Astron. Astrophys. 324, 556-565 (1997)

Synthetic Hertzsprung-Russell diagrams of open clusters

Lionel Siess , Manuel Forestini and Catherine Dougados

Laboratoire d'Astrophysique, Observatoire de Grenoble, Université Joseph Fourier, BP 53, F-38041 Grenoble Cedex 9, France

Received 22 July 1996 / Accepted 28 January 1997

Abstract

New evolutionary models for pre-main sequence (PMS) stars (0.4 M 5 ) of three different metallicities (, , and ) have been computed. Improvements have been made in the constitutive physics with the use of new opacity tables at low temperatures and new atmospheric models. We first compare our mass tracks with other recent PMS models and show that important deviations occur below 0.5 , due to different treatments of the electrostatic corrections in cool degenerated matter. We first check the ability of our models to reproduce the ZAMS location and the low-mass end of the isochrones. From the turn-off in the versus (B-V) diagram, we estimate ages of for Per cluster and between and yr for the Pleiades, ages comparable or lower than recent determinations including overshooting.

These evolutionary models are used to generate synthetic Hertzsprung-Russell Diagrams (HRD). We show that each of the different parameters, namely the initial mass function (IMF), the star formation rate (SFR) and the binary fraction affect in a specific way the morphology of the derived HRD. We propose to use these synthetic diagrams to constrain some of these quantities in observed open clusters. Applying this procedure to the Pleiades cluster, the observed morphology is best reproduced by invoking a large age dispersion ( 3 10⁷ yr) and a binary fraction for F-G stars of 40 % 5 %.

Key words: stars: evolution of - stars: pre-main sequence - stars: Hertzsprung-Russell diagram - stars: fundamental parameters

Send offprint requests to: Lionel Siess

Contents

• 1. Introduction
• 2. HR diagrams: computed models and comparisons with observations
  • 2.1. New PMS models
    • 2.1.1. Physical ingredients
    • 2.1.2. Computed models
    • 2.1.3. Comparisons with other works
  • 2.2. From theoretical to observational HRD
  • 2.3. The ZAMS location
  • 2.4. Isochrones and cluster dating
• 3. Synthetic HRD and comparison with standard clusters
  • 3.1. The overall problem
    • 3.1.1. The transformation method
    • 3.1.2. The Initial Mass Function
    • 3.1.3. The binarity rate
    • 3.1.4. Observational uncertainties
  • 3.2. Influence of the parameters
  • 3.3. Application to the Pleiades
• 4. Conclusions
• Acknowledgements
• Appendix
• References

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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