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

• Published in: arXiv.org 2007-01
• Main Authors: Gratton, R G, Lucatello, S, Bragaglia, A, Carretta, E, Cassisi, S, Momany, Y, Pancino, E, Valenti, E, Caloi, V, Claudi, R, D'Antona, F, Desidera, S, François, P, James, G, Moehler, S, Ortolani, S, Pasquini, L, Piotto, G, Recio-Blanco, A
• Format: Article
• Language: English
• Subjects: Calcium; Collapse; Globular clusters; Horizontal branch stars; Interstellar extinction; Iron; Magnesium; Spectra; Stars; Titanium
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.0701179

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).