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

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 γ-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.

24 pages, accepted in AJ

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• The Kinematic Properties of BHB and RR Lyrae stars towards the Anticentre and the North Galactic Pole: The Transition between the Inner and the Outer Halo
• Stellar streams around the Magellanic Clouds in 4D