ON THE CALIBRATION OF RED-GIANT BRANCH METALLICITY INDICATORS IN GLOBULAR-CLUSTERS - NEW RELATIONS BASED ON AN IMPROVED ABUNDANCE SCALE

We present an improved calibration of photometric metallicity indicato rs, derived from the new metallicity scale for Globular Clusters prese nted by Carretta & Gratton (1997) and based on direct high resolution spectroscopy of 160 stars in 24 globular clusters. We have carefully r ecalibrated the traditional abundance indices based upon the red giant branch (RGB) morphology, both in the V, B - V and V,V - I planes, nam ely the dereddened colour at the luminosity level of the horizontal br anch (HB), and the magnitude difference between the HB and the RBG at a given dereddened colour. Finally, we give new accurate relations to employ in the Simultaneous Metallicity Reddening method by Sarajedini (1994), also tied to the Carretta & Gratton (1997) abundance scale.