Globular clusters and their contribution to the formation of the Galactic halo

This is a "biased" review because I will show recent evidence on the contribution of globular clusters (GCs) to the halo of our Galaxy seen through the lens of the new paradigm of multiple populations in GCs. I will show a few examples where the chemistry of multiple populations helps to answer hot questions including whether and how much GCs did contribute to the halo population, if we have evidence of the GCs-halo link, what are the strengths and weak points concerning this contribution.Comment: invited review to appear in Proc. IAU Symp. 317, XXIXth IAU General Assembly, August 2015, Honolulu (HI, USA); eds. A. Bragaglia, M. Arnaboldi, M. Rejkuba, D. Romano, (7 pages, 4 figures, LaTeX, using iau.cls

Five groups of red giants with distinct chemical composition in the globular cluster NGC 2808

The chemical composition of multiple populations in the massive globular cluster (GC) NGC~2808 is addressed with the homogeneous abundance re-analysis of 140 red giant branch (RGB) stars. UVES spectra for 31 stars and GIRAFFE spectra for the other giants were analysed with the same procedures used for about 2500 giants in 23 GCs in our FLAMES survey, deriving abundances of Fe, O, Na, Mg, Si, Ca, Ti, Sc, Cr, Mn, and Ni. Iron, elements from alpha-capture, and in the Fe-group do not show intrinsic scatter. On our UVES scale the metallicity of NGC~2808 is [Fe/H]=-1.129+/-0.005+/-0.034$ (+/-statistical +/-systematic error) with sigma=0.030 (31 stars). Main features related to proton-capture elements are retrieved, but the improved statistics and the smaller associated internal errors allow to uncover five distinct groups of stars along the Na-O anticorrelation. We observe large depletions in Mg, anticorrelated with enhancements of Na and also Si, suggestive of unusually high temperatures for proton-captures. About 14% of our sample is formed by giants with solar or subsolar [Mg/Fe] ratios. Using the [Na/Mg] ratios we confirm the presence of five populations with different chemical composition, that we called P1, P2, I1, I2, and E in order of decreasing Mg and increasing Na abundances. Statistical tests show that the mean ratios in any pair of groups cannot be extracted from the same parent distribution. The overlap with the five populations recently detected from UV photometry is good but not perfect, confirming that more distinct components probably exist in this complex GC.Comment: 16 pages, 9 tables, 16 figures; accepted for publication on the Astrophysical Journa

Empirical estimates of the Na-O anti-correlation in 95 Galactic globular clusters

Large star-to-star abundance variations are direct evidence of multiple stellar populations in Galactic globular clusters (GCs). The main and most widespread chemical signature is the anti-correlation of the stellar Na and O abundances. The interquartile range (IQR) of the [O/Na] ratio is well suited to quantifying the extent of the anti-correlation and to probe its links to global cluster parameters. However, since it is quite time consuming to obtain precise abundances from spectroscopy for large samples of stars in GCs, here we show empirical calibrations of IQR[O/Na] based on the O, Na abundances homogeneously derived from more than 2000 red giants in 22 GCs in our FLAMES survey. We find a statistically robust bivariate correlation of IQR as a function of the total luminosity (a proxy for mass) and cluster concentration c. Calibrated and observed values lie along the identity line when a term accounting for the horizontal branch (HB) morphology is added to the calibration, from which we obtained empirical values for 95 GCs. Spreads in proton-capture elements O and Na are found for all GCs in the luminosity range from Mv=-3.76 to Mv=-9.98. This calibration reproduces in a self-consistent picture the link of abundance variations in light elements with the He enhancements and its effect on the stellar distribution on the HB. We show that the spreads in light elements seem already to be dependent on the initial GC masses. The dependence of IQR on structural parameters stems from the well known correlation between c and Mv, which is likely to be of primordial origin. Empirical estimates can be used to extend our investigation of multiple stellar populations to GCs in external galaxies, up to M31, where even integrated light spectroscopy may currently provide only a hint of such a phenomenon.Comment: 18 pages, 2 tables, 16 figures; accepted for publication on Astronomy and Astrophysic

Potassium abundances in multiple stellar populations of the globular cluster NGC 4833

NGC 4833 is a metal-poor Galactic globular cluster (GC) whose multiple stellar populations present an extreme chemical composition. The Na-O anti-correlation is quite extended, which is in agreement with the long tail on the blue horizontal branch, and the large star-to-star variations in the [Mg/Fe] ratio span more than 0.5 dex. Recently, significant excesses of Ca and Sc with respect to field stars of a similar metallicity were also found, signaling the production of species forged in H-burning at a very high temperature in the polluters of the first generation in this cluster. Since an enhancement of potassium is also expected under these conditions, we tested this scenario by analysing intermediate resolution spectra of 59 cluster stars including the K I resonance line at 7698.98 A. We found a wide spread of K abundances, anti-correlated to Mg and O abundances, as previously also observed in NGC 2808. The abundances of K are found to be correlated to those of Na, Ca, and Sc. Overall, this chemical pattern confirms that NGC 4833 is one of the relatively few GCs where the self-enrichment from first generation polluters occurred at such high temperatures that proton-capture reactions were able to proceed up to heavier species such as K and possibly Ca. The spread in K observed in GCs appears to be a function of a linear combination of cluster total luminosity and metallicity, as other chemical signatures of multiple stellar populations in GCs.Comment: 13 pages, 12 figures, 3 tables; accepted for publication on Astronomy and Astrophysic

Chemistry of multiple stellar populations in the mono-metallic, in situ, bulge globular cluster NGC 6388

We present the homogeneous abundance analysis for a combined sample of 185 giants in the bulge globular cluster (GC) NGC 6388. Our results are used to describe the multiple stellar populations and differences or analogies with bulge field stars. Proton-capture elements indicate that a single class of first-generation polluters is sufficient to reproduce both the extreme and intermediate parts of the anti-correlations among light elements O, Na, Mg, and Al, which is at odds with our previous results based on a much smaller sample. The abundance pattern of other species in NGC 6388 closely tracks the trends observed in bulge field stars. In particular, the alpha-elements, including Si, rule out an accreted origin for NGC 6388, confirming our previous results based on iron-peak elements, chemo-dynamical analysis, and the age-metallicity relation. The neutron-capture elements are generally uniform, although the [Zr/Fe] ratio shows an intrinsic scatter, correlated to Na and Al abundances. Instead, we do not find enhancement in neutron-capture elements for stars whose photometric properties would classify NGC 6388 as a type II GC. Together with the homogeneity in [Fe/H] we found in a previous paper, this indicates we need to better understand the criteria to separate classes of GCs, coupling photometry, and spectroscopy. These results are based on abundances of 22 species (O, Na, Mg, Al, Si, Ca, Ti, Sc, V, Cr, Mn, Fe, Co, Ni, Zn, Y, Zr, Ba, La, Ce, Nd, and Eu) from UVES spectra sampling proton-, alpha-, neutron-capture elements, and Fe-peak elements. For 12 species, we also obtain abundances in a large number of giants (up to 150) from GIRAFFE spectra.Comment: 19 pages plus 6 pages of Appendix, 23 figures, 5 tables; accepted for publication on Astronomy and Astrophysic