AGB subpopulations in the nearby globular cluster NGC6397

It has been well established that Galactic Globular clusters (GCs) harbour more than one stellar population, distinguishable by the anticorrelations of light-element abundances (C-N, Na-O, and Mg-Al). These studies have been extended recently to the asymptotic giant branch (AGB). Here, we investigate the AGB of NGC6397 for the first time. We have performed an abundance analysis of high-resolution spectra of 47 red giant branch (RGB) and eight AGB stars, deriving Fe, Na, O, Mg, and Al abundances. We find that NGC6397 shows no evidence of a deficit in Na-rich AGB stars, as reported for some other GCs - the subpopulation ratios of the AGB and RGB in NGC6397 are identical, within uncertainties. This agrees with expectations from stellar theory. This GC acts as a control for our earlier work on the AGB of M4 (with contrasting results), since the same tools and methods were used.Based in part on data acquired through the AAO, via programme 15A/21 (PI Campbell). Part of this work was supported by the DAAD (PPP project 57219117) with funds from the German Federal Ministry of Education and Research (BMBF). BTM acknowledges the financial support of his Australian Postgraduate Award scholarship. SWC acknowledges federal funding from the Australian Research Council though the Future Fellowship grant entitled ‘Where are the Convective Boundaries in Stars?’ (FT160100046). VD acknowledges support from the AAO distinguished visitor programme 2016. LC gratefully acknowledges support from the Australian Research Council (grants DP150100250, FT160100402)