Stellar Evolution On The Asypmtotic Giant Branch

Thorsten Arndt

Studienarbeit, Technische Universität Berlin, 1994

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Abstract:

This work treats the possible ways of describing stellar evolution. The deciding feature for the future evolution is found in the stellar mass, therefore the first chapter presents the different, mass dependent paths of stellar evolution. Combining the obtainable informations by observations, with the insights of stellar models allows to gain a general view.

The fundamental problem is the observation itself. Only the outermost layers of a star are accessible, and the large distances of the stars reduce the informations to their mean values over the visible stellar surface, so further reducing their testimony of the stellar conditions. Adapting the outer border to the observed features is the main criterion for modeling the inner structure of a star.

The view of stellar evolution is mainly based on the results of modeling the physical processes inside the star. The second chapter gives a short introduction into the foundation of these modells. Some processes in these models still depend on an appropriate parametrisation. Reducing these, in some cases nearly free choosable parameters is a tempting target. The models of course inherit growing uncertainties by such parameters, sometimes there is even a lack of understanding the involved physical processes.

One of these above mentioned parameters -- the mixing length -- is a main part of the mixing length theory of convection. The influence of this treatment of convection on the stellar evolution is shown in the third chapter.

All processes leading to a perceptible mass loss are of high influence on the stellar evolution. The fourth chapter deals with the observed high mass loss rates by stellar winds on the asymptotic giant branch (AGB) in the Hertzsprung-Russell diagram and their grave consequences for the further development of the star.

The description of these winds included a rather free parameter, because the mass loss rate was obtained by a fit-formula following D. Reimers (Circumstellar Absorption Lines and Mass Loss from Red Giants in "Problèmes d"Hydrodynamique Stellairè", 1975, Mem. Soc. R. Sci. Liège, 6eme, 8).

If a star is cool enough, the conditions in its surrounding shell can allow a condensation of dust grains. Radiation pressure on these dust grains is capable of driving a stellar wind with high mass loss rates via momentum coupling of the grains with the surrounding gas. The free Reimers-parameter can be excluded from the calculation of stellar evolution, by combining this calculation on the AGB, with a selfconsistent model of a dust driven wind.