Confirming the presence of second population stars and the iron
  discrepancy along the AGB of the globular cluster NGC 6752
  Asymptotic giant branch (AGB) stars in the globular cluster NGC6752 have been
found to exhibit some chemical peculiarities with respect to the red giant
branch (RGB) stars. A discrepancy between [FeI/H] and [FeII/H] (not observed in
RGB stars) has been detected adopting spectroscopic temperatures. Moreover, a
possible lack of second-population stars along the AGB was claimed. The use of
photometric temperatures based on (V-K) colors was proposed to erase this iron
discrepancy. Also, ad hoc scenarios have been proposed to explain the absence
of second-population AGB stars. Here we analyzed a sample of 19 AGB and 14 RGB
stars of NGC6752 observed with the spectrographs UVES. The two temperature
scales agree very well for the RGB stars while for the AGB stars there is a
systematic offset of ~100 K. We found that even if the photometric temperatures
alleviate the iron discrepancy with respect to the spectroscopic ones, a
systematic difference between [FeI/H] and [FeII/H] is still found among the AGB
stars. An unexpected result is that the photometric temperatures do not satisfy
the excitation equilibrium in the AGB stars. This suggests that standard 1D-LTE
model atmospheres are unable to properly describe the thermal structure of AGB
stars, at variance with the RGB stars. The use of photometric temperatures
confirms the previous detection of second-population AGB stars in this cluster,
with the presence of clear correlations/anticorrelations among the light
element abundances. This firmly demonstrates that both first and
second-population stars evolve along the AGB of NGC6752.
