Vertical velocities from proper motions of red clump giants

Abstract

We derive the vertical velocities of disk stars in the range of Galactocentric radii of R=5-16 kpc within 2 kpc in height from the Galactic plane. This kinematic information is connected to dynamical aspects in the formation and evolution of the Milky Way, such as the passage of satellites and vertical resonance and determines whether the warp is a long-lived or a transient feature. We used the proper motions of the PPMXL survey, correcting of systematic errors with the reference of quasars. From the color-magnitude diagram K versus (J-K) we selected the standard candles corresponding to red clump giants and used the information of their proper motions to build a map of the vertical motions of our Galaxy. We derived the kinematics of the warp both analytically and through a particle simulation to fit these data. Complementarily, we also carried out the same analysis with red clump giants spectroscopically selected with APOGEE data, and we predict the improvements in accuracy that will be reached with future Gaia data. A simple model of warp with the height of the disk z_w(R,phi)=gamma (R-R_sun) sin(phi-phi_w) fits the vertical motions if d(gamma)/dt/gamma=-34+/-17 Gyr^{-1}; the contribution to d(gamma)/dt comes from the southern warp and is negligible in the north. The vertical motion in the warp apparently indicates that the main S-shaped structure of the warp is a long-lived feature, whereas the perturbation that produces an irregularity in the southern part is most likely a transient phenomenon. With the use of the Gaia end-of-mission products together with spectroscopically classified red clump giants, the precision in vertical motions can be increased by an order of magnitude at least.