Kinematics of the Galaxy from Hipparcos Proper Motions

Abstract

From Hipparcos proper motions, we have analysed the local kinematics of the Milky Way from young O-B5 stars, classical Cepheids, and the late-type K-M giants. The components of solar motion along the direction pointing to the galactic center from the Sun u_0 and along the direction pointing to the north galactic pole w_0 are approximately 10 km s^-1 and 8 km s^-1 for all types of stars, while the component pointing to the direction of the galactic rotation varies over a wide range from 13 to 21 km s^-1, depending on the type of stars. The galactic rotation derived from O-B5 stars is 269 +/- 12 km s^-1, which is apparently larger than that recommended by the IAU in 1985 (220 km s^-1). Analysing the proper-motion data of O-B5 stars, we clarified a clear stellar warping motion that is a systematic rotation, partialV_z /partial theta /R = -3.79 +/- 1.05 km s^-1 kpc^-1, of stars around the axis pointing to the galactic center. Taking the kinematical solution from the K-M giants into account, and considering the possible systematic errors remaining in the Hipparcos proper-motion system, the warping motion of O-B5 stars might even be pronounced. The solution from classical Cepheids seems to show a slight contracting motion of the Galaxy in the solar neighbourhood, partialV _theta /partial theta /R = -2.60 +/- 1.07 km s^-1 kpc^-1, which is along the solar circle. By comparing the thus-derived results, the kinematical discrepancies between each subset of stars are analysed, and the possible reasons for the kinematical differences are discussed.