Other
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002phdt.........3r&link_type=abstract
Ph.D. Thesis, Katholieke Universiteit Leuven
Other
16
Agb Stars, Post-Agb Stars, Abundances, Carbon, S-Process, Nucleosynthesis, Chemically Peculiar, Line Identification
Scientific paper
The final evolutionary stage of a low mass stellar object is a complex phase which is still poorly understood. In this thesis we contribute to a better understanding of the nucleosynthesis and dredge-up phenomena that occur in such objects during their ascent on the AGB by means of a detailed study of high-resolution optical spectra of post-AGB objects.
In the first four chapters we mainly focus on the photospheric abundances of eight carbon and s-process enriched post-AGB objects. The carbon enrichment clearly proves that products of the helium burning shell were brought to the surface in the so-called third dredge-up. Moreover, also products of the (slow) neutron nucleosynthesis (the s-process) are brought to the surface, which allows us to characterize this nucleosynthesis. A detailed study of the chemical pattern displayed by these elements, including a comparison with up-to-date nucleosynthetic AGB stellar models, reveals that the expected anti-correlation between metallicity and neutron nucleosynthesis efficiency is hardly seen (if at all). The anti-correlation is expected since in a lower metallicity object, more neutrons are available per iron seed and hence heavier nucleons can be built up, assuming a similar primary production rate of the neutrons. Instead, a large spread in efficiency is seen. On the other hand, a clear correlation was found between the total enrichment and the nucleosynthesis efficiency, indicating that the dredge-up efficiency is strongly linked to the neutron production. Furthermore, detailed abundances of elements beyond the Ba-peak (Gd, Yb, Lu and possibly W) were obtained for the first time in intrinsically enriched objects for three stars of the sample, a result which was possible due to the combination of the high quality VLT+UVES spectra and newly released atomic data in both VALD and DREAM (Database on Rare Earths At Mons University). Finally, a new identification was found for the line at 670.8 nm in the spectra of the enriched objects. This line was previously identified as the lithium resonance line, but we propose that it is due to a CeII transition, implying that there is no need to invoke a lithium producing scenario for these objects.
In the last two chapters an extensive observational study is given of two remarkable lithium rich objects: HD172481 and HD190390 (HR7671). The former turns out to be a highly exceptional double lined spectroscopic binary, consisting of a F-type post-AGB star and a M-type AGB star. For the latter, a conclusive evolutionary status is not found, but in the variability analysis of this object, we found evidence for a beating and a slight frequency increase. The genuine high lithium content of these two objects is not understood.
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