Do the nearby BHB stars belong to the Thick Disk or the Halo?

TL;DR

This study investigates whether nearby blue horizontal branch stars belong to the Milky Way's thick disk or halo by analyzing their kinematics and metallicities, finding most are characteristic of the halo.

Contribution

It provides new kinematic and metallicity data for BHB stars, clarifying their association with the Galactic halo rather than the thick disk.

Findings

Most BHB stars have halo-like velocities and dispersions.

Few BHB stars show disk-like rotation or metallicity.

Nearby RR Lyrae stars mostly exhibit disk kinematics.

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,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 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 Century Survey BHB stars. The local samples of BHB stars and metal-weak red giants are too incomplete for a similar comparison.