Astron. Astrophys. 358, 943-955 (2000)

Comparison between observed and theoretical Red Giant Branch luminosity functions of galactic globular clusters ^*

M. Zoccali and G. Piotto

Dipartimento di Astronomia - Università di Padova, Padova, Italy (zoccali,piotto@pd.astro.it)

Received 25 January 2000 / Accepted 26 April 2000

Abstract

V-band luminosity functions (LF) have been obtained for the upper main-sequence (MS), sub-giant branch (SGB) and red giant branch (RGB) of 18 galactic globular clusters (GGC) from HST data. A comparison with four sets of theoretical models has been performed. In contrast with what was found in several previous works, a good general agreement has been found between the observed and theoretical LF at any metallicity [M/H]. Possible discrepancies at higher metallicity, in the upper part of the RGB, need to be confirmed with further observational data and by extending all the models to the most metal rich regime. The SGB shape has been used to set an upper limit to the cluster age, and consequently a lower limit on the cluster distance. A discussion on the still open problem of the mismatch between the observed and theoretical RGB bump location is also presented.

Key words: stars: distances stars: evolution stars: luminosity function, mass function stars: Population II Galaxy: globular clusters: general

* Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555, and on observations retrieved from the ESO ST-ECF Archive.

Send offprint requests to: M. Zoccali (zoccali@pd.astro.it)

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Contents

• 1. Introduction
• 2. The theoretical LFs
  • 2.1. Position of the SGB break
  • 2.2. Shape of the SGB break
  • 2.3. Choice of the reference models
• 3. The data
• 4. Comparison between observational and theoretical LFs
• 5. Discussion
  • 5.1. The SGB hump
  • 5.2. The ratio of MS/RGB stars
  • 5.3. The upper RGB
  • 5.4. Distances and Ages
  • 5.5. The RGB bump
• 6. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 2000

Online publication: June 20, 2000
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