Revised Stroemgren metallicity calibration for red giants

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A new calibration of the Stroemgren (b-y),m₁ diagram in terms of iron abundance of red giants is presented. This calibration is based on a homogeneous sample of giants in the globular clusters omega Centauri, M22, and M55 as well as field giants from the list of Anthony-Twarog & Twarog (1998). Towards high metallicities, the new calibration is connected to a previous calibration by Grebel & Richtler (1992), which was unsatisfactory for iron abudances lower than -1.0 dex. The revised calibration is valid for CN-weak/normal red giants in the abundance range of -2.0 < [Fe/H]r < 0.0 dex, and a color range of 0.5 < (b-y) < 1.1 mag. As shown for red giants in ω Centauri, CN-weak stars with Stroemgren metallicities higher than -1.0 dex cannot be distinguished in the (b-y),m₁ diagram from stars with lower iron abundances but higher CN band strengths.