Galactic Interior Motions Derived from HIPPARCOS Proper Motions. I. Young Disk Population

Abstract

Analyzing Hipparcos proper motions of 1352 O-B5 stars other than the Gould belt stars, which are representative of the young disk population, we have found a clear stellar warping motion that is a systematic rotation +3.8 +/- 1.1 km s^-1 kpc^-1 of stars about the axis pointing to the Galactic center in the sense of increasing the inclination of the H i warp, and a remarkably large Galactic rotation of (V_0)_O-B5 = 268.7 +/- 11.9 km s^-1, compared with the IAU recommendation (V_0 = 220 +/- 20 km s^-1), given the Galactocentric distance of the Sun R_0 = 8.5 kpc. We have carried out a similar analysis for 170 Hipparcos Cepheids as well and obtained a solution that apparently shows neither the rotation nor the shear, other than the Oort differential rotation. The Cepheids are purely rotating around the Galactic center with the velocity (V_0)_Cep = 243.3 +/- 12.0 km s^-1, again larger than the IAU recommendation. However, the solution for the Cepheids, which are considered the same young disk population as the O-B5 stars, seems to be different from that for the O-B5 stars. In order to find the above systematic stellar motion as generally as possible, we apply the Ogorodnikov-Milne model to the Hipparcos proper motions, and solve for nine kinematic parameters: three components of solar motion, three components of vorticity (rotation), and three components of strain velocity (shear). This paper discusses the systematic difference in the proper-motion systems between the ground-based catalogs and the Hipparcos Catalogue, in order to examine one of the main causes of the large difference between the present (270 km s^-1) and previous (220 km s^-1) Galactic rotations.