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We present the iron abundance of 24 asymptotic giant branch (AGB) starsmembers of the globular cluster 47 Tucanae, obtained with high-resolutionspectra collected with the FEROS spectrograph at the MPG/ESO-2.2m Telescope. Wefind that the iron abundances derived from neutral lines (with mean value [FeI/H] = -0.94 +/- 0.01, {\sigma} = 0.08 dex) are systematically lower than thosederived from single ionized lines ([Fe II/H] = -0.83 +/- 0.01, {\sigma} = 0.05dex). Only the latter are in agreement with those obtained for a sample of redgiant branch (RGB) cluster stars, for which Fe I and Fe II lines provide thesame iron abundance. This finding suggests that Non Local ThermodynamicalEquilibrium (NLTE) effects driven by overionization mechanisms are present inthe atmosphere of AGB stars and significantly affect Fe I lines, while leavingFe II features unaltered. On the other hand, the very good ionizationequilibrium found for RGB stars indicates that these NLTE effects may depend onthe evolutionary stage. We discuss the impact of this finding both on thechemical analysis of AGB stars, and on the search for evolved blue stragglers.

NLTE effects on AGB stars in 47 Tucanae