A detailed study of the main sequence of the globular cluster NGC 6397: can we derive constraints on the existence of multiple populations?
Osservatorio Astronomico di Roma, via di Frascati 33, 00040, Monte Porzio Catone, Rome, Italy e-mail: [dicrisci;dantona;ventura]@oa-roma.inaf.it
Accepted: 15 December 2009
Context. Globular clusters can no longer be regarded as examples of “simple stellar populations” as all those so far examined contain an important fraction of “second generation” stars, in which the light elements are processed through the hot CNO cycle, and helium variations may be present. Clusters apparently “simple” contain a majority of second generation stars.
Aims. If NGC 6397 contains a large fraction of “second generation” stars (>70% according to recent analysis), the helium abundance of its stars might also be affected, show some star-to-star variation, and be larger than the standard Big Bang abundance . Can we derive constraints on this issue from the analysis of the main sequence width and from its luminosity function?
Methods. We build up new models for the turnoff masses and the main sequence down to the hydrogen burning minimum mass, adopting two versions of an updated equation of state (EOS) including the OPAL EOS. Models consider different initial helium and CNO abundances to cover the range of possible variations between the first and second generation stars. We compare the models with the observational main sequence. We also make simulations of the theoretical luminosity function, for different choices of the mass function and of the mixture of first and second generation stars and compare them with the observed luminosity function by means of the Kolmogorov Smirnov – KS-test.
Results. The new models for very low mass stars compare well with previous models and show that the OPAL EOS is a good description in all the region of temperature and densities of very low mass stars for which it is computable. The analysis of the main sequence width shows that any helium variation must be confined within ~ 0.02 in the case of a CNO increase as suggested by literature, and we discuss the consequent implications for the model of self-enrichment. We also show that the KS test on the luminosity functions allows us to derive a best distance modulus for each age. For a population all made by stars with standard helium Y=0.24, standard CNO abundances, and an age of 12 Gyr, choosing a double power law mass function for M > 0.18 and for M<0.18 , the resulting theoretical luminosity function agrees well with the observed one (KS ~ 0.75 for a distance modulus = 12.31 ± 0.05 mag). Using non-standard CNO abundance for all the stars or for a fraction of 70%, the KS test provides comparable agreement (~KS > 0.55) with the observed luminosity function.
Conclusions. The study of the width of the main sequence at a different interval of magnitude is consistent with the hypothesis that both generations are present in the cluster. If the CNO increase suggested by spectroscopic observation is taken into account the small helium spread of the main sequence in NGC 6397 implies a substantial helium uniformity ( ~ 0.02) between first and second generation stars. The possible spread in helium doubles if an higher larger increase of CNO is considered. The luminosity function is in any case well consistent with the observed data.
Key words: stars: general / stars: evolution / stars: low-mass / Hertzsprung-Russell (HR) and C-M diagrams / brown dwarfs
© ESO, 2010