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

4. Conclusions

We compute the early PMS and MS phases of 0.4 to 5 stars with , 0.02 and 0.04 ². We compare our models with those provided by DM94 and S94 and we showed important deviations in PMS tracks for stellar masses below 0.5 , most probably due to differences in the treatment of electrostatic corrections in cool degenerated matter.

Special attention has been paid to the conversion relations from theoretical (log L, log ) HRD to observational (magnitude-color) diagrams. We adopted calibration relations based on empirical methods. Uncertainties are still large in the O-B and M star domains.

Comparison between our predicted and observed location of the ZAMS and TAMS in the versus (B-V) and versus (V-I) diagrams is very good for masses above 0.5 . Discrepancy observed below cannot be fully accounted for by the uncertainties in the color to temperature conversion relations and could be due to our improper treatment of electrostatic corrections for such low-mass objects. In the versus (V-I) diagram, our 10⁸ yr isochrone is in good qualitative agreement with the low-mass end distribution of the Pleiades and Per clusters.

From the versus (B-V) turn-off, we find ages of for Per cluster and between and yr for the Pleiades cluster. These ages are somewhat lower than previous determinations including overshooting contrary to us.

We generate synthetic HRD diagrams including different IMF, star formation rates and accounting for binary stars and observational uncertainties. The synthetic HRD diagrams prove to be a useful tool to study the morphology of young clusters and constrain the above-mentioned parameters. We apply this procedure to the Pleiades cluster. We best reproduce its morphology with an age dispersion of yr and a fraction of binary systems with companion mass greater than 0.5 for F-G stars of 40% 5%. We also favor the multiple power law IMF but we cannot clearly discriminate at this point between different IMF relations because of possible incompleteness of observational data.

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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