Evidence of the inhomogeneity of the stellar population in the differentially reddened globular cluster NGC 3201

We report on evidence of the inhomogeneity (multiplicity) of the stellar population in the Galactic globular cluster (GC) NGC 3201, which is irregularly reddened across its face. We carried out a more detailed and careful analysis of our recently published new multi-color photometry in a wide field of the cluster with particular emphasis on the U band. Using the photometric data corrected for differential reddening, we found for the first time two key signs of the inhomogeneity in the cluster's stellar population and of its radial variation in the GC. These are (1) an obvious trend in the color-position diagram, based on the (U-B) color-index, of red giant branch (RGB) stars, which shows that the farther from the cluster's center, the bluer on average the (U-B) color of the stars is; and (2) the dependence of the radial distribution of sub-giant branch (SGB) stars in the cluster on their U magnitude, where brighter stars are less centrally concentrated than their fainter counterparts at a confidence level varying between 99.2% and 99.9% depending on the color-index used to select the stars. The same effects were recently found by us in the GC NGC 1261. However, contrary to NGC 1261, we are not able to unambiguously suggest which of the sub-populations of SGB/RGB stars can be the progenitor of blue and red horizontal branch stars of the cluster. Apart from M4, NGC 3201 is another GC very probably with an inhomogeneous stellar population, which has essentially lower mass than the most massive Galactic GCs where multiple stellar populations were unambiguously detected for the first timeComment: 5 pages, 4 figure

Horizontal-branch morphology and multiple stellar populations in the anomalous globular cluster M22

M22 is an anomalous globular cluster that hosts two groups of stars with different metallicity and s-element abundance. The star-to-star light-element variations in both groups, with the presence of individual Na-O and C-N anticorrelations, demonstrates that this Milky-Way satellite has experienced a complex star-formation history. We have analysed FLAMES/UVES spectra for seven stars covering a small color interval, on the reddest horizontal-branch (HB) portion of this cluster and investigated possible relations between the chemical composition of a star and its location along the HB. Our chemical abundance analysis takes into account effects introduced by deviations from the local-thermodynamic equilibrium (NLTE effects), that are significant for the measured spectral lines in the atmospheric parameters range spanned by our stars. We find that all the analysed stars are barium-poor and sodium-poor, thus supporting the idea that the position of a star along the HB is strictly related to the chemical composition, and that the HB-morphology is influenced by the presence of different stellar populations.Comment: 21 pages, 3 figures, accepted for publication in Ap

A Double Main Sequence in the Globular Cluster NGC 6397

High-precision multi-band HST photometry reveals that the main sequence (MS) of the globular cluster NGC 6397 splits into two components, containing ~30% and ~70% of the stars. This double sequence is consistent with the idea that the cluster hosts two stellar populations: (i) a primordial population that has a composition similar to field stars, and containing ~30% of the stars, and (ii) a second generation with enhanced sodium and nitrogen, depleted carbon and oxygen, and a slightly enhanced helium abundance (Delta Y~0.01). We examine the color difference between the two sequences across a variety of color baselines and find that the second sequence is anomalously faint in m_F336W. Theoretical isochrones indicate that this could be due to NH depletion.Comment: 19 pages, 11 figures, accepted for pubblication in Ap

The oxygen vs. sodium (anti)correlation(s) in omega Cen

Recent exam of large samples of omega Cen giants shows that it shares with mono-metallic globular clusters the presence of the sodium versus oxygen anticorrelation, within each subset of stars with iron content in the range -1.9<~[Fe/H]<~-1.3. These findings suggest that, while the second generation formation history in omega Cen is more complex than that of mono-metallic clusters, it shares some key steps with those simpler cluster. In addition, the giants in the range -1.3<[Fe/H]<~-0.7 show a direct O--Na correlation, at moderately low O, but Na up to 20 times solar. These peculiar Na abundances are not shared by stars in other environments often assumed to undergo a similar chemical evolution, such as in the field of the Sagittarius dwarf galaxy. These O and Na abundances match well the yields of the massive asymptotic giant branch stars (AGB) in the same range of metallicity, suggesting that the stars at [Fe/H]>-1.3 in omega Cen are likely to have formed directly from the pure ejecta of massive AGBs of the same metallicities. This is possible if the massive AGBs of [Fe/H]>-1.3 in the progenitor system evolve when all the pristine gas surrounding the cluster has been exhausted by the previous star formation events, or the proto--cluster interaction with the Galaxy caused the loss of a significant fraction of its mass, or of its dark matter halo, and the supernova ejecta have been able to clear the gas out of the system. The absence of dilution in the metal richer populations lends further support to a scenario of the formation of second generation stars in cooling flows from massive AGB progenitors. We suggest that the entire formation of omega Cen took place in a few 10^8yr, and discuss the problem of a prompt formation of s--process elements.Comment: The Astrophysical Journal, in pres

The helium spread in the Globular cluster 47 Tuc

Spectroscopy has shown the presence of the CN band dicothomy and the Na-O anticorrelations for 50--70% of the investigated samples in the cluster 47 Tuc, otherwise considered a "normal" prototype of high metallicity clusters from the photometric analysis. Very recently, the re-analysis of a large number of archival HST data of the cluster core has been able to put into evidence the presence of structures in the Sub Giant Branch: it has a brighter component with a spread in magnitude by $\sim$0.06 mag and a second one, made of about 10% of stars, a little fainter (by $\sim$0.05 mag). These data also show that the Main Sequence of the cluster has an intrinsic spread in color which, if interpreted as due to a small spread in helium abundance, suggests $\Delta$Y$\sim$0.027. In this work we examine in detail whether the Horizontal Branch morphology and the Sub Giant structure provide further independent indications that a real --although very small-helium spread is present in the cluster. We re--analyze the HST archival data for the Horizontal Branch of 47 Tuc, obtaining a sample of $\sim$500 stars with very small photometric errors, and build population synthesis based on new models to show that its particular morphology can be better explained by taking into account a spread in helium abundance of 2% in mass. The same variation in helium is able to explain the spread in luminosity of the Sub Giant Branch, while a small part of the second generation is characterized by a small C+N+O increase and provides an explanation for the fainter Sub Giant Branch. We conclude that three photometric features concur to form the paradigm that a small but real helium spread is present in a cluster that has no spectacular evidence for multiple populations like those shown by other massive clusters.Comment: Accepted for publication in the MNRAS on 2010 June 8. Received 2010 May 19; in original form 2010 February 9. 7 pages and 3 figures. No table

New HST WFC3/UVIS observations augment the stellar-population complexity of omega Centauri

We used archival multi-band Hubble Space Telescope observations obtained with the Wide-Field Camera 3 in the UV-optical channel to present new important observational findings on the color-magnitude diagram (CMD) of the Galactic globular cluster omega Centauri. The ultraviolet WFC3 data have been coupled with available WFC/ACS optical-band data. The new CMDs, obtained from the combination of colors coming from eight different bands, disclose an even more complex stellar population than previously identified. This paper discusses the detailed morphology of the CMDs.Comment: 17 pages, 14 figures (11 in low res), 3 tables. Accepted for publication in AJ on June 19, 201

A unique model for the variety of multiple populations formation(s) in globular clusters: a temporal sequence

We explain the multiple populations recently found in the 'prototype' Globular Cluster (GC) NGC 2808 in the framework of the asymptotic giant branch (AGB) scenario. The chemistry of the five -or more- populations is approximately consistent with a sequence of star formation events, starting after the supernovae type II epoch, lasting approximately until the time when the third dredge up affects the AGB evolution (age ~90-120Myr), and ending when the type Ia supernovae begin exploding in the cluster, eventually clearing it from the gas. The formation of the different populations requires episodes of star formation in AGB gas diluted with different amounts of pristine gas. In the nitrogen-rich, helium-normal population identified in NGC 2808 by the UV Legacy Survey of GCs, the nitrogen increase is due to the third dredge up in the smallest mass AGB ejecta involved in the star formation of this population. The possibly-iron-rich small population in NGC 2808 may be a result of contamination by a single type Ia supernova. The NGC 2808 case is used to build a general framework to understand the variety of 'second generation' stars observed in GCs. Cluster-to-cluster variations are ascribed to differences in the effects of the many processes and gas sources which may be involved in the formation of the second generation. We discuss an evolutionary scheme, based on pollution by delayed type II supernovae, which accounts for the properties of s-Fe-anomalous clusters.Comment: 20 pages, 7 figures, in press on MNRA

The Near Infrared NaI Doublet Feature in M Stars

The NaI near-infrared doublet has been used to indicate the dwarf/giant population in composite systems, but its interpretation is still a contentious issue. In order to understand the behaviour of this controversial feature, we study the observed and synthetic spectra of cool stars. We conclude that the NaI infrared feature can be used as a dwarf/giant discriminator. We propose a modified definition of the NaI index by locating the red continuum at 8234 angstrons and by measuring the equivalent width in the range 8172-8197 angstrons, avoiding the region at lambda > 8197 angstrons, which contains VI, ZrI, FeI and TiO lines. We also study the dependence of this feature on stellar atmospheric parameters.Comment: 9 pages, (TeX file) + 7 Figures in Postscript format. Accepted for publication in The Astrophysical Journa