THE KINEMATICS OF LATE-TYPE STARS IN THE SOLAR CYLINDER STUDIED WITH SDSS DATA

We study the velocity distribution of Milky Way disk stars in a kiloparsec-sized region around the Sun, based on {approx}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 {approx}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{sup -1} and (U) = -9 {+-} 1 km s{sup -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{sup -1} in the mid-plane to (V) = -32 km s{sup -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,more » most notably {sigma} {sub ZZ}, which grows from 18 km s{sup -1} (Z = 0) to 40 km s{sup -1} (at Z = 800 pc). We determine the orientation of the velocity ellipsoid, and find a significant vertex deviation of 20{sup 0}-25{sup 0}, 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{sup 0} at Z = 800 pc and which is not easily explained. Finally, we determine the ratio {sigma}{sup 2} {sub {phi}}{sub {phi}}/{sigma}{sup 2} {sub RR} near the mid-plane, which in the epicyclic approximation implies an almost perfectly flat rotation curve at the solar radius.« less