THE ABSOLUTE MAGNITUDE OF FIELD METAL-POOR HORIZONTAL-BRANCH STARS

Hipparcos satellite parallaxes for 22 metal-poor field horizontal bran ch stars with V-0<9 are used to derive their absolute magnitude. The w eighted mean value is M-v= + 0.69 +/- 0.10 for an average metallicity of [Fe/H]= -1.41; a somewhat brighter average magnitude of M-v = + 0.6 0 +/- 0.12 for an average metallicity of [Fe/H] = -1.51 is obtained by eliminating HD 17072, which might be on the first ascent of the giant branch rather than on the horizontal branch. The present values agree with the determinations based on proper motions and application of th e Baade-Wesselink method to field RR Lyraes; they are 0.1-0.2 mag fain ter than those based on calibration of cluster distances obtained by u sing local subdwarfs and on alternative distance calibrators for the L arge Magellanic Cloud (LMC). The possibility that there is a real diff erence between the luminosity of the horizontal branch for clusters an d the field is briefly commented on.