The Na-O anticorrelation in horizontal branch stars. I. NGC 2808

Globular clusters have been recognized to host multiple stellar populations. A spectacular example of this is the massive cluster NGC2808, where multiple populations have been found along the horizontal branch (HB) and the main sequence (MS). Studies of red giants showed that this cluster appears homogeneous insofar Fe abundance is concerned, but it shows an extended anticorrelation between Na and O abundances. The Na-poor, O-rich population can be identified with the red MS, and the Na-rich, O-poor one with the blue one. This may be understood in terms of different He content, He being correlated with Na. A prediction of this scenario is that He-rich, Na-rich He-core burning stars, because they are less massive, will end up on the bluer part of the HB, while He-poor, Na-poor stars will reside on the red HB. The aim of this paper is to verify this prediction. To this purpose, we acquired high-resolution spectra of regions including strong O and Na lines in several tens of HB stars of NGC2808, sampling both the red and blue parts of the HB. We limited our analysis to those blue HB stars cooler than the gap at 11,500 K, because diffusion and radiative pressure are known to strongly modify the atmospheric composition of warmer stars. We indeed found a strict correspondence between the colour of the HB stars and their Na and O abundances: all blue HB stars are very O-poor and Na-rich. In addition, we found that while all the red HB stars are more O-rich and Na-poor than the blue ones, there is a moderate Na-O anticorrelation among them as well. This anticorrelation is in turn related to the colour of the red HB stars. These results reinforce the connection between Na and O abundances and the second parameter phenomenon, and show that there are more than three stellar populations in NGC 2808 because only a fraction of the red HB stars belong to the primordial population of this cluster.