Revised Strömgren metallicity calibration for red giants

A new calibration of the Strömgren (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 ω Centauri, M22, and M55 as well as field giants from the list of AnthonyTwarog & 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]< 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 Strömgren 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.