Asymptotic Giant Branch stars viewed up-close and far-off : The physics, chemistry, and evolution of their circumstellar envelopes

The asymptotic giant branch (AGB) is the last stage of stellar evolution for stars with masses between 0.8-8 M(sun). This phase is characterised by an intense mass loss, which builds up a circumstellar envelope of dust and gas(CSE). 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.Observations of circumstellar H2O are of particular interest, as it is expected to be one of the most abundant molecules in the inner envelopes of M-type AGB stars (with C/O<1). 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, H2O abundances in these stars are determined and compared with theoretical chemical models. The importance of resolved H2O line profiles and excitation through different vibrationally excited states are 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 C/O>1), believed to be an effect of highly time-variable mass loss during a thermal pulse. The detached shells around three sources were observed in stellar light scattered by dust and gas in the shells using ground-based and space telescopes. The observations allow a separation of the scattering agents, and reveal information on the detached shells in unprecedented detail.