The absolute magnitude of field metal-poor horizontal branch stars

Abstract

HIPPARCOS satellite parallaxes for 22 metal-poor field horizontal branch stars with Vo<9 are used to derive their absolute magnitude. The weighted mean value is Mv=+0.69+/-0.10 for an average metallicity of [Fe/H]=-1.41; a somewhat brighter average magnitude of Mv=+0.60+/-0.12 for an average metallicity of [Fe/H]=-1.51 is obtained eliminating HD17072, that might be on the first ascent of the giant branch rather than on the horizontal branch. The present values agree with determinations based on proper motions and application of the Baade-Wesselink method to field RR Lyraes; they are from 0.1 to 0.2 mag fainter than those based on calibration of cluster distances obtained by using local subdwarfs, and on alternative distance calibrators for the LMC. The possibility that there is a real difference between the luminosity of the horizontal branch for clusters and the field is briefly commented.