A detailed study of the main sequence of the Globular Cluster NGC 6397: can we derive constraints on the existence of multiple population?

TL;DR

This study models the main sequence of NGC 6397 to investigate the presence of multiple stellar populations and constrain helium abundance variations, finding evidence for both generations with limited helium spread.

Contribution

It introduces new stellar models considering different helium and CNO abundances and compares them with observations to constrain multiple populations in NGC 6397.

Findings

Main sequence width supports presence of multiple populations.

Helium spread between populations is limited (~0.02) under CNO enhancement.

Luminosity function matches observational data across models.

Abstract

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 Y~0.24. Can we derive constraints on this issue from the analysis of the main sequence width and from its luminosity function? 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 functions, 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. The study of the width of the main sequence at 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 (DY~0.02) between first and second generation stars. The possible spread in helium doubles if an even larger increase of CNO is considered. The luminosity function is in any case well consistent with the observed data.