Computer Science
Scientific paper
Jun 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997phdt........20a&link_type=abstract
PhD Thesis, Univ. Uppsala, (1997)
Computer Science
4
Stellar Evolution, Stellar Abundances, Stellar Atmospheres, Instabilities, R Coronae Borealis Stars, Post-Agb Stars
Scientific paper
The R Coronae Borealis (RCrB) stars are characterized by their severe hydrogen-deficiency and drastic visual variability. This thesis is devoted to both these issues using theoretical, numerical and observational investigations. A large sample of the stars has been analysed to determine their elemental abundances, which trace both their ancestry and the history of stellar nucleosynthesis. For the purpose, line-blanketed, hydrogen-deficient model atmospheres have been constructed. The peculiar compositions of the stars indicate that they are in fact born-again giants, for which two explanations have been proposed: either a merger of two white dwarfs or a final He-shell flash in a post-AGB star, which briefly re-inflates the star back to giant dimensions. According to their composition, the stars can be divided into a homogeneous majority group and a diverse minority, which might reflect two different evolutionary backgrounds or the effect of dust-gas separation. The atmospheres bear witness of H- and He-burning in different phases as well as s-processing, but the high Si/Fe and S/Fe ratios of in particular the minority remain unexplained. The inability of the models to reproduce the CI lines suggests that standard model atmospheres are far from adequate descriptions of supergiants such as the RCrB stars. An analysis of Sakurai's object, which has likely recently experienced a final He-shell flash, reveals similarities with the RCrB stars as regards chemical composition. More spectacular, the star shows evidence of very rapid evolution and nucleosynthesis, most notably a decrease in the H abundance and an increase in the Li and s-element contents within only five months. The star represents an impressively fast case of stellar evolution very rarely encountered. The variability of the stars with fadings of up to 8 magnitudes is unique and not yet explained. It is probably due to obscuration events of the stars by newly formed dust clouds. Possible instabilities due to large radiative forces in the stellar atmospheres, which could be the unknown trigger mechanism for the variability by ejecting condensible material, are searched for. A connection between such instabilities and the declines is supported by the proximity of the stars to the theoretical opacity-modified Eddington limit.
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