Detection of the Halo Component of the Field sdB Population

Abstract

A new, complete sample of 20 faint sdB stars isolated from the US survey is reported. Because the sample contains stars with magnitudes as faint as B = 18, it is uniquely situated to probe the field sdB spatial distribution at relatively large distances out of the Galactic plane. A V/Vmax analysis provides strong evidence that the majority of these stars are not part of a disklike stellar density distribution. Using a statistical treatment for population membership, a further V/Vmax analysis shows that the spatial distribution of the US sdB stars is consistent with a combined disk-plus-halo stellar density law. This analysis also provides a preliminary value for the sdB halo-to-disk density ratio in the solar neighborhood that is larger by a factor of 7 than that for the field stars in general. It is likely that even relatively bright samples of sdB stars, such as those isolated from the PG survey, contain significant numbers of halo stars.