Astron. Astrophys. 358, 651-664 (2000)

Circumstellar dust shells around long-period variables

VII. The role of molecular opacities

Ch. Helling ¹, J.M. Winters ^1,2 and E. Sedlmayr <sup>1

1</sup> Institut für Astronomie und Astrophysik, TU Berlin, Sekr. PN 8-1, Hardenbergstrasse 36, 10623 Berlin, Germany
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Received 10 October 1999 / Accepted 7 March 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 [500K, 10 000K] and [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 (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 , lower terminal wind velocities and lower dust-to-gas ratios . Models with lower metalicity (LMC) form by far the smallest amount of dust and show therefore the lowest , , and . 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.

Key words: infrared: stars molecular processes hydrodynamics stars: late-type stars: circumstellar matter

Send offprint requests to: chris@physik.tu-berlin.de

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Contents

• 1. Introduction
• 2. Molecular mean opacities
  • 2.1. Chemical and opacity data
  • 2.2. Mean gas opacities in the (T, ) plane
  • 2.3. Mean gas opacities for different metalicities
• 3. Model calculations
• 4. Results
  • 4.1. Initial models
  • 4.2. Time-dependent hydrodynamic models
  • 4.3. Wind properties
  • 4.4. Multi-periodicity
• 5. Pressure inversions
  • 5.1. Hydrostatic case
  • 5.2. Hydrodynamic case
• 6. Discussion
  • 6.1. Wind properties
  • 6.2. Pressure inversions
• 7. Conclusions
• Acknowledgements
• Appendices
  • A: Element abundances
  • B: TiC equilibrium constant
• References

© European Southern Observatory (ESO) 2000

Online publication: June 8, 2000
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