Surface Gravities of Metal-Poor Stars Derived from HIPPARCOS Parallaxes

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Surface gravities of metal-poor stars have been derived from the basic relations g ~ M / R^2 and R^2 ~L / T_(eff)^4 using Hipparcos parallaxes to determine luminosities. The method has been applied to 54 metal-poor stars with [Fe/H] < -1.0 and sigma_pi / pi < 0.2 selected from the photometric study of Schuster & Nissen (1989). T_eff is determined from the Stroemgren color index (b-y) and the mass is derived by fitting 14 Gyr isochrones to the distribution of the stars in the M_V - T_eff plane. The gravities have statistical errors sigma(log g) < 0.20 dex and systematic errors within +/- 0.10 dex. Comparison with gravities, derived from the requirement that FeI and FeII lines should provide the same iron abundance, shows that such `spectroscopic' gravities are often in error by a factor of 2 to 3. The importance of the parallax-based gravities for setting the metallicity scale of metal-poor stars and for deriving accurate abundances of key elements in nucleosynthesis studies is emphasized.

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