly from -20 ± 2 km s-1 in the mid-plane to 〈V〉 = -32 km s-1 at Z = 800 pc, reflecting an asymmetric drift of the stellar mean velocity that increases with height. All three components of the M-star velocity dispersion show a strong linear rise away from the mid-plane, most notably σZZ, which grows from 18 km s-1 (Z = 0) to 40 km s-1 (at Z = 800 pc). We determine the orientation of the velocity ellipsoid, and find a significant vertex deviation of 20°-25°, which decreases only slightly to heights of Z = 800 pc. Away from the mid-plane, our sample exhibits a remarkably large tilt of the velocity ellipsoid toward the Galactic plane, which reaches 20° at Z = 800 pc and which is not easily explained. Finally, we determine the ratio σφφ2/σRR2 near the mid-plane, which in the epicyclic approximation implies an almost perfectly flat rotation curve at the solar radius. © 2009. The American Astronomical Society. All rights reserved.