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

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

I. Saviane ¹, A. Rosenberg ^2,3, G. Piotto ¹ and A. Aparicio <sup>4

1</sup> 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

Abstract

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 bands. To this aim, two key points are developed in the course of the analysis.

(a) Using our photometric 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, and indices in the 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 , , and indices are consistent with existing relations.

(b) Using a grid of selected RGB fiducial points, we define 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). 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 coordinates into 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

SIMBAD Objects

Contents

• 1. Introduction
  • 1.1. Metallicity indices
  • 1.2. Old stellar populations in Local Group galaxies
  • 1.3. Layout of the paper
• 2. The observational sample
• 3. Metallicity indices
  • 3.1. Definitions
  • 3.2. Measurement procedures
  • 3.3. Internal consistency checks
• 4. New standard globular cluster giant branches
  • 4.1. Selection of clusters
  • 4.2. Determination of the fiducial loci
  • 4.3. Analytic fits to the fiducial loci
• 5. Calibration of the indices. Introduction
• 6. Calibration of the indices. Discussion
  • 6.1. S
  • 6.2.
  • 6.3.
  • 6.4. The family and
• 7. A test of the "model" RGBs; comparison with the observed [Fe/H] indices
• 8. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 2000

Online publication: April 3, 2000
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