NLTE effects on AGB stars in 47 Tucanae

TL;DR

This study investigates how NLTE effects influence iron abundance measurements in AGB stars of 47 Tucanae, revealing that overionization causes underestimation of Fe I lines, which impacts chemical analysis and stellar evolution studies.

Contribution

It demonstrates the presence of NLTE effects in AGB stars and compares their impact to RGB stars, highlighting the importance of considering evolutionary stage in chemical abundance analyses.

Findings

Fe I lines underestimate iron abundance in AGB stars due to NLTE effects.

Fe II lines provide consistent iron abundance across AGB and RGB stars.

NLTE effects are likely dependent on stellar evolutionary stage.

Abstract

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.