We use high-precision photometry of red-giant-branch (RGB) stars in 57 Galactic globular clusters (GCs), mostly from the "Hubble Space Telescope (HST ) UV Legacy Survey of Galactic globular clusters", to identify and characterize their multiple stellar populations. For each cluster the pseudo two-color diagram (or 'chromosome map') is presented, built with a suitable combination of stellar magnitudes in the F275W, F336W, F438W and F814W filters that maximizes the separation between multiple populations. In the chromosome map of most GCs (Type I clusters), stars separate in two distinct groups that we identify with the first (1G) and the second generation (2G). This identification is further supported by noticing that 1G stars have primordial (oxygen-rich, sodium-poor) chemical composition, whereas 2G stars are enhanced in sodium and depleted in oxygen. This 1G-2G separation is not possible for a few GCs where the two sequences have apparently merged into an extended, continuous sequence. In some GCs (Type II clusters) the 1G and/or the 2G sequences appear to be split, hence displaying more complex chromosome maps. These clusters exhibit multiple SGBs also in purely optical color-magnitude diagrams, with the fainter SGB joining into a red RGB which is populated by stars with enhanced heavy-element abundance. We measure the RGB width by using appropriate colors and pseudo-colors. When the metallicity dependence is removed, the RGB width correlates with the cluster mass. The fraction of 1G stars ranges from ∼8% to ∼67% and anticorrelates with the cluster mass, indicating that incidence and complexity of the multiple population phenomenon both increase with cluster mass.

Aims. We present abundance analysis based on high resolution spectra of 105 isolated red giant branch (RGB) stars in the Galactic Globular Cluster NGC 6121 (M 4). Our aim is to study its star population in the context of the multi-population phenomenon recently discovered to affect some Globular Clusters. Methods. The data have been collected with FLAMES+UVES, the multi-fiber high resolution facility at the ESO/VLT@UT2 telescope. Analysis was performed under LTE approximation for the following elements: O, Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Ni, Ba, and NLTE corrections were applied to those (Na, Mg) strongly affected by departure from LTE. Spectroscopic data were coupled with high-precision wide-field UBVI C photometry from WFI@2.2 m telescope and infrared JHK photometry from 2MASS. Results. We derived an average [Fe/H] = −1.07 ± 0.01 (internal error), and an α enhancement of [α/Fe] = +0.39 ± 0.05 dex (internal error). We confirm the presence of an extended Na-O anticorrelation, and find two distinct groups of stars with significantly different Na and O content. We find no evidence of a Mg-Al anticorrelation. By coupling our results with previous studies on the CN band strength, we find that the CN strong stars have higher Na and Al content and are more O depleted than the CN weak ones. The two groups of Na-rich, CN-strong and Na-poor, CN-weak stars populate two different regions along the RGB. The Na-rich group defines a narrow sequence on the red side of the RGB, while the Na-poor sample populate a bluer, more spread portion of the RGB. In the U vs. U − B color magnitude diagram the RGB spread is present from the base of the RGB to the RGB-tip. Apparently, both spectroscopic and photometric results imply the presence of two stellar populations in M 4. We briefly discuss the possible origin of these populations.

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We present a chemical abundance analysis based on high resolution UVES spectra of seventeen bright giant stars in the Globular Cluster (GC) M 22. We obtained an average iron abundance of [Fe/H] = −1.76 ± 0.02 (internal errors only) and an α enhancement of 0.36 ± 0.04 (internal errors only). Na and O, and Al and O follow the well known anticorrelations found in many other GCs. We identified two groups of stars with significantly different abundances of the s-process elements Y, Zr, and Ba. The relative numbers of the two group members are very similar to the ratio of the number of stars in the two sub giant branches (SGB) of M 22. Y and Ba abundances do not correlate with Na, O, and Al. The s-element rich stars are also richer in iron and have higher Ca abundances. The results from high resolution spectra were confirmed by analyses of lower resolution GIRAFFE spectra of fourteen additional M 22 stars. The analyses of the GIRAFFE spectra also show that the Eu -a pure r-process element -abundance is not related to the iron content. We discuss the chemical abundance pattern of M 22 stars in the context of GC multiple stellar populations phenomenon.

We present a detailed chemical composition analysis of 35 red giant stars in the globular cluster M 22. High resolution spectra for this study were obtained at five observatories, and analyzed in a uniform manner. We have determined abundances of representative light proton-capture, α, Fe-peak and neutron-capture element groups. Our aim is to better understand the peculiar chemical enrichment history of this cluster, in which two stellar groups are characterized by a different content in iron, neutron capture elements Y, Zr and Ba, and α element Ca

We use Hubble Space Telescope (HST) and ground-based imaging to study the multiple populations of 47 Tuc, combining high-precision photometry with

We build on the evidence provided by our Legacy Survey of Galactic globular clusters (GC) to submit to a crucial test four scenarios currently entertained for the formation of multiple stellar generations in GCs. The observational constraints on multiple generations to be fulfilled are manifold, including GC specificity, ubiquity, variety, predominance, discreteness, supernova avoidance, p-capture processing, helium enrichment and mass budget. We argue that scenarios appealing to supermassive stars, fast rotating massive stars and massive interactive binaries violate in an irreparable fashion two or more among such constraints. Also the scenario appealing to AGB stars as producers of the material for next generation stars encounters severe difficulties, specifically concerning the mass budget problem and the detailed chemical composition of second generation stars. We qualitatively explore ways possibly allowing one to save the AGB scenario, specifically appealing to a possible revision of the cross section of a critical reaction rate destroying sodium, or alternatively by a more extensive exploration of the vast parameter space controlling the evolutionary behavior of AGB stellar models. Still, we cannot ensure success for these efforts and totally new scenarios may have to be invented to understand how GCs formed in the early Universe.

Recent work, based on data from the Hubble Space Telescope (HST ) UV Legacy Survey of Galactic Globular Clusters (GCs), has revealed that all the analyzed clusters host two groups of first-(1G) and second-generation (2G) stars. In most GCs, both 1G and 2G stars host substellar populations with different chemical composition.We compare multi-wavelength HST photometry with synthetic spectra to determine for the first time the average helium difference between the 2G and 1G stars in a large sample of 57 GCs and the maximum helium variation within each of them. We find that in all clusters 2G stars are consistent with being enhanced in helium with respect to 1G. The maximum helium variation ranges from less than 0.01 to more than 0.10 in helium mass fraction and correlates with both the cluster mass and the color extension of the horizontal branch (HB). These findings demonstrate that the internal helium variation is one of the main (second) parameters governing the HB morphology.