High-precision multi-band HST photometry reveals that the main sequence (MS) of the globular cluster NGC 6397 splits into two components, containing ~30% and ~70% of the stars. This double sequence is consistent with the idea that the cluster hosts two stellar populations: (i) a primordial population that has a composition similar to field stars, and containing ~30% of the stars, and (ii) a second generation with enhanced sodium and nitrogen, depleted carbon and oxygen, and a slightly enhanced helium abundance (Delta Y~0.01). We examine the color difference between the two sequences across a variety of color baselines and find that the second sequence is anomalously faint in m_F336W. Theoretical isochrones indicate that this could be due to NH depletion.
@article{arxiv.1110.1077,
title = {A Double Main Sequence in the Globular Cluster NGC 6397},
author = {A. P. Milone and A. F. Marino and G. Piotto and L. R. Bedin and J. Anderson and A. Aparicio and S. Cassisi and R. M. Rich},
journal= {arXiv preprint arXiv:1110.1077},
year = {2015}
}
Comments
19 pages, 11 figures, accepted for pubblication in ApJ