First evidence of multiple populations along the AGB from Strömgren photometry

Spectroscopic studies have demonstrated that nearly all Galactic globular clusters (GCs) harbour multiple stellar populations with different chemical compositions. Moreover, colour-magnitude diagrams based exclusively on Strömgrem photometry have allowed us to identify and characterise multiple populations along the RGB of a large number of clusters. In this paper we show for the first time that Strömgren photometry is also very efficient at identifying multiple populations along the AGB, and demonstrate that the AGB of M 3, M 92, NGC 362, NGC 1851, and NGC 6752 are not consistent with a single stellar population. We also provide a catalogue of RGB and AGB stars photometrically identified in these clusters for further spectroscopic follow-up studies. We combined photometry and elemental abundances from the literature for RGB and AGB stars in NGC 6752 where the presence of multiple populations along the AGB has been widely debated. We find that, while the MS, SGB, and RGB host three stellar populations with different helium and light element abundances, only two populations of AGB stars are present in the cluster. These results are consistent with standard evolutionary theory.P.G. acknowledges support from grant No. 2011- 5042 from the Swedish Research Council. S.F acknowledge the grant The New Milky Way from the Knut and Alice Wallenberg Foundation. P.G. and S.F. acknowledges support from the Swedish National Space Board. AS acknowledges support from MINECO (ESP2015-66134-R) and Generalitat de Catalunya (SGR2014-1458). L.C gratefully acknowledge support from the Australian Research Council (grants DP150100250, FT160100402). A. P. M. acknowledges support by the Australian Research Council through Discovery Early Career Researcher Award DE150101816