Getting intimate with AGB stars: the chemistry, dynamics and evolution of CSEs
by
Matthias Maercker(SU)
→
Europe/Stockholm
FA31
FA31
Description
The dominant process that determines the life time of evolved stars on the AGB is the mass loss from the stellar surface, with winds corresponding to rates up to 10e-4 Msun/year. Nuclear processed material is mixed up from the stellar interior to the surface through dredge-up caused by thermal pulses and is expelled into the interstellar medium through the circumstellar envelopes (CSEs) formed by the wind. AGB stars therefore contribute to the chemical enrichment of galaxies. Understanding the mass loss and its mechanism is essential in order to understand the evolution on the AGB and its effect on the surrounding environment. I will present two methods aimed at gaining a deeper understanding of the mass-loss and its properties. The modelling of water vapour emission in the CSEs of M-type AGB stars, based on unresolved ISO observations, indicate a higher amount of H2O than expected from equilibrium models. Our models of the spectrally resolved 557 GHz line observed with Odin show that such data gives important information on the chemistry, dynamics, and structure throughout the entire CSE. Upcoming observations with Herschel/HIFI will provide spectrally resolved data of H2O line emission from the CSEs of a larger number of AGB stars. Another way of examining the mass loss, and in particular its variations, are the observations of thin, detached shells observed around eight carbon AGB stars. I will present new data in scattered stellar light of the detached shells around the carbon star U Ant. The data strongly suggests a period during which the mass-loss rate of the star and the epxansion velocity of the wind increase, creating a thin shell, possibly affected by interaction with an earlier, slower wind. This suggests that the detached shells are linked to changes in the mass loss caused by changes in the stellar parameters during the thermal pulse cycle, and offer a unique way of studying the evolution of the star during these events. The presence of multiple shells further indicates that the change in mass loss during a thermal pulse is more complicated than previously assumed (Host: Sofia Ramstedt).
