DO THE NEARBY BLUE HORIZONTAL BRANCH STARS BELONG TO THE THICK DISK OR THE HALO?

Abstract

We study the Milky Way region (|Z|< 3.0 kpc) where the thick disk and inner halo overlap by using the kinematics of local blue horizontal branch (BHB) stars (within 1 kpc) and new samples of BHB stars and A-type stars from the Century Survey. We derive Galactic U, V, and W velocities for these BHB and A-type star samples using proper motions from the NOMAD catalog. The mean velocities and the velocity dispersions of the BHB samples (|Z| < 3 kpc) are characteristic of the halo, while those of the Century Survey A-type stars are characteristic of the thick disk. There is no evidence from our samples that the BHB stars rotate with the thick disk in the region |Z| < 3 kpc. Nearly a third of the nearby local RR Lyrae stars have disk kinematics and are more metal rich than [Fe/H] ~ –1. Only a few percent of the Century Survey BHB (CBHB) stars have these properties. Only one nearby BHB star (HD 130201) is likely to be such a disk star but selection based on high proper motions will have tended to exclude such stars from the local sample. The scale height derived from a sample of local RR Lyrae stars agrees with that of the CBHB stars. The local samples of BHB stars and metal-weak red giants are too incomplete for a similar comparison.