Chemical Characteristics of Galactic bulge AGB stars

The Galactic bulge is the major component of the Milky Way galaxy whose formation is least understood. Detailed investigations of the Bulge stellar population and its chemical characteristics commenced with the advent of high-resolution spectrographs mounted to 8m class telescopes. Despite these efforts, it is still unclear whether the Bulge is a merger-driven "classical" bulge, or rather a "pseudo-bulge" formed by secular evolution of the Disk. The chemical properties of Bulge stars provide constraints on the possible formation scenarios. Among the stellar types that are accessible to spectroscopic study by present-day instrumentation, Asymptotic Giant Branch (AGB) stars have been excluded so far, mainly because of the complexity of their optical spectra. AGB stars are the evolutionary link between K-giants and Planetary Nebulae, object types that have already been studied extensively in the Bulge. The present project aims at filling this gap by analysing high-resolution near-infrared spectra of a considerable sample of AGB stars located in the Galactic bulge. The analysis focuses on the determination of the abundances of iron and the alpha-elements (O, Mg, Si, S, Ca, and Ti). AGB stars are being used as probes of the Bulge for a long time because of their high luminosities, and an analysis of their chemical properties would be enormously important. The proposed measurements will clarify how the AGB stars fit in the picture drawn by the other Bulge objects, and eventually provide crucial constraints on the formation and evolution of the Bulge, and thus of the Galaxy itself.
Central questions that will be adressed by the proposed project are: What are the chemical properties of the Bulge AGB population, and are they the same as those of other stellar types in the Bulge? Can we find similarity with the chemical properties of stars in the (thick) Disk? Does the metallicity distribution depend on the stage of stellar evolution, i.e. do metal-rich stars get "lost" at some point in their evolution? How do the AGB stars fit in the picture drawn by their possible precursors (micro-lensed dwarfs and sub-giants, K-type giants) and their successors (PNe)? And finally, how did the Bulge of the Milky Way galaxy form? The analysis of a sample of Bulge AGB stars can deliver important constraints to that last question, which also relates to the formation of the Galaxy as a whole, and the formation of galaxies in general.
The aim of the present project is to find answers to the questions above, by analysing a set of NIR spectra of AGB stars located in the Galactic bulge, obtained with the CRyogenic high-resolution InfraRed Echelle Spectrograph
(CRIRES) mounted to one of ESO's Very Large Telescopes. Elemental abundances will be derived from them by spectral synthesis techniques with the help of state-of-the-art model spectra based on the COMARCS atmosphere code, and detailed comparisons with abundances measured in other stellar types in the Bulge, as well as thin and thick Disk stars, will be performed.