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