ner_tags: O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Survey O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation O O O O B-Citation I-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-CelestialObject O O O O O O O O O O O O O O O O O O O O O B-Formula I-Formula I-Formula I-Formula O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-CelestialObject I-CelestialObject O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O
tokens: with indirect estimates of the helium spread, based on the morphology of the HB. For the clusters for which these estimates were available, it was possible to verify a satisfactory consistency. The APOGEE data used in this work are of paramount importance, because they confirm that the material from which SG stars formed was exposed to a very advanced nucleosynthesis, with the activation of Mg burning and, in the most metal-poor clusters, the synthesis of silicon. This is of great help in the way towards the understanding of the possible GC polluters, because the possibility that either fast-rotating massive stars (Krause et al. 2013 ) or massive binaries (De Mink et al. 2009 ) played a role in this context can be ruled out. Indeed in neither of the two cases the temperatures of the nuclear burning regions are sufficiently hot to allow such an advanced nucleosynthesis. The only possibility left besides the massive AGB scenario is super massive main sequence stars. As shown by Denissenkov Hartwick ( 2014 ), the core temperature during H-burning reach ∼70 MK, thus triggering the activation of Mg burning and the production of aluminium. A straight comparison between the predictions of the AGB hypothesis and the chemical patterns expected when pollution from super massive stars is considered is cumbersome, because while it was shown that in the interiors of this class of objects the ignition of Mg burning occurs, the trend with metallicity, if any, needs further investigation. While in the AGB case the sensitivity of the HBB nucleosynthesis to the metallicity is a direct result of stellar evolution modelling, it is not clear how the chemical composition would affect the core H-burning of super massive stars. At the moment, this problem is still open. 6 CONCLUSIONS We discuss the chemical composition of giant stars in nine Galactic GCs, for which the abundances of C, N, O, Mg, Al, and Si are available. The wide range of metallicities ( − 2.4 [Fe/H] -0.7) and the simultaneous knowledge of the elements mentioned above offer a valuable opportunity to test the origin of multiple populations in these complex stellar systems. We concentrate on the possibility that the polluters of the intra-cluster medium, providing the gas from which SG stars formed, were massive AGB stars (), which experienced HBB at the base of their external envelope. To this aim, we calculated ad hoc AGB models, with the same metallicity and the same mixture of the stars belonging to the FG of each cluster. In agreement with previous studies on GCs in the Milky Way, we find that all the clusters show the presence of SG stars, as deduced on the star-to-star chemical differences, which define abundance patterns in the different observational planes. We find that the chemical patterns observed (Mg, Al, Si, and O in particular) are in good agreement with the theoretical expectations: (a) on the qualitative side, the spread observed for the different species follows the results from AGB