Properties of red giant branches of star clusters in the Magellanic Clouds and their relation with cluster metallicity. II. Mean photometric colors of cluster RGBs

Abstract

We derive new calibrations that relate the mean J-Ks photometric colors of red giant branch (RGB) stars at MKs=-5.5 and -5.0 with cluster metallicity. The new calibrations are derived using a sample of intermediate age (1--8 Gyr) clusters in the Large and Small Magellanic Clouds, with the JHKs photometry taken from the SIRIUS photometric survey of the Magellanic Clouds. Cluster metallicities are literature data, obtained either from the high resolution or infrared calcium triplet spectroscopy of individual RGB stars. We find systematic differences between the RGB color vs. metallicity relations derived in this work and those determined by Valenti et al. (2004), the latter ones obtained for a sample of old Galactic globular clusters. In terms of age, this discrepancy corresponds to ˜ 5 Gyr and therefore can be attributed to the age difference between the two cluster samples used in the derivation of the corresponding RGB color vs. metallicity relations.