Astron. Astrophys. 324, 617-623 (1997)

Mid- and far-infrared properties of dynamical models of carbon-rich long-period variables

W. Windsteig ¹, E.A. Dorfi ¹, S. Höfner ^{1, 2}, J. Hron ¹ and F. Kerschbaum <sup>1

1</sup> Institut für Astronomie der Universität Wien, Türkenschanzstraße 17, A-1180 Wien, Austria
² Niels Bohr Institute, University Observatory, Juliane Maries Vej 30, DK-2100 Copenhagen, Denmark

Received 20 September 1996 / Accepted 3 February 1997

Abstract

We have calculated spectral energy distributions and synthetic IRAS colours of carbon-rich long-period variables. On top of dynamical models for the stellar atmosphere and circumstellar envelope which consistently treat the time-dependent hydrodynamics, the formation of amorphous carbon grains and grey radiative transfer, frequency-dependent radiative transfer calculations have been carried out in the range between m and m. Calculating the IRAS colours we find that the models lie in regions I (stars without circumstellar shells) and VII (variable stars with carbon-rich circumstellar shells) of the IRAS two colour diagram in accordance with observational results for carbon stars. They form an almost linear sequence (near the black body line) reflecting the different mass loss rates. We compare our results to empirical formulae which link the mass loss rate to the observed flux at m. Furthermore, we have also investigated qualitatively the effects of dust with -SiC grains and of detached shells. The SiC-contribution causes mainly a blue-shift of about mag in the colour. Adding a detached shell results in higher values partly shifting the models into region VIa (non-variable stars with relatively cold dust at large distances) of the IRAS two colour diagram as expected from observations.

Key words: stars: AGB and post-AGB stars: variables stars: mass-loss stars: circumstellar matter stars: carbon hydrodynamics

Send offprint requests to: W. Windsteig, (windsteig@auro.ast.univie.ac.at)

Contents

• 1. Introduction
• 2. Modelling method
  • 2.1. Dynamical models
  • 2.2. Frequency-dependent radiative transfer
• 3. IRAS two-colour diagram: dynamical models and observations
  • 3.1. Spectral energy distributions
  • 3.2. Synthetic IRAS colours
  • 3.3. Influence of SiC and detached shells
  • 3.4. Comparison with observations and previous works
• 4. Observed and theoretical IRAS LRS classes
• 5. Discussion and conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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