Proton capture elements in the globular cluster NGC 2808 - I. First detection of large variations in sodium abundances along the Red Giant Branch

We have used spectra obtained as part of the Science Verification program of the FLAMES multi-object spectrograph mounted on Kueyen (VLT-UT2) to perform an abundance analysis of stars along the giant branch (RGB) in the globular cluster NGC 2808. Sodium abundances are derived from Na D lines for a sample of 81 cluster stars spanning a range of about 2 mag from the tip of the RGB. Our results show that a large star-to-star scatter does exist at all positions along the RGB, suggesting large variations in the abundance of proton capture elements down to luminosities comparable to the red horizontal branch (HB). The distribution of Na abundances along the RGB seems to point out that in this cluster most of the observed spread has a primordial origin. Overimposed evolutionary effects, if any, must be only at "noise" level, at odds with results from a similar analysis in M 13 (Pilachowski et al. 1996). This study is a first step towards ascertaining if a link exists between the distribution of chemical anomalies in light elements along the RGB and the global properties of globular clusters, in particular the HB morphology.