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arXiv · astro-ph/0701179

Na-O Anticorrelation and Horizontal Branches. V. The Na-O anticorrelation in NGC 6441 from Giraffe spectra

R. G. Gratton·S. Lucatello·A. Bragaglia·E. Carretta·S. Cassisi·Y. Momany·E. Pancino·E. Valenti·V. Caloi·R. Claudi·F. D'Antona·S. Desidera·P. François·G. James·S. Moehler·S. Ortolani·L. Pasquini·G. Piotto·A. Recio-Blanco
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Abstract

We present an analysis of FLAMES-Giraffe spectra for several bright giants in NGC 6441, to investigate the presence and extent of the Na-O anticorrelation in this anomalous globular cluster. The field of NGC 6441 is very crowded, with severe contamination by foreground (mainly bulge) field stars. Appropriate membership criteria were devised to identify a group of 25 likely cluster members among the about 130 stars observed. Combined with the UVES data obtained with the same observations, high dispersion abundance analyses are now available for a total of 30 stars in NGC 6441, 29 of them having data for both O and Na. The spectra were analyzed by a standard line analysis procedure; care was taken to minimize the impact of the differential interstellar reddening throughout the cluster, and to extract reliable information from crowded, and moderately high S/N (30-70), moderately high resolution (R ~ 23,000) spectra. NGC 6441 is very metal-rich ([Fe/H]=$-0.34\pm 0.02\pm0.04$ dex). There is no clear sign of star-to-star scatter in the Fe-peak elements. The alpha-elements Mg, Si, Ca, and Ti are overabundant by rather large factors, suggesting that the cluster formed from material enriched by massive core collapse SNe. The O-Na anticorrelation is well defined, with about 1/4 of the stars being Na-rich and O-poor. One of the stars is a Ba-rich and moderately C-rich star. The distribution of [Na/O] ratios among RGB stars in NGC 6441 appears similar to the distribution of colors of stars along the horizontal branch. The fraction of Na-poor, O-rich stars found in NGC 6441 agrees well with that of stars on the red horizontal branch of this cluster (in both cases about 80%), with a sloping distribution toward lower values of [O/Na] (among RGB stars and bluer colors (among HB stars).

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R. G. Gratton, S. Lucatello, A. Bragaglia, E. Carretta, S. Cassisi, Y. Momany, E. Pancino, E. Valenti, V. Caloi, R. Claudi, F. D'Antona, S. Desidera, P. François, G. James, S. Moehler, S. Ortolani, L. Pasquini, G. Piotto, A. Recio-Blanco. 2007-01-08. Na-O Anticorrelation and Horizontal Branches. V. The Na-O anticorrelation in NGC 6441 from Giraffe spectra. https://doi.org/10.1051/0004-6361%3A20066061

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