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).
