The BHB stars in the Survey Fields of Rodgers et al. (1993): New Observations and Comparisons with other Recent Surveys

TL;DR

This study analyzes blue horizontal branch and RR Lyrae stars in specific survey fields, comparing their distributions and velocities with other surveys, revealing insights into the structure and composition of the Milky Way's halo.

Contribution

It provides new observations of BHB and RR Lyrae stars, assesses survey efficiencies, and clarifies the spatial and kinematic properties of the Milky Way's inner halo.

Findings

At least twice as many BHB stars as RR Lyrae stars in the northern field.

No evidence of an anomalous thick disk in the southern field.

BHB stars mostly belong to a flattened, non-rotating inner halo.

Abstract

We study blue horizontal branch (BHB) and RR Lyrae stars in the Rogers et al. (1993a) fields and compare their velocity and density distributions with other surveys in the same part of the sky. Photometric data are given for 176 early-type stars in the northern field. We identify fourteen BHB stars and four possible BHB stars, and determine the selection efficiency of the Century Survey, the HK Survey, and the SDSS survey for BHB stars. We give light curves and \gamma -radial velocities for three type-ab RR Lyrae stars in the northern field; comparison with the nearby LONEOS Survey shows that there is likely to be an equal number of lower-amplitude type-ab RR Lyrae stars that we do not find. There are therefore at least twice as many BHB stars as type-ab RR Lyrae stars in the northern field--similar to the ratio in the solar neighborhood. The velocity distribution of the southern field shows no evidence for an anomalous thick disk that was found by Gilmore et al. (2002); the halo velocity peaks at a slightly prograde rotational velocity but there is also a significant retrograde halo component in this field. The velocity distribution in the northern field shows no evidence of Galactic rotation for |Z|>4 kpc and a slight prograde motion for |Z|<4 kpc. The space densities of BHB stars in the northern field agree with an extrapolation of the power-law distribution recently derived by de Propris et al. (2010). For |Z|<4 kpc, however, we observe an excess of BHB stars compared with this power-law. We conclude that these BHB stars mostly belong to a spatially flattened, non-rotating inner halo component of the Milky Way in confirmation of the Kinman et al. (2009) analysis of Century Survey BHB stars.