We present a new theoretical calibration of the Strömgren metallicity index hk using \alpha -enhanced evolutionary models transformed into the observational plane by using atmosphere models with the same chemical mixture .
We apply the new Metallicity–Index–Color ( MIC ) relations to a sample of 85 field red giants ( RGs ) and find that the difference between photometric estimates and spectroscopic measurements is on average smaller than 0.1 dex with a dispersion of \sigma = 0.19 dex .
The outcome is the same if we apply the MIC relations to a sample of eight RGs in the bulge globular cluster NGC 6522 , but the standard deviation ranges from 0.26 ( hk, \hbox { \it v - - y\ / } ) to 0.49 ( hk, \hbox { \it u - - y\ / } ) .
The difference is mainly caused by a difference in photometric accuracy .
The new MIC relations based on the Ca–y color provide metallicities systematically more metal-rich than the spectroscopic ones .
We found that the Ca -band is affected by Ca abundance and possibly by chromospheric activity .