A&A Supplement Series, Vol. 126, November II 1997, 39-65
Received December 9, 1996; accepted February 5, 1997
M. Steffen
-
R. Szczerba
-
A. Men'shchikov
-
D. Schönberner
Send offprint request: M. Steffen
Astrophysikalisches Institut Potsdam,
D-14473 Potsdam, Germany
e-mail: MSteffen@aip.de
Institut für Astronomie und Astrophysik der Universität Kiel,
D-24098 Kiel, Germany
e-mail: supas048@astrophysik.uni-kiel.de
Nicolaus Copernicus Astronomical Center,
PL-87-100 Torun, Poland
e-mail: szczerba@ncac.torun.pl
Department of Physics and Astronomy,
University of Calgary, Calgary, AB T2N 1N4, Canada
Max-Planck-Gesellschaft, AG ``Dust in star forming regions'',
D-07745 Jena, Germany
e-mail: sascha@georg.astro.uni-jena.de
Astrophysikalisches Institut Potsdam,
D-14473 Potsdam, Germany
e-mail: Deschoenberner@aip.de
We present a sample of hydrodynamical steady state models of circumstellar
gas/dust shells around
late type giants together with computed spectral energy distributions (SEDs).
In these models, the stellar wind is driven by radiation pressure on dust
grains and subsequent momentum transfer to the gas molecules via collisions.
Given the fundamental stellar parameters (
, 
, 
), the mass
loss rate (
), and the dust properties, a self-consistent physical model
of the circumstellar gas/dust shell is obtained from the numerical solution of
the coupled equations of hydrodynamics and radiative transfer.
The computed outflow velocities and infrared fluxes of the circumstellar
envelopes can be compared directly with the observed properties of stars on
asymptotic giant branch.
Plotting the positions of our steady state models in different
IRAS two-color-diagrams, we confirm that,
for fixed dust properties, all models fall on a simple color-color relation
with (or optical depth) as the only parameter. Surprisingly, we find a
good agreement between the synthetic spectra resulting from the
self-consistent hydrodynamical approach and those obtained from much simpler
models based on a constant outflow velocity and ignoring drift of dust relative
to the gas.
Our models are compared with the results of similar calculations by Netzer
& Elitzur (1993). We find significant differences which are probably the
result of some unrealistic approximations in the treatment of radiative transfer
underlying the model calculations of Netzer & Elitzur. Moreover, our results
demonstrate that, in general, gas pressure cannot be neglected for winds with
relatively low expansion velocities (
km/s). For given stellar
parameters and dust properties, the theoretical minimum (maximum) mass loss
rate decreases (increases) significantly when gas pressure is taken into
account.
keywords: stars: AGB and post-AGB -- circumstellar
matter -- stars: mass loss
-- dust, extinction -- hydrodynamics -- radiative transfer