Kinematics of the Galaxy from Hipparcos Proper Motions

Astronomy and Astrophysics – Astronomy

Scientific paper

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Astrometry, Galaxy: Fundamental Parameters, Galaxy: Kinematics And Dynamics, Galaxy: Solar Neighborhood

Scientific paper

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.

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