An Abundance Analysis for Five Red Horizontal Branch Stars in the Extremely Metal Rich Globular Cluster NGC 6553

We provide a high dispersion line-by-line abundance analysis of five red HB stars in the extremely metal rich galactic globular cluster NGC 6553. These red HB stars are significantly hotter than the very cool stars near the tip of the giant branch in such a metal rich globular cluster and hence their spectra are much more amenable to an abundance analysis than would be the case for red giants. We find that the mean [Fe/H] for NGC 6553 is -0.16 dex, comparable to the mean abundance in the galactic bulge found by McWilliam & Rich (1994) and considerably higher than that obtained from an analysis of two red giants in this cluster by Barbuy etal (1999). The relative abundance for the best determined alpha process element (Ca) indicates an excess of alpha process elements of about a factor of two. The metallicity of NGC 6553 reaches the average of the Galactic bulge and of the solar neighborhood.Comment: 29 pages, 6 figures, accepted for publication in the Ap

Nucleosynthesis in Type II supernovae and the abundances in metal-poor stars

We explore the effects on nucleosynthesis in Type II supernovae of various parameters (mass cut, neutron excess, explosion energy, progenitor mass) in order to explain the observed trends of the iron-peak element abundance ratios ([Cr/Fe], [Mn/Fe], [Co/Fe] and [Ni/Fe]) in halo stars as a function of metallicity for the range $-4 \le$ [Fe/H] $\le -2.5$. [Cr/Fe] and [Mn/Fe] decrease with decreasing [Fe/H], while [Co/Fe] behaves the opposite way and increases. We show that such a behavior can be explained by a variation of mass cuts in Type II supernovae as a function of progenitor mass, which provides a changing mix of nucleosynthesis from an alpha-rich freeze-out of Si-burning and incomplete Si-burning. This explanation is consistent with the amount of ejected $^{56}$Ni determined from modeling the early light curves of individual supernovae. We also suggest that the ratio [H/Fe] of halo stars is mainly determined by the mass of interstellar hydrogen mixed with the ejecta of a single supernova which is larger for larger explosion energy and the larger Str\"omgren radius of the progenitor.Comment: 17 pages, LaTeX, Accepted for publication in the Astrophysical Journal, more discussion on the Galactic chemical evolutio

Na-O Anticorrelation and HB. IV. Detection of He-rich and He-poor stellar populations in the globular cluster NGC 6218

We used the multifiber spectrograph FLAMES on the ESO Very Large Telescope UT2 to derive atmospheric parameters, metallicities and abundances of O and Na for 79 red giant stars in the Galactic globular cluster NGC 6218 (M 12). We analyzed stars in the magnitude range from about 1 mag below the bump to the tip of the Red Giant Branch. The average metallicity we derive is [Fe/H]=-1.31+/-0.004+/-0.028 dex (random and systematic errors, respectively), with a very small star-to-star scatter (rms=0.033 dex), from moderately high-resolution Giraffe spectra. This is the first extensive spectroscopic abundance analysis in this cluster. Our results indicate that NGC 6218 is very homogeneous as far as heavy elements are concerned. On the other hand, light elements involved in the well known proton-capture reactions of H-burning at high temperature, such as O and Na, show large variations, anticorrelated with each other, at all luminosities along the red giant branch. The conclusion is that the Na-O anticorrelation must be established in early times at the cluster formation. We interpret the variation of Na found near the RGB-bump 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 GC where such a signature has been spectroscopically detected, when combined with consistent and homogeneous data obtained for NGC 6752 to gain in statistical significance.Comment: 15 pages, 10 figures; fig.5 degraded in resolution; tables 2,3,5 available at CDS. Accepted for publication on A&

Aluminum abundances of multiple stellar generations in the globular cluster NGC 1851

We study the distribution of aluminum abundances among red giants in the peculiar globular cluster NGC 1851. Aluminum abundances were derived from the strong doublet Al I 8772-8773 A measured on intermediate resolution FLAMES spectra of 50 cluster stars acquired under the Gaia-ESO public survey. We coupled these abundances with previously derived abundance of O, Na, Mg to fully characterize the interplay of the NeNa and MgAl cycles of H-burning at high temperature in the early stellar generation in NGC 1851. The stars in our sample show well defined correlations between Al,Na and Si; Al is anticorrelated with O and Mg. The average value of the [Al/Fe] ratio steadily increases going from the first generation stars to the second generation populations with intermediate and extremely modified composition. We confirm on a larger database the results recently obtained by us (Carretta et al. 2011a): the pattern of abundances of proton-capture elements implies a moderate production of Al in NGC 1851. We find evidence of a statistically significant positive correlation between Al and Ba abundances in the more metal-rich component of red giants in NGC 1851.Comment: Astronomy and Astrophysics, in pres

Star-to-star Na and O abundance variations along the red giant branch in NGC 2808

We report for the first time Na and O abundances from high-resolution, high S/N echelle spectra of 20 red giants in NGC 2808, taken as part of the Science Verification program of the FLAMES multi-object spectrograph at the ESO VLT. In these stars, spanning about 3 mag from the red giant branch (RGB) tip, large variations are detected in the abundances of oxygen and sodium, anticorrelated with each other; this is a well known evidence of proton-capture reactions at high temperatures in the ON and NeNa cycles. One star appears super O-poor; if the extension of the Na-O anticorrelation is confirmed, NGC 2808 might reach O depletion levels as large as those of M 13. This result confirms our previous findings based on lower resolution spectra (Carretta et al. 2003) of a large star-to-star scatter in proton capture elements at all positions along the RGB in NGC 2808, with no significant evolutionary contribution. Finally, the average metallicity for NGC 2808 is [Fe/H]= -1.14 +/- 0.01 dex (rms=0.06) from 19 stars.Comment: 12 pages, 3 figures, accepted for publication in ApJ Letter

Observational evidence for a different IMF in the early Galaxy

The unexpected high incidence of carbon-enhanced, s-process enriched unevolved stars amongst extremely metal-poor stars in the halo provides a significant constraint on the Initial Mass Function (IMF) in the early Galaxy. We argue that these objects are evidence for the past existence of a large population of intermediate-mass stars, and conclude that the IMF in the early Galaxy was different from the present, and shifted toward higher masses.Comment: 14 pages, 1 color figure, accepted for publication on Ap