The chemical diversity of post-AGB stars in the Galaxy and the Magellanic Clouds
De chemische diversiteit van post-AGB sterren in de melkweg en de magellaanse wolken.
De Smedt, Kenneth; S0171444
One of the most fascinating stages in the life of low- and intermediate mass stars is the asymptotic giant branch (AGB) phase due to its strong mass loss and internal nucleosynthesis. The slow neutron capture process (s-process) nucleosynthesis is the origin of approximately half of all elements heavier than iron. Low-mass AGB stars are expected to be the main contributors to the cosmic carbon and s-process abundances. Carbon and s-process elements are synthesised in the AGB interior and different mixing processes transport these newly-synthesised material to the stellar surface, enriching the surface abundances. However, the physical mechanisms driving the internal AGB processes are to date poorly understood. Current AGB evolution and nucleosynthesis models are used to gain insight in the different processes, but these simulations contain large uncertainties. Observational constraints are necessary for improving our knowledge of the AGB nucleosynthesis and associated mixing processes. Post-AGB stars are ideal testbeds for improving theoretical AGB evolution and nucleosynthesis models as the chemical composition of their photospheres display the signature of the different dredge-up processes and internal nucleosynthesis that take place during the total stellar lifetime. A systematic abundance study of optically visible post-AGB stars in a wide range in metallicity and luminosity, hence initial masses, will help to constrain AGB models. The sample of studied post-AGB stars in the Milky Way Galaxy displays a wide chemical diversity. Unfortunately, the distance to Galactic objects is unknown, hampering accurate luminosity determinations and hence initial mass estimates, which is a key parameter of AGB models. Post-AGB stars in the Magellanic Clouds do not have this distance drawback. In this thesis, we perform detailed photospheric abundance studies of post-AGB stars in the Magellanic Clouds and the Galaxy to gain insight in the different processes that took place during the AGB phase. The known distances to the Magellanic Clouds allow to estimate the initial mass of studied post-AGB stars and to compare the observed abundances with theoretical model predictions. A key element in our abundance studies is lead (Pb), which is the end-product of the s-process nucleosynthesis and hence the Pb abundance is sensitive to the different processes that take place during the TP-AGB. Our studies show that our sample of s-process enriched post-AGB stars in the Magellanic Clouds are all low-mass stars with low C/O ratios and high s-process enrichments. Comparison with AGB model predictions shows that the observed C/O ratios are overestimated in model predictions. In addition, we find an increasing discrepancy between observed and predicted Pb abundances towards lower metallicities: for more metal-rich stars, the observed Pb abundances confirm theoretical predictions while for lower-metallicity objects, the Pb abundances are strongly overestimated. We find a chemical diversity of post-AGB stars in the Magellanic Clouds, which is similar to the chemical diversity of their Galactic counterparts. Our abundance studies show that very similar objects with similar masses and metallicities can have large differences in s-process abundances. Furthermore, our abundance results confirm the mild intrinsic s-process enrichment of the Magellanic Clouds from previous studies. This enrichment should be taken into account when comparing observations with theoretical model predictions, which can help to improve current stellar evolution models.