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AGB (Asymptotic Giant Branch) stars generate a massive dust driven stellar wind at the end
of their lives. Ideally, this mass loss is spherical if the physical conditions are homogeneous
at the stellar surface (e.g. temperature) and the stellar vicinity (e.g. density). Indeed,
several physical processes induce deviations from these ideal conditions. This will affect
the condensation of dust and therefore the mass loss rate. Inhomogeneities can also caused
by cool spots at the stellar surface. These inhomogeneities of the temperature are able to
emanate from a magnetic field or a huge convection cell within the stellar envelope. Both
options are possible at the surface of AGB-stars.
This thesis introduces a model for the investigation of the mass loss above cool spots.
For that purpose a radiation hydrodynamic simulation (including a gas, a dust and a radiation
component) has been used and modified for the special purposes of this problem. A
flux tube
geometry has been chosen which could have been produced by a magnetic field in the lower
stellar atmosphere. Finally, a discussion has been carried out about the creation of dense
knots in planetary nebula as a result of cool regions at the stellar surface.
The result supports the theory that stellar spots generate significant inhomogeneities of
the mass loss. But the formation of dense knots in planetary nebulae
(e.g. in Helix
Nebula or Escimo Nebula)
have to be interpreted
as a combination of inhomogeneities in the mass loss together with hydrodynamical
instabilities. The model investigated describes the formation of initial inhomogeneities
which can be later amplified by an interaction of the slow AGB wind with the fast tenuous
wind of the hot central star of the planetary nebula.
View the "trumpet star" .
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