Circumstellar dust shells around long-period variables: VII. The role of molecular opacities

Ch. Helling, J.M. Winters and E. Sedlmayr

Astronomy & Astrophysics, 358, 651-664 (2000)

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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 in [500K, 10000K] and n<H> in [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 <dot{M}>, lower terminal wind velocities <vinfty> and lower dust-to-gas ratios <
hodust /
hogas>. Models with lower metalicity (LMC) form by far the smallest amount of dust and show therefore the lowest <dot{M}>, <vinfty>, and <
hodust /
hogas>. 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.

Keywords:

Infrared: stars -- molecular processes -- hydrodynamics -- stars: late type -- circumstellar matter