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 NGC 6752 have been found to exhibit some chemical peculiarities with respect to the red giant branch (RGB) stars. A discrepancy between [Fe i/H] and [Fe ii/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 NGC 6752 observed with the spectrograph's 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 [Fe i/H] and [Fe ii/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 NGC 6752.