ON A NEW THEORETICAL CALIBRATION OF THE STROMGREN hk METALLICITY INDEX: NGC 6522 AS A FIRST TEST CASE

We present a new theoretical calibration of the Stromgren 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, v-y) to 0.49 (hk, 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.