3 research outputs found
Thresholds for the Dust Driven Mass Loss from C-rich AGB Stars
It is well established that mass loss from AGB stars due to dust driven winds
cannot be arbitrarily low. We model the mass loss from carbon rich AGB stars
using detailed frequency-dependent radiation hydrodynamics including dust
formation. We present a study of the thresholds for the mass loss rate as a
function of stellar parameters based on a subset of a larger grid of such
models and compare these results to previous observational and theoretical
work. Furthermore, we demonstrate the impact of the pulsation mechanism and
dust formation for the creation of a stellar wind and how it affects these
thresholds and briefly discuss the consequences for stellar evolution.Comment: 2 pages, 1 figure. To be published in the proceedings of IAU Symp.
241 on Stellar Populations as Building Blocks of Galaxies, ed. A. Vazdekis et
al. (2007). Replaced to match edited versio
Dust driven mass loss from carbon stars as a function of stellar parameters - I. A grid of Solar-metallicity wind models
[Abridged] We have computed a grid of 900 numeric dynamic model atmospheres
(DMAs) using a well-tested computer code. This grid of models covers most of
the expected combinations of stellar parameters, which are made up of the
stellar temperature, the stellar luminosity, the stellar mass, the abundance of
condensible carbon, and the velocity amplitude of the pulsation. The resultant
mass-loss rates and wind speeds are clearly affected by the choice of stellar
temperature, mass, luminosity and the abundance of available carbon. In certain
parts of the parameter space there is also an inevitable mass-loss threshold,
below which a dust-driven wind is not possible. Contrary to some previous
studies, we find a strong dependence on the abundance of free carbon, which
turns out to be a critical parameter. Furthermore, we have found that the dust
grains that form in the atmosphere may grow too large for the commonly used
small particle approximation of the dust opacity to be strictly valid. This may
have some bearing on the wind properties, although further study of this
problem is needed before quantitative conclusions can be drawn. The wind
properties show relatively simple dependences on stellar parameters above the
mass-loss threshold, while the threshold itself is of a more complicated
nature. Hence, we chose not to derive any simplistic mass-loss formula, but
rather provide a mass-loss prescription in the form of an easy-to-use FORTRAN
routine. Since this mass-loss routine is based on data coming from an
essentially self-consistent model of mass loss, it may therefore serve as a
better mass-loss prescription for stellar evolution calculations than empirical
formulae. Furthermore, we conclude that there are still some issues that need
to be investigated, such as the role of grain-sizes.Comment: 27 pages, 11 figures, to appear in Astronomy & Astrophysics.
Corrections by language editor included in this new versio