Multiple stellar populations in Globular Clusters

People involved at OAB: Bragaglia, Carretta, Bellazzini, Pancino

Figure: The Na-O anticorrelation in 20 GCs (adapted from Carretta et al. 2009a, 2010d): the universality of this feature is the confirmation of the existence of multiple populations in GCs.

Our group was fundamental in providing the chemical evidence that GCs are not simple stellar populations.

While the metallicity (i.e., [Fe/H]) is remarkably homogeneous in each GC, the large star-to-star variations in light elements (see e.g. the Na-O and Mg-Al anticorrelations) are explained by the formation of (at least) a second generation of stars from material polluted by a previous population. All GCs studied so far support this vision; in particular we have analysed about 2000 RGB stars in 20 GCs, observed with FLAMES@VLT. We are now extending the project adding other elements, like Li, and observing other GCs (e.g., in P84 we obtained X-shooter spectra of two MS stars in NGC 2808, under the Italian GTO programme).

The results have been presented in a series of papers: Carretta et al. 2009a, 2009b on the ubiquitous Na-O anticorrelation (see Fig. ); Carretta et al. 2009c on a new [Fe/H] scale for GCs; Bragaglia et al. 2010a on the He content of RGB stars (see Fig. ); Gratton et al. 2010 on the relation between the anticorrelations, He and the HB; Carretta et al. 2010a on the Ca abundance; Carretta et al. 2009b on the properties of GCs; Carretta et al. 2010c on the high-mass GC M54 and its similarities with $\omega$ Cen (see Fig. ); Bragaglia et al. 2010b on the abundances of two MS stars in NGC 2808; Carretta et al. 2010d on NGC 1851.

Figure: He-rich (i.e., Na-rich, second generation) and He-poor (i.e., Na-poor, first generation) stars have different luminosities at the RGB bump. We show here the results from our survey (Bragaglia et al. 2010a).

Figure: M54 and $\omega$ Cen show interesting similarities. We show here the Na-O anticorrelations we found in M54 (Carretta et al. 2010c) compared to literature data on $\omega$ Cen. In both cases the leftmost panels show the whole samples, the intermediate the metal-rich components, and the rightmost panels the metal-poor populations. It is clear that the extension of the anticorrelation is different in the two sub-samples in both stellar systems.

This work is in collaboration with R. Gratton, S. Lucatello, Y. Momany, V. D'Orazi (INAF-Padova Obs.), G. Piotto (Univ. of Padova), F. D'Antona (INAF-Roma Obs.), F. Leone (Univ. Catania), G. Catanzaro (INAF-Catania Obs.), S. Cassisi (INAF-Teramo Obs.), P. François (Obs. Paris), A. Recio-Blanco (Obs. Nice) and many more. This project received funding by INAF-PRIN 2005 and by the PRIN-MIUR 2007.