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Astron. Astrophys. 355, 966-978 (2000)

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The red giant branches of Galactic globular clusters in the [FORMULA] plane: metallicity indices and morphology

I. Saviane 1, A. Rosenberg 2,3, G. Piotto 1 and A. Aparicio 4

1 Dipartimento di Astronomia, Università di Padova, Vicolo dell'Osservatorio 5, 35122 Padova, Italy
2 Telescopio Nazionale Galileo, Vicolo dell'Osservatorio 5, 35122 Padova, Italy
3 Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5, 35122 Padova, Italy
4 Instituto de Astrofísica de Canarias, Via Lactea, 38200 La Laguna, Tenerife, Spain

Received 6 August 1999 / Accepted 21 December 1999


The purpose of this study is to carry out a thorough investigation of the changes in morphology of the red giant branch (RGB) of Galactic globular clusters (GGC) as a function of metallicity, in the [FORMULA] bands. To this aim, two key points are developed in the course of the analysis.

(a) Using our photometric [FORMULA] database for Galactic globular clusters (the largest homogeneous data sample to date; Rosenberg et al. 1999a) we measure a complete set of metallicity indices, based on the morphology and position of the red-giant branch. In particular, we provide here the first calibration of the S, [FORMULA] and [FORMULA] indices in the [FORMULA] 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 Carretta & Gratton (1997) scales. Our new calibrations of the [FORMULA], [FORMULA], [FORMULA] and [FORMULA] indices are consistent with existing relations.

(b) Using a grid of selected RGB fiducial points, we define a function in the [FORMULA] space which is able to reproduce the whole set of 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 metallicity and distances within the Local Group, since it allows to easily map [FORMULA] coordinates into [FORMULA] ones. To this aim the "synthetic" RGB distribution is generated both for the currently used Lee et al. (1990) distance scale, and for the most recent results on the RR Lyr distance scale.

Key words: stars: abundances – stars: Population II – galaxies: abundances – Galaxy: globular clusters: general

Send offprint requests to: Ivo Saviane

Correspondence to: saviane@pd.astro.it

© European Southern Observatory (ESO) 2000

Online publication: April 3, 2000