Star-to-Star Na and O Abundance Variations along the Red Giant Branch in NGC 2808

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Abstract
We report for the first time Na and O abundances from high-resolution, high signal-to-noise ratio echelle spectra of 20 red giants in NGC 2808, taken as part of the Science Verification program of the FLAMES multiobject 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 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 M13. This result confirms our previous findings based on lower resolution spectra (Carretta et al.) 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.
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