We present a new theoretical calibration of the Stroemgren metallicity index
hk by 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 NGC6522,
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.Comment: Accepted for publication on The Astrophysical Journal Letter
