Na-O anti-correlation and HB: a clue to GC formation

People involved at OAB: Bragaglia, Carretta, Pancino, Valenti.

In recent years, many studies discovered that the mono-metallic nature of Galactic Globular Clusters (GGCs) is affected by a number of exceptions. The only elements showing very homogeneous abundances in GC stars are those produced by explosions of Supernovae, both core-collapse Type II Supernovae and Type Ia Supernovae from lower-mass, long-living progenitors. The first enrich the interstellar medium in oxygen and other $\alpha$-elements, whereas the bulk of iron-peak elements is provided by the second class of objects.

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 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 observed variations observed for Li, C and N are partly explained by the normal evolution of low mass giants, where changes in surface abundances are due to the standard first dredge-up after the Main Sequence phase, plus a second mixing mechanism that occurs when the advancing H-burning shell crosses the chemical discontinuity left behind by the retreating convective envelope (the RGB-bump point). Both mixing episodes bring to the surface products of an incomplete CNO-cycle, lowering the Li, C abundances and the isotopic ratio of $\rm ^{12}C/^{13}C$ and increasing the N abundance (see Gratton, Sneden and Carretta 2004 for a recent review). However, variations of heavier elements cannot be explained with this scenario.

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 $10^8$ 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 have observed about 19 Galactic GCs selected to span the whole range of different physical parameters (metallicity, concentration, density, HB morphology, mass, etc.); in more than 70 hours at VLT-UT2 with FLAMES we collected about 100 high resolution spectra of RGB stars in each GC. Analysing this large and homogeneous dataset, we plan to answer fundamental questions such as: (i) Were/Are GC stars really born in a single ``instantaneous'' burst? (ii) How do abundance anomalies within each individual GC relate to the formation and early evolution of the GC itself and of each individual cluster member?

Our analysis of the first clusters shows that:

a) in NGC 2808 the bulk of stars is O-rich, but the cluster does contain a lot of super O-poor stars, likely the counterparts of blue and extremely blue HB stars; O-poor and O-rich groups show hints of a different He content (Carretta et al. 2006). This cluster is the only one, apart from M 13, in which extreme O-depletion (down to $\rm [O/Fe]\sim -1$ dex is found.

b) the distribution of stars along the Na-O anti-correlation is different in NGC 6752, where more Na-poor/O-rich stars are found. This is somewhat at odds to the very blue population of this cluster (Carretta et al. 2007a), indicating that a simple link between HB morphology and chemical anomalies may be unrealistic.

c) NGC 6218 shows an anti-correlation too, similar to that observed in NGC 6752, in spite of the differences in the HB morphology. Furthermore, it is possible to see the presence of distinct Na-poor and Na-rich populations at the level of the RGB bump: we interpret this variation as the effect of two distinct populations having different bump luminosities, as predicted for different He content. To our knowledge, NGC 6218 is the first GGC where such a signature has been spectroscopically detected on the Red Giant Branch (Carretta et al. 2007b).

d) the two peculiar bulge clusters NGC 6441 and NGC 6388 present a Na-O anti-correlation, seen for the first time in this GC population (Gratton et al. 2006, Carretta et al. 2007c).

In addition to the GIRAFFE data on NGC 2808, Science Verification spectra taken with UVES/FLAMES@VLT of a sample of 20 RGB stars in a range of about 3 mag from the RGB tip, were used to derive the detailed chemical composition of this peculiar and massive cluster. Results for light proton-capture elements (O, Na, Mg, Al) were combined with literature samples for other clusters to study the relation of the anti-correlation extent with structural and orbital parameters (Carretta 2006). The main result is that chemical anomalies are more pronounced in GGCs with more eccentric orbits in the Galaxy. As a consequence, they spend most of their lifetime in a relatively undisturbed environment. To the contrary, clusters near the bulge or the Galactic plane show a modest extent of anti-correlation.

This work is in collaboration with Gratton, Lucatello, Desidera, Momany (INAF-Padova Obs.), Piotto (Univ. of Padova), Ferraro (Univ. of Bologna), D'Antona (INAF-Roma Obs.), Leone, Catanzaro (INAF-Catania Obs.), Cassisi (INAF-Teramo Obs.), Moehler (ESO), François (Obs. Paris), Recio-Blanco (Obs. Nice) and many more. This project received funding by INAF-PRIN 2005.