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PI: Annalisa Calamida (INS/WFC3), RW303, 667 218 6428

Galactic globular clusters (GGCs) have long been considered the most simple stellar populations, with their stars located at the same distance, with typically the same extinction, and the same bona fide age and chemical composition. However, during the last two decades, high-resolution spectroscopic studies have shown that most GGCs display a large spread of light elements and anti-correlations.This evidence suggests the presence of more than one generation of stars in these systems, with the younger generation born from a stellar medium enriched by the material ejected by the previous one (Carretta et al.\ 2010a, and references therein). Some of the proposed polluters are intermediate-mass Asymptotic Giant Branch stars or massive rotating stars but a consensus on the mechanisms has not been reached yet (D'Antona \& Ventura 2007; Decressin et al.\ 2007;
Bastian et al.\ 2013). For a critical discussion on different hypotheses on the origin of abundance dishomogeneities in GGCs see Renzini et al. (2015).

Thanks to the high spatial resolution and depth of Hubble Space Telescope (HST) imaging, it was possible to discover different sequences in the color-magnitude diagrams (CMDs) of several GGCs. A stunning case is Omega Cen, the most massive GGC, where at least 15 stellar sub-populations were identified (Bellini et al.\ 2017, Milone et al.\ 2017a). However, Omega Cen is very different from all other GGCs, since it shows an iron spread of more than 1 dex and dispersions of most of the heavy elements and a possible age and helium spread. (Piotto et al.\ 2005; Calamida et al.\ 2009; Marino et al.\ 2011; Johnson \& Pilachowski 2010).
Omega Cen is then thought to be the nucleus of a dwarf galaxy accreted by the Milky Way or the result of the merger of two clusters (Norris 1997; Bekki \& Freeman 2003; Bekki \& Norris 2007).

NGC2808 is the second most massive Galactic globular cluster and a very peculiar object. HST photometric studies showed that the cluster main-sequence splits in a blue, an intermediate and a red sequence (see Fig. 1) and that stars on the intermediate and blue main sequences are more centrally concentrated compared to stars on the red main-sequence. It was suggested that NGC2808 experienced different episodes of star formation with a significant helium enrichment, with the bluest main-sequence being the most enhanced. However, high-resolution spectroscopic measurements of red-giant stars indicate that, while expected in the case of different star formation episodes, no spread in iron content is present in NGC2808. 

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