In this work, we have provided the first calibration of a few metallicity indices in the plane, namely the indices S, and . Calibrations on both the Zinn & West (1984) and Carretta & Gratton (1997) scales have been obtained. The metallicity indices , , and have been also calibrated on both scales, and we have shown that our new relations are consistent with existing ones. In the case of the latter two indices, we have obtained the first calibration on the CG scale; for both scales, we have also obtained the first calibration that takes into account new results on the RR Lyr distances. The accuracy of the calibrations is generally better than 0.2 dex, regardless of the metallicity scale that is used.
Our results are an improvement over previous calibrations, since a new approach in the definition of the RGB is used, and since our formulae are based on the largest homogeneous photometric database of Galactic globular clusters.
The availability of such database also allowed us a progress towards the definition of a standard description of the RGB morphology and location. We were able to obtain a function in the space which is able to reproduce the whole set of GGC giant branches in terms of a single parameter (the metallicity). We suggest that the usage of this function will improve the current determinations of metallicity and distances within the Local Group, extending the methods of Lee et al. (1993).
© European Southern Observatory (ESO) 2000
Online publication: April 3, 2000