A sequence of nitrogen-rich very red giants in the globular cluster NGC 1851

We present the abundances of N in a sample of 62 stars on the red giant branch (RGB) in the peculiar globular cluster NGC 1851. The values of [N/Fe] ratio were obtained by comparing the flux measured in the observed spectra with that from synthetic spectra for up to about 15 features of CN. This is the first time that N abundances are obtained for such a large sample of RGB stars from medium-resolution spectroscopy in this cluster. With these abundances we provide a chemical tagging of the split red giant branch found from several studies in NGC 1851. The secondary, reddest sequence on the RGB is populated almost exclusively by N-rich stars, confirming our previous suggestion based on Stromgren magnitudes and colours. These giants are also, on average, enriched in s-process elements such as Ba, and are likely the results of pollution from low mass stars that experienced episodes of third dredge-up in the asymptotic giant branch phase.Comment: Version to match the one in press on Astronomy and Astrophysic

An aluminium tool for multiple stellar generations in the globular clusters 47 Tuc and M 4

We present aluminium abundances for a sample of about 100 red giant stars in each of the Galactic globular clusters 47 Tuc (NGC 104) and M 4 (NGC 6121). We have derived homogeneous abundances from intermediate-resolution FLAMES/GIRAFFE spectra. Aluminium abundances are from the strong doublet Al I at 8772-8773 A as in previous works done for giants in NGC 6752 and NGC 1851, and nitrogen abundances are extracted from a large number of features of the CN molecules, by assuming a suitable carbon abundance. We added previous homogeneous abundances of O and Na and newly derived abundances of Mg and Si for our samples of 83 stars in M 4 and 116 stars in 47 Tuc to obtain the full set of elements from proton-capture reactions produced by different stellar generations in these clusters. By simultaneously studying the Ne-Na and Mg-Al cycles of H-burning at high temperature our main aims are to understand the nature of the polluters at work in the first generation and to ascertain whether the second generation of cluster stars was formed in one or, rather, several episodes of star formation. Our data confirm that in M 4 only two stellar populations are visible. On the other hand, for 47 Tuc a cluster analysis performed on our full dataset suggests that at least three distinct groups of stars are present on the giant branch. The abundances of O, Na, Mg and Al in the intermediate group can be produced within a pollution scenario; results for N are ambiguous, depending on the C abundance we adopt for the three groups.Comment: 11 pages, 9 figures, 2 on-line tables: accepted for publication on Astronomy and Astrophysic

Calcium and light-elements abundance variations from high resolution spectroscopy in globular clusters

We use abundances of Ca, O, Na, Al from high resolution UVES spectra of 200 red giants in 17 globular clusters (GCs) to investigate the correlation found by Lee et al. (2009) between chemical enrichment from SN II and star-to-star variations in light elements in GC stars. We find that (i) the [Ca/H] variations between first and second generation stars are tiny in most GCs (~0.02-0.03 dex, comparable with typical observational errors). In addition, (ii) using a large sample of red giants in M 4 with abundances from UVES spectra from Marino et al. (2008), we find that Ca and Fe abundances in the two populations of Na-poor and Na-rich stars are identical. These facts suggest that the separation seen in color-magnitude diagrams using the U band or hk index (as observed in NGC 1851 by Han et al. 2009) are not due to Ca variations. Small differences in [Ca/H] as associated to hk variations might be due to a small systematic effect in abundance analysis, because most O-poor/Na-rich (He-rich) stars have slightly larger [Fe/H] (by 0.027 dex on average, due to decreased H in the ratio) than first generation stars and are then located at redder positions in the V,hk plane. While a few GCs (M 54, omega Cen, M 22, maybe even NGC 1851) do actually show various degree of metallicity spread, our findings eliminate the need of a close link between the enrichment by core-collapse SNe with the mechanism responsible for the Na-O anticorrelation.Comment: Uses emulateapj, 3 figures, 2 tables (1 only available in electronic form), accepted for publication on ApJ Letter

Reading the book: from "chemical anomalies" to "standard composition" of globular clusters

It is now commonly accepted that globular clusters (GCs) have undergone a complex formation and that they host at least two stellar generations. This is a recent paradigm and is founded on both photometric and spectroscopic evidence. We concentrate on results based on high-resolution spectroscopy and on how we moved from single to multiple stellar populations concept for GCs. We underline that the peculiar chemical composition of GC stars is fundamental in establishing the multiple populations scenario and briefly outline what can be learned from observations. Finally, recent observational results on large samples of stars in different evolutionary phases are discussed.Comment: 5 pages, 1 figure. To appear in the proceedings of "Reading the book of globular clusters with the lens of stellar evolution", in the Memorie della Societa Astronomica Italian

Diagnostics of Stellar Modelling from Spectroscopy and Photometry of Globular Clusters

We conduct a series of comparisons between spectroscopic and photometric observations of globular clusters and stellar models to examine their predictive power. Data from medium-to-high resolution spectroscopic surveys of lithium allow us to investigate first dredge-up and extra mixing in two clusters well separated in metallicity. Abundances at first dredge-up are satisfactorily reproduced but there is preliminary evidence to suggest that the models overestimate the luminosity at which the surface composition first changes in the lowest-metallicity system. Our models also begin extra mixing at luminosities that are too high, demonstrating a significant discrepancy with observations at low metallicity. We model the abundance changes during extra mixing as a thermohaline process and determine that the usual diffusive form of this mechanism cannot simultaneously reproduce both the carbon and lithium observations. Hubble Space Telescope photometry provides turnoff and bump magnitudes in a large number of globular clusters and offers the opportunity to better test stellar modelling as function of metallicity. We directly compare the predicted main-sequence turn-off and bump magnitudes as well as the distance-independent parameter $\Delta M_V ~^{\rm{MSTO}}_{\rm{bump}}$. We require 15 Gyr isochrones to match the main-sequence turn-off magnitude in some clusters and cannot match the bump in low-metallicity systems. Changes to the distance modulus, metallicity scale and bolometric corrections may impact on the direct comparisons but $\Delta M_V ~^{\rm{MSTO}}_{\rm{bump}}$, which is also underestimated from the models, can only be improved through changes to the input physics. Overshooting at the base of the convective envelope with an efficiency that is metallicity dependent is required to reproduce the empirically determined value of $\Delta M_V ~^{\rm{MSTO}}_{\rm{bump}}$.Comment: 20 pages, 11 Figures, 4 Tables, Accepted for publication in MNRA

Terzan 8: a Sagittarius-flavoured globular cluster

Massive globular clusters (GCs) contain at least two generations of stars with slightly different ages and clearly distinct light elements abundances. The Na-O anticorrelation is the best studied chemical signature of multiple stellar generations. Low-mass clusters appear instead to be usually chemically homogeneous. We are investigating low-mass GCs to understand what is the lower mass limit where multiple populations can form, mainly using the Na and O abundance distribution. We used VLT/FLAMES spectra of giants in the low-mass, metal-poor GC Terzan 8, belonging to the Sagittarius dwarf galaxy, to determine abundances of Fe, O, Na, alpha-, Fe-peak, and neutron-capture elements in six stars observed with UVES and 14 observed with GIRAFFE. The average metallicity is [Fe/H]=-2.27+/-0.03 (rms=0.08), based on the six high-resolution UVES spectra. Only one star, observed with GIRAFFE, shows an enhanced abundance of Na and we tentatively assign it to the second generation. In this cluster, at variance with what happens in more massive GCs, the second generation seems to represent at most a small minority fraction. We discuss the implications of our findings, comparing Terzan 8 with the other Sgr dSph GCs, to GCs and field stars in the Large Magellanic Cloud, Fornax, and in other dwarfs galaxies.Comment: 15 pages, 12 figures, 10 tables; accepted for publication on Astronomy and Astrophysic

The beneficial effect of Zinc(II) on low-dose chemotherapeutic sensitivity involves p53 activation in wild-type p53-carrying colorectal cancer cells

BACKGROUND: Activation of wild-type p53 in response to genotoxic stress occurs through different mechanisms including protein conformation, posttranslational modifications, and nuclear localization, leading to DNA binding to sequence-specific promoters. Zinc ion plays a crucial role in stabilizing p53/DNA binding to induce canonical target genes. Mutant p53 proteins undergo protein misfolding that can be counteracted by zinc. However, whether zinc supplementation might have a beneficial antitumor effect in wild-type p53-carrying cells in combination with drugs, has not been addressed so far. METHODS: In this study we compared the effect of two antitumor treatments: on the one hand wild-type p53-carrying colon cancer cells were treated with low and high doses of chemotherapeutic agent Adriamycin and, on the other hand, Adriamycin was used in combination with ZnCl2. Biochemical and molecular analyses were applied to evaluate p53 activity and biological outcomes in this setting. Finally, the effect of the different combination treatments were applied to assess tumor growth in vivo in tumor xenografts. RESULTS: We found that low-dose Adriamycin did not induce p53 activation in wtp53-carrying colon cancer cells, unless in combination with ZnCl2. Mechanistically, ZnCl2 was a key determinant in inducing wtp53/DNA binding and transactivation of target genes in response to low-dose Adriamycin that used alone did not achieve such effects. Finally, in vivo studies, in a model of wtp53 colon cancer xenograft, show that low-dose Adriamycin did not induce tumor regression unless in combination with ZnCl2 that activated endogenous wtp53. CONCLUSIONS: These results provide evidence that ZnCl2 might be a valuable adjuvant in chemotherapeutic regimens of colorectal cancer harboring wild-type p53, able to both activate p53 and reduce the amount of drugs for antitumor purposes

Chemical characterization of the globular cluster NGC 5634 associated to the Sagittarius dwarf spheroidal galaxy

As part of our on-going project on the homogeneous chemical characterization of multiple stellar populations in globular clusters (GCs), we studied NGC 5634, associated to the Sagittarius dwarf spheroidal galaxy, using high-resolution spectroscopy of red giant stars collected with FLAMES@VLT. We present here the radial velocity distribution of the 45 observed stars, 43 of which are member, the detailed chemical abundance of 22 species for the seven stars observed with UVES-FLAMES, and the abundance of six elements for stars observed with GIRAFFE. On our homogeneous UVES metallicity scale we derived a low metallicity [Fe/H]=-1.867 +/-0.019 +/-0.065 dex (+/-statistical +/-systematic error) with sigma=0.050 dex (7 stars). We found the normal anti-correlations between light elements (Na and O, Mg and Al), signature of multiple populations typical of massive and old GCs. We confirm the associations of NGC 5634 to the Sgr dSph, from which the cluster was lost a few Gyr ago, on the basis of its velocity and position and the abundance ratios of alpha and neutron capture elements.Comment: 16 pages, 10 figures, 11 tables; accepted for publication on Astronomy and Astrophysic

Rubidium abundances in the globular clusters NGC 6752, NGC 1904 and NGC 104 (47 Tuc)

Large star-to-star variations of the abundances of proton-capture elements, such as Na and O, in globular clusters (GCs) are interpreted as the effect of internal pollution resulting from the presence of multiple stellar populations. To better constrain this scenario we investigate the abundance distribution of the heavy element rubidium (Rb) in NGC 6752, NGC 1904, and NGC 104 (47 Tuc). Combining the results from our sample with those in the literature, we found that Rb exhibits no star-to-star variations, regardless the cluster metallicity, with the possible intriguing, though very uncertain, exception of the metal-rich bulge cluster NGC 6388. If no star-to-star variations will be confirmed for all GCs, it implies that the stellar source of the proton-capture element variations must not have produced significant amounts of Rb. This element is observed to be enhanced at extremely high levels in intermediate-mass AGB (IM-AGB) stars in the Magellanic Clouds (i.e., at a metallicity similar to 47 Tuc and NGC 6388). This may present a challenge to this popular candidate polluter, unless the mass range of the observed IM-AGB stars does not participate in the formation of the second-generation stars in GCs. A number of possible solutions are available to resolve this conundrum, also given that the Magellanic Clouds observations are very uncertain and may need to be revised. The fast rotating massive stars scenario would not face this potential problem as the slow mechanical winds of these stars during their main-sequence phase do not carry any Rb enhancements; however, these candidates face even bigger issues such as the production of Li and the close over-imposition with core-collapse supernova timescales. Observations of Sr, Rb, and Zr in metal-rich clusters such as NGC 6388 and NGC 6441 are sorely needed to clarify the situation.Comment: Accepted for publication in The Astrophysical Journa