Kinematics of RHB stars to trace the structure of the Galaxy

Abstract

Red horizontal-branch (RHB) stars have been selected from the Hipparcos catalogue to investigate their kinematics and spatial distribution. Hipparcos parallaxes, literature radial velocities and Hipparcos proper motions, together with models for the gravitational potential of the Milky Way allow a calculation of the actual velocity vectors and the orbits of the RHB stars. The velocity characteristics are used to define a halo population sample (HPS) in the collection of RHBs. The orbits lead statistically to an overall z-distance probability distribution, showing that the RHBs exhibit two populations, a disk one having a scale height of h_disk ≃ 0.6 kpc and a halo one of ≃4 kpc. We have investigated the influence on our results of parallax accuracy and of a demarcation line in the HRD between the RHB and the red-giant (RG) star region. Neither of them show marked effects. We have performed the orbit analysis using the potential model of Allen & Santillan as well as of Dehnen & Binney. The results differ only slightly for the disk population, showing that these potential models are not a critical part of such orbit investigations. RHB scale height values are smaller than those found earlier for sdB stars, most likely because the samples of stars used had different spatial distributions a priori. The data do not allow us to specify a trend in the kinematic behaviour of star types along the horizontal branch.