Molecules and Dust around AGB stars : Mass-loss rates and molecular abundances
Sammanfattning: All stars with a stellar mass of about 0.8–8 MSun will end their lives as asymptotic giant branch (AGB) stars. Through their extensive mass loss the AGB stars constitutes an important source of nuclear processed material. They also provide us with fascinating systems where an interchange of different physical and chemical processes occur, making them excellent astrophysical laboratories.Being the most important process for the evolution of an AGB star, the mass loss is well established, but its details are less well known. On the AGB, the mass-loss rate can span several orders of magnitude, reaching 10?4 MSun yr?1 toward the end of the AGB. It is challenging to ?nd reliable methods to estimate the mass-loss rates of individual objects. Nevertheless, it is important, since the mass-loss rate affects the derived abundances of other molecules in the circumstellar envelope, and therefore the estimates of the amount and composition of the recycled material. In the ?rst part of the thesis the reliability of mass-loss rate estimates is evaluated using two main methods; the observations and radiative transfer analysis of CO radio line emission, and dust radiative transfer combined with a dynamical model.The second part of the thesis focuses on a particular chemical type of AGB stars; the S-type. The S-stars are believed to have approximately the same amount of carbon as oxygen in the photosphere, and to be an intermediate evolutionary stage as the star evolves from an oxygen-rich M-star into a carbon star. As possible transition objects the S-stars might give important clues to the mass-loss mechanisms and to the chemical evolution along the AGB. Using observations of circumstellar radio line emission in combination with detailed radiative transfer analysis, we have estimated mass-loss rates and abundances of chemically important molecules for a sample of 40 S-stars. The results are compared to previous results for M- and carbon stars.
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