The red giant branches of Galactic globular clusters in the [(V-I)(0), M-V] plane: metallicity indices and morphology

The purpose of this study is to carry out a thorough investigation of the c hanges in morphology of the red giant branch (RGB) of Galactic globular clu sters (GGC) as a function of metallicity, in the V, I bands. To this aim, t wo key points are developed in the course of the analysis. (a) Using our photometric V, I database for Galactic globular clusters (the largest homogeneous data sample to date; Rosenberg et al. 1999a) we measur e a complete set of metallicity indices, based on the morphology and positi on of the red-giant branch. In particular, we provide here the first calibr ation of the S, Delta V-1.1 and Delta V-1.4 indices in the (V - I, V) plane . We show that our indices are internally consistent, and we calibrate each index in terms of metallicity, both on the Zinn & West (1984) and the Carr etta & Gratton (1997) scales. Our new calibrations of the (V - I)(0,g), Del ta V-1.2, (V - I)(-3.0) and (V - I)(-3.5) indices are consistent with exist ing relations. (b) Using a grid of selected RGB fiducial points, Me define a function in t he (V - I)(0), M-I, [Fe/H] space which is able to reproduce the whole set o f GGC giant branches in terms of a single parameter (the metallicity). As a first test, we show that the function is able to predict the correct trend of our observed indices with metallicity. The usage of this function will improve the current determinations of metal licity and distances within the Local Group, since it allows to easily map (V - I)(0), M-I coordinates into [Fe/H], M-I ones. To this aim the "synthet ic" RGB distribution is generated both for the currently used Lee et al. (1 990) distance scale, and for the most recent results on the RR Lyr distance scale.