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Scientific paper
Oct 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007phdt.........3u&link_type=abstract
PhD Thesis, Univ. of Vienna, Vienna, Austria
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Nuclear Reactions, Nucleosynthesis Abundances, Stars: Agb And Post-Agb, Stars: Evolution, Stars: Abundances, Stars: Late-Type
Scientific paper
This thesis deals with stars on the asymptotic giant branch (AGB) located in the Galactic Bulge and the mixing and nucleosynthetic processes therein. These processes have been studied by means of the occurrence and the abundances of key elements such as Technetium (Tc), Lithium (Li), and Fluorine (F). For this purpose high-resolution spectra in the visual and infrared spectral regions were analysed with respect to the presence and strength of absorption lines connected to these elements. The spectra have been obtained with instruments of ESO's Very Large Telescope (VLT) in Chile.
Because of its radioactive nature, Tc serves well as an indicator of recent deep mixing (third dredge-up, 3DUP) to where nucleosynthetic processes are taking place within a star. In combination with other observables such as the luminosity and the pulsation period, conclusions could be drawn on the occurrence of 3DUP in the course of the stars' evolution on the AGB. One result of the studies is that the minimum luminosity required for 3DUP to occur, as predicted by theoretical evolution models, is in agreement with observations. Furthermore, masses of the stars could be estimated from their luminosity and pulsation period. The derived masses hint towards a relatively young age of the stars, in contrast to what is found from studies of other stellar types in the bulge, but well in line with a number of works on bulge AGB stars.
Surprisingly, Li was found in a few sample stars, and different scenarios for its enrichment in AGB stars are discussed. As the most promising explanation Cool Bottom Processing (CBP), another mixing process that may operate on the AGB, was identified. A relatively uncharted field of research was entered with this study, and the knowledge of CBP on the AGB could be expanded by our work.
The abundance of F was measured in one of the sample stars showing both Tc and Li. F is sensitive to the temperature in the nuclearly active zone of the star. Abundance determinations of F, particularly in genuine AGB stars, are rare in the literature. The measurements are of astrophysical interest because the star's mass can be estimated rather accurately and thus serves as a benchmark for the F production in rather low mass AGB stars. The measurements are based on infrared spectra obtained with the CRIRES spectrograph at the VLT.
An additional Chapter is devoted to a comparison of stellar model atmospheres and synthetic spectra based on the MARCS and PHOENIX atmospheric codes, respectively. The precise reason for the differences found in high-resolution spectra could not be identified, nevertheless useful hints could be found.
The final Chapter deals with the author's various technical contributions to the CRIRES project in the course of the ESO studentship. In the Appendices can be found a comparison of spectra of Bulge Mira variables, taken with the CRIRES and Phoenix infrared spectrographs, as well as the source code to describe the polarisation properties of an optical element for the spectro-polarimetric mode of CRIRES.
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