Thanks to the new efficient optical and IR spectrographs with
high resolution and multi-object capabilities mounted on
4m- and 10m-class telescopes, high-quality spectra can be obtained
for tens to hundreds of stars in each GC in very reasonable
exposure times.
Detailed and precise chemical abundances of many key elements (Fe, light,
, proton-capture and neutron-capture elements) can now be
measured for stars in Galactic GCs from the RGB tip to the MS Turn-Off.
In this regard, the recent investigation in this field revealed several clear
exceptions to the
classical view of Galactic Globular Clusters as purely mono-metallic populations.
The only elements showing very homogeneous abundances in GC stars are those
produced by explosions of Supernovae, in particular iron peak elements and, in
some cases, elements.
However, large star-to-star intrinsic variations for abundances of the lightest elements (from Li and C to Mg and Al) are known to exist in every GGC examined so far. Part of these chemical anomalies (those related to Li, C, N, and their isotopes) share the same behaviour of field stars of similar metallicity, but heavier nuclei (noticeably Na, O, Mg, Al) present in GGC a peculiar pattern not seen in halo field analogs and still not well explained.
The emerging picture is that globular clusters are not a true example of Simple Stellar Population, and that their early evolution was probably not very simple. This is indicated by stars that populate side-by-side the same evolutionary locus from the late RGB down to the unevolved main sequence and show very different surface abundances of light elements (C, N, O, Na, Al).
The star-to-star anti-correlation between the O and Na abundances (see Gratton,
Sneden, Carretta 2004) is the main sign of the (unexpected)
presence of material processed through the complete CNO cycle in GC stars, most
likely from thermally pulsing intermediate-mass AGB (IM-AGB) stars of an early
stellar generation, undergoing hot bottom burning and/or fast rotating massive
stars losing material at the end of their main sequence phase.
The age difference between
the two populations (a few 
yr) is too small to be directly detectable as
different Turn-Offs (TO's), but may be unveiled by a careful abundance analysis
of the relics of now-extinct first generation stars, whose nucleosynthetic
yields are possibly incorporated in the present observed GC stars.
We completed the analysis of about 2000 red giant stars in 19 GCs of different metallicity, mass, HB morphology, etc. Using this homogeneous and unprecedented database, we studied the Na-O anticorrelation in each cluster, finding that 2 stellar generations co-exist in every GC: (i) a Primordial component of first generation stars, present in all clusters with a constant fraction of one third and (ii) two components of second generation stars distinct from their Intermediate or Extremely modified composition. The run of the slopes of the Na-O and Mg-Al anticorrelations change from cluster to cluster, and are well reproduced by a combination of cluster metallicity and luminosity. We show in Fig. 7, as an example, the run of Na and Al, very different in the various GCs. We interpret this as evidence that the properties of polluters in GCs change regularly with these two main global cluster parameters (Carretta et al. 2009a,b). Using high resolution spectra we established also a new metallicity scale for GCs (Carretta et al. 2009c).
 |
The relation between the detailed chemistry of first and second generation stars and global cluster parameters was extensively analysed in a study leading to: (i) a new definition of GCs (those stellar aggregates able to develop the Na-O anticorrelation), (ii) a new classification of sub-populations of GCs, (iii) a qualitative scenario for the formation of GCs that constitutes a first benchmark to test the new evidences we are finding, and (iv) the finding that most of the ``phenotypes'' of GCs are fairly well reproduced by variations of the main parameters mass, metallicity and age. A paper is submitted to A&A.
We sampled the extremes of the mass range of typical GCs: 1) we analysed M54, the second most massive cluster in the Galaxy, associated to the nucleus of the Sagittarius dwarf galaxy (analysis completed, paper to be written), and 2) we tested our definition of GC by looking for the Na-O anticorrelation in the old open cluster NGC 6791 (data acquired, analysis in progress).
Finally, we started a study to derive abundances of Li, Na and O in main sequence stars in GCs. Lithium is the best tracer of the dilution with primordial matter in GCs. We collected and started to analyse archival ESO data for NGC 6397, NGC 6752 and 47 Tuc. Moreover, we submitted a new proposal to ESO telescopes to gather new data on M 22, M 4 and NGC 6752.
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, 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.