Horizontal branch morphology and multiple stellar populations in the anomalous globular cluster M 22

M 22 is an anomalous globular cluster that hosts two groups of stars with different metallicity and s-element abundance. The star-to-star light-element variations in both groups, with the presence of individual Na-O and C-N anticorrelations, demonstrates that this Milky Way satellite has experienced a complex star formation history. We have analyzed FLAMES/UVES spectra for seven stars covering a small color interval on the reddest horizontal branch (HB) portion of this cluster and investigated possible relations between the chemical composition of a star and its location along the HB. Our chemical abundance analysis takes into account effects introduced by deviations from the local thermodynamic equilibrium (NLTE effects), which are significant for the measured spectral lines in the atmospheric parameters range spanned by our stars. We find that all the analyzed stars are barium-poor and sodium-poor, thus supporting the idea that the position of a star along the HB is strictly related to the chemical composition, and that the HB morphology is influenced by the presence of different stellar populations.