Synthetic Hertzsprung-Russell Diagrams Of Open Clusters

New evolutionary models for pre-main sequence (PMS) stars (0.4M(circle dot)less than or equal to M less than or equal to 5M(circle dot)) of three different metallicities (Z = 0.005, Z = 0.02, and Z = 0.04) 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 0.5M(circle dot), 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 My versus (B-V) diagram, we estimate ages of 4 10(7) for alpha Per cluster and between 8 10(7) and 10(8) 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 (sigma(t) similar or equal to 3 x 10(7) yr) and a binary fraction for F-G stars of 40 % +/- 5 %.