When stars with masses between approx. 0.8 and 8 times the mass of our Sun have ceased nuclear burning in their centre, they expand and become red giants. This late phase of stellar evolution is known as the asymptotic giant branch (AGB). The AGB is characterised by an intense mass loss, which builds up a circumstellar envelope (CSE) of dust and gas. It is through this process that the AGB stars contribute to the chemical evolution of galaxies. In addition, a rich and varied chemistry is active within the CSEs.

Water is one of the most abundant molecules in the inner envelopes of M-type AGB stars (AGB stars with more oxygen than carbon in their atmospheres). The first part of this thesis concerns the modelling of water vapour emission lines from CSEs around M-type AGB stars. Using satellite observations and detailed radiative transfer models, abundances of water vapour in these stars are determined and compared with theoretical chemical models. The importance of resolved high quality observations and detailed models is also demonstrated.

The second part of the thesis has its focus on the detached shells of dust and gas observed around a handful of carbon AGB stars (with more carbon than oxygen in their atmospheres), believed to be an effect of highly time-variable mass loss during a thermal pulse, an event during which the star rapidly burns helium in a thin shell around the core. The detached shells around three sources were observed in stellar light scattered by dust and gas in the shells using ground-based and spaceborne telescopes. The observations reveal information on the detached shells in unprecedented detail.


Monday, January 18, 10.00
Place: FB52, AlbaNova University Centre, Roslagstullsbacken 21

Thesis information Contact:

Matthias Maercker, Tel: 08-5537 8550, maercker@astro.su.se