Galactic kinematics of Cepheids from Hipparcos proper motions

Abstract

The Hipparcos proper motions of 220 Galactic Cepheids, together with relevant ground-based photometry, have been analysed. The effects of Galactic rotation are very clearly seen. Mean values of the Oort constants, A = 14.82 ± 0.84 km s-1 kpc-1, and B = -12.37 ± 0.64 km s-1 kpc-1, and of the angular velocity of circular rotation at the Sun, Ωo=27.19 ± 0.87 km s-1 kpc-1, are derived. A comparison of the value of A with values derived from recent radial velocity solutions confirms, within the errors, the zero-points of the period-luminosity and period-luminosity-colour relations derived directly from the Hipparcos trigonometrical parallaxes of the same stars. The proper motion results suggest that the Galactic rotation curve is declining slowly at the solar distance from the Galactic Centre [(dΘ/dR)o= -2.4 ± 1.2 km s-1 kpc-1]. The component of the solar motion towards the North Galactic Pole is found to be + 7.61 ± 0.64 km s-1. Based on the increased distance scale deduced in the present paper, the distance to the Galactic Centre derived in a previous radial velocity study is increased to Ro=8.5 ±0.5 kpc. © 1997 RAS.