Revised Stromgren metallicity calibration for red giants

A new calibration of the Stromgren (b - y), mi diagram in terms of iron abu ndance of red giants is presented. This calibration is based on a homogeneo us sample of giants in the globular clusters omega Centauri, M 22, and M 55 as well as field giants from the list of Anthony-Twarog & Twarog (1998). T owards high metallicities, the new calibration is connected to a previous c alibration by Grebel & Richtler (1992), which was unsatisfactory for iron a budances 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 co lor range of 0.5 < (b - y) < 1.1 mag. As shown for red giants in omega Centauri, CN-weak stars with Stromgren met allicities higher than -1.0 dex cannot be distinguished in the (b - y), m(1 ) diagram from stars with lower iron abundances but higher CN band strength s.