Issue |
A&A
Volume 381, Number 3, JanuaryIII 2002
|
|
---|---|---|
Page(s) | 1015 - 1025 | |
Section | Stellar structure and evolution | |
DOI | https://doi.org/10.1051/0004-6361:20011526 | |
Published online | 15 January 2002 |
The outer evolution of instability-generated structure in radiatively driven stellar winds
1
Royal Observatory of Belgium, Ringlaan 3, 1180 Brussel, Belgium
2
Bartol Research Institute, University of Delaware, Newark, DE 19716, USA
Corresponding author: M. C. Runacres, Mark.Runacres@oma.be
Received:
7
August
2001
Accepted:
29
October
2001
We investigate stochastic structure in hot-star winds.
The structure (i.e. inhomogeneities such as clumps and shocks)
is generated by the instability of the line driving mechanism in the inner wind.
It is self-excited in the
sense that it persists even in the absence of explicit
perturbations.
The evolution of structure as it moves out with the flow
is quantified by the radial dependence of statistical properties such as
the clumping factor and the velocity dispersion.
We find that structure evolves under the influence of two competing mechanisms.
Dense clumps pressure-expand into the rarefied gas that separates them, but
this expansion is counteracted by supersonic collisions among the clumps,
which tend to compress them further.
Because of such ongoing collisions, clumps can survive over an
extended region out of pressure equilibrium with the rarefied
surrounding gas.
Moreover, the line-driving force has little rôle in
maintaining the structure beyond about 20–30 , implying that the outer
evolution can be simplified as a pure gasdynamical problem.
In modelling the distant wind structure we find it is necessary to
maintain a relatively fine constant grid spacing to resolve the often quite
narrow dense clumps.
We also find that variations in the heating and cooling, particularly
the “floor” temperature to which shock-compressed gas is allowed to
cool, can affect both the density and temperature variation.
Finally, we find that increasing the value of the line-driving cut-off
parameter
can significantly enhance the level of
flow structure.
Overall, the results of our work suggest that structure initiated in the
inner wind acceleration region can survive to substantial distances
(
), and thus can have an important influence on observational
diagnostics (e.g. infrared and radio emission) formed in the outer
wind.
Key words: stars: early-type / stars: mass-loss / stars: winds, outflows / hydrodynamics / instabilities
© ESO, 2002
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