Na-O anticorrelation and HB. I. The Na-O anticorrelation in NGC 2808

We derived the atmospheric parameters and elemental abundances of Fe, O, and Na for about 120 red giant stars in the Galactic globular cluster NGC 2808. Our results are based on the analysis of medium-high resolution (R=22 000{-}24 000) GIRAFFE spectra acquired with the FLAMES spectrograph at VLT-UT2 as a part of a project aimed at studying the Na-O anticorrelation as a function of physical parameters in globular clusters. We present the anticorrelation of Na and O abundances in NGC 2808 here, and discuss the distribution function of stars along this relation. Besides a bulk of O-normal stars with the typical composition of field halo stars, NGC 2808 seems to host two other groups of O-poor and super O-poor stars. In this regard, NGC 2808 is similar to M 13, the template cluster for the Na-O anticorrelation. However, in contrast to M 13, most stars in NGC 2808 are O-rich. This might be related to the horizontal branch morphologies that are very different in these two clusters. The average metallicity we found for NGC 2808 is [Fe/H]=-1.10 (rms = 0.065 dex, from 123 stars). We also found some evidence of a small intrinsic spread in metallicity, but more definitive conclusions are hampered by the presence of a small differential reddening.