Calibrating horizontal-branch stars with Hipparcos.

Abstract

Parallaxes from the Hipparcos satellite allow for the first time to calibrate parameters of horizontal-branch (HB) like stars based on geometric distances. The authors present absolute magnitudes and luminosities for 8 such stars. Using T_eff and log g values available from the literature, they also can calculate the mass of the HB stars. They find an average value of M_HB = 0.38±0.07 M_sun for stars with 7500 < T_eff < 9000 K, well below the value from stellar evolution theory, which predicts masses near 0.6 M_sun. Recent investigations of HB stars in globular clusters indicated similarly low mass values. The likely cause for the discrepancy of the mass values lies in the low gravities derived from Balmer line profiles.