Astron. Astrophys. 334, 953-968 (1998)

7. Isochrones and the age of young open clusters

We have computed isochrones from our models and have transformed them into the observational - plane. The tracks are transformed by adopting Kurucz & Castelli (1996, private communication) model atmosphere relations between and . The semiempirical transformations by Flower (1996) are similar, but provide bluer main sequence colors -by mag - at , while the main sequence colors become definitely too red at .

Detailed comparisons with data in the literature will be postponed to a subsequent work. Current open problems of datation demand a preliminary application of the computed isochrones to the Pleiades cluster. There are in fact some recent problems regarding this cluster. First of all, the Hipparcos parallaxes have revised its distance modulus to from the previously accepted value of and this revision opens problems of compatibility with the other open clusters main sequences (Mermilliod et al. 1997). We will show here the comparisons made with the "classic" distance modulus , with a reddening (e.g. Meynet et al. 1993). If we reduce the distance modulus to the Hipparcos value but at the same time bring down the reddening to the still acceptable value of 0.02, the main sequence location is still accurately reproduced (in this case, the agreement is better if we adopt Flower (1996) semiempirical correlations and the age is increased by 15% with respect to the present fit.

Second, the Pleiades have received a recent datation based on the luminosity location of the transition between stars without lithium in ther spectrum (HHJ3) and showing lithium (the candidate brown dwarf PPL 15). This age is (Basri et al. 1996), noticeably larger than what generally inferred for this cluster. Although the first datations of the Pleiades gave an age of , more recent values (e.g. Meynet et al. 1993 models with overshooting) provided an age of , closer, but not yet fully consistent with the brown dwarf dating. Actually, Basri et al. (1996) datation could result to be a lower limit, as PPL15 is a binary, so it could not be an optimum marker for the luminosity at which lithium reappears.

We show in Fig. 9 the HR diagram of the Pleiades and our isochrones with and without overshooting. The continuous line is the isochrone of including overshooting, while the two dashed lines are the isochrones of and without overshooting. Noticeably, the isochrone with overshooting very reasonably fits the stellar locus, and its age is compatible with the result from the brown dwarfs observations. The age derived by Meynet et al. (1993), also on the basis of models including overshooting, is yr: as we have discussed above, the differences in the nuclear evolutionary scheme and in the chemistry (they adopt a larger helium content and metallicity ) provide somewhat shorter evolutionary times in the mass range which defines the Pleiades turnoff. The uncertainty on the age is in any case considerable: if we define the turnoff by requiring the full inclusion of the most luminous star, the age is smaller also in the models with overshooting. Neverthless, it is important that consistency between the brown dwarf dating and the turnoff dating is obtained. Given that the brown dwarf should provide more precise information than the turnoff datation (Basri 1997, D'Antona & Mazzitelli 1997, Bildsten et al. 1997), we regard this result as an indication that the models with overshooting are to be preferred to the standard models.

Fig. 9. Top figure: the Pleiades HR diagram data are taken from Iriarte (1969) photoelectric data (open triangles), and from a list by Stauffer 1984 and reference therein, (open squares). Stauffer's data are shown for the stars in common between the two lists. Superimposed we show the isochrone of 1.2 yr with diffusive overshooting (full line) and the isochrones of and yr without overshooting. Although the turn-off region is scarcely defined by only six stars, the isochrone with overshooting provides a reasonable fit of the data, although the corresponding one without overshooting can not be excluded. Distance modulus and reddening are the pre-Hipparcos values. An age % larger is obtained by assuming the Hipparcos distance modulus [ ] but reducing the reddening to 0.02mag. In the bottom figure, the Persei diagram is compared with our isochrones including overshooting for ages of 70 and 80Myr, and with the isochrone of 70Myr with no overshooting

We also show in Fig. 9 the turnoff fitting of the cluster Persei. Meynet et al. (1993) provide an age of 50 Myr, based on the fit of both the turnoff and of the subgiant star Per itself, interpreted as a star belonging to the blue loop of post-helium ignition evolution. Limiting ourselves to the turnoff, the resulting age can be as large as 80 Myr in our models with overshooting, but probably smaller than 70 Myr in the models without overshooting. Observations of Lithium in the brown dwarf candidates belonging to this cluster will provide another interesting constraints on the turnoff models.

© European Southern Observatory (ESO) 1998

Online publication: June 2, 1998

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