5. Comparison between theoretical isochrones and observational data
We present in Fig. 2a and in Fig. 2b the HR diagram of the 33 stars of Sample 1. We have split the sample in two subsamples: Fig. 2a corresponds to stars of solar metallicity or close to it () while Fig. 2b is constituted of the moderately metal deficient stars (). We have superimposed on those diagrams theoretical isochrones associated to the limits of the metallicity ranges considered. Isochrones with [Fe/H] values of -0.5 and -1.0 have been derived from models calculated with an -elements enriched mixture and have solar-scaled helium values, Y=0.256 and 0.236 respectively. The metal-rich isochrone has a solar -elements ratio and Y=0.32. We have also plotted the position of the solar ZAMS on Fig. 2a. Isochrones are given for ages in the range 8-10 Gyr representative of the age of the galactic disk.
Fig. 2a shows that the stars of solar metallicity or close to it lie in the region defined by the theoretical models corresponding to their associated metallicity range. We notice the particular position of Lep (HIC 27072) which is a very young star lying close to the ZAMS.
However, it is worth to note that the slope of the theoretical main sequence agrees well with the observational slope. This, once more, is in favor of the uniqueness of the mixing-length parameter in low-mass stars. Furthermore the width of the global observed main-sequence band for metal-rich stars, which is around 0.3 mag, and the theoretical width corresponding to the same metallicity range with solar-scaled helium abundances are quite similar (see also Fig. 6).
Turning now to the moderately deficient stars plotted in Fig. 2b we note that all the stars but one are outside the theoretical band corresponding to their metallicity . Stars are located on theoretical isochrones with a metallicity higher than their observed metallicity.
The same behaviour is seen in the two other selected samples in which the effective temperatures of stars have been determined independently by different authors (see Sect. 3). Fig. 3a and 3b are similar to Fig. 2a and 2b but for the 64 stars with effective temperatures derived from detailed spectroscopic analysis (Sample 2) while Fig. 4a and 4b represent the situation of the 15 stars of Sample 3. Although the error bars on effective temperature are greater in Fig. 3a and 3b and although they are few objects in Fig. 4a and 4b, the same tendency is found, strengthening the conclusion that classical theoretical isochrones and actual observations of moderately metal deficient stars do not match.
Note that, in the confrontation of observational position of stars with stellar isochrones, errors on [Fe/H] and [/Fe] which are in the range 0.10-0.15 dex, are responsible for a hidden enlargment of the error boxes, because the chemical composition of the models to be compared with the observed stars are not known exactly.
© European Southern Observatory (ESO) 1999
Online publication: October 4, 1999