Year: 2009
Type: article
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 σ ZZ , 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 σ 2 ϕϕ /σ 2 RR near the mid-plane, which in the epicyclic approximation implies an almost perfectly flat rotation curve at the solar radius.
Source: The Astronomical Journal
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: bronze
Funders: National Science Foundation, U.S. Department of Energy, National Aeronautics and Space Administration, Alfred P. Sloan Foundation, Princeton University, Ohio State University, University of Washington, Johns Hopkins University, University of Pittsburgh, Case Western Reserve University, Drexel University, Universität Basel, University of Portsmouth, New Mexico State University, Chinese Academy of Sciences, Max-Planck-Gesellschaft, Max-Planck-Institut für Astronomie, U.S. Naval Observatory, Fermilab, Los Alamos National Laboratory
OpenAlex ID: W2089973285