Paper detail

COSMIC-LAB: Unexpected Results from High-resolution Spectra of AGB Stars in Globular Clusters

This thesis is aimed at clarifying one of the least studied phases of stellar evolution: the asymptotic giant branch (AGB). Recent results obtained for Galactic globular clusters (GCs) suggest that the AGB stage may contain crucial information about the evolutionary history of exotic stars (Beccari et al. 2006) and multiple-populations (Campbell et al. 2013) in the parent cluster. The thesis presents the analysis of a large sample of high-resolution spectra of AGB stars in four Galactic GCs, acquired at the Very Large Telescope (ESO) and the 2.2 meter telescope (MPG). The obtained results provide evidence of a previously unknown physical mechanism affecting the neutral species of some chemical elements in the atmosphere of most AGB stars: because of it, the abundances derived from neutral lines are systematically underestimated, while those measured from ionized lines remain unaffected. Such a behaviour exactly corresponds to what expected in the case of non-local thermodynamic equilibrium (NLTE) conditions in the star atmosphere. For this reason, in this work we refer to it as "NLTE effect", with the caveat that this could be not the case. In fact, the current NLTE models are unable to account for the observed effect, thus demonstrating that either our comprehension of NLTE is not adequate enough, or that some more complex physical phenomena are occurring in AGB atmospheres. This effect has been found in all the investigated GCs. It affects most (but not all) AGB stars and, in some cases, also some RGB stars. It is particularly evident for iron and titanium lines (i.e., the elements providing the largest number of both neutral and ionized lines). The deep understanding of the detected phenomenon is of paramount importance since it has a huge impact on the proper determination of GC chemistry and enrichment history (for instance, it can mimic spurious iron spreads).