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.

This project dealt with the measurement of abundances of selected chemical elements in a kind of red giant stars located in the central parts of our Milky Way galaxy, also called the bulge of the Milky Way (Galactic bulge). The aim of the project was to understand in more detail the content of stars of the bulge, as well as its formation history and evolution. The elemental abundances were measured from high-resolution spectra in the infra-red domain, which were obtained with the spectrograph CRIRES that is mounted to one of the unit telescopes of the Very Large Telescope (VLT) operated by the European Southern Observatory (ESO) in Chile. The spectra were mainly analysed with the help of numerical model atmospheres of the stars, on the basis of which we computed model spectra to be compared with the real, observed spectra. In particular, we determined the general abundances of heavy elements (such as iron) relative to that in the Sun, as well as the ratio of the abundances of carbon to oxygen in the red giant stars. Furthermore, the isotopic abundance ratio of carbon (12C/13C) and the abundances of the individual elements Al, Si, Ti, and Y were measured. Our observations constitute abundance determinations at very low stellar temperatures (as cool as 3100 Kelvin), which is much cooler than the kind of stars that is usually analysed in abundance studies.The main result of the investigations is that red giants in the Galactic bulge contain, on average, slightly less heavy elements than the Sun. Few individual stars contain somewhat more heavy elements than the Sun does. Also the ratio of carbon to oxygen abundance was found to be close to the solar value, none of the stars has an appreciably higher ratio. This finding is surprising since red giant stars are expected to produce carbon in their interior during their late stages of evolution, and so it was expected that at least some of them have an increased carbon to oxygen ratio. The same holds for the carbon isotopic ratio that was found to be lower than expected from internal carbon production. It seems that the stars lack the mixing event that brings up the carbon (and other heavy elements) from regions close to the star's core to the outer atmosphere. One reason for this could be a too low total mass of the stars in combination with too high total metal content for the mixing to take place.Another important result is that stars closer to the mid-plane of the Milky Way have on average a higher metal content than stars further away from the plane. That is, there is a gradient in metal content within the bulge. This was known already from other stars in the outer regions of the bulge, here we found that this gradient continues to the inner regions.In summary, we could gain further insight into the history and evolution of our cosmic home, the Milky Way galaxy.