Publications
of the
MPIfR
Optical & Infrared
Interferometry Group
C. Helling, J.M. Winters, and E.
Sedlmayr:
Circumstellar dust shells around long-period
variables:
VII. The role of molecular opacities
Astronomy and Astrophysics 358, 651-664 (2000)
Abstract.
The role of molecular opacities for the structure and dynamics of winds
of carbon-rich AGB stars is investigated in the frame work of
time-dependent hydrodynamic models of dust forming circumstellar shells
around cool pulsating stars. New Rosseland and Planck mean gas opacity
tables have been calculated for T=[500K...10000K] and
n=[105cm-3...1015cm-3]
for
solar, LMC and SMC abundances. Carbon-rich, static and time-dependent
models have been computed using either the Planck mean or the Rosseland
mean for solar and LMC metalicity or a constant gas opacity
(chig = 2 10-4 cm2g-1,
Bowen 1988).
In the model calculations, a large gas opacity (Planck mean) generally
causes a
less dense atmosphere than a small gas opacity (Rosseland mean,
constant
gas opacity) which leads to smaller amounts of dust formed, and
consequently to smaller mass loss rates <Mdot>, lower
terminal wind velocities < vinfty> and lower
dust-to-gas ratios <rhodust / rhogas>.
Models with lower metalicity (LMC) form by far the smallest amount of
dust and show therefore the lowest <Mdot>, <
vinfty>, and <rhodust / rhogas>.
Counteracting to the global density reduction due to strong gas
absorption, the density might locally increase due to a
pressure
inversion. These pressure inversions are preserved even in the
hydrodynamic models where the atmosphere is disturbed by the
propagation
of shock waves. Due to the present determination of the temperature
structure by grey opacities in the time-dependent models, the
occurrence
of pressure inversions deserves, however, further investigations by
means
of a more elaborate treatment of the radiative transfer in dynamic
model
atmospheres.
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