Astron. Astrophys. 326, 305-317 (1997)

Circumstellar dust shells around long-period variables

V. A consistent time-dependent model for the extreme carbon star AFGL 3068

J.M. Winters ¹, A.J. Fleischer ¹, T. Le Bertre ² and E. Sedlmayr <sup>1

1</sup> Technische Universität Berlin, Institut für Astronomie und Astrophysik, Sekr. PN 8-1, Hardenbergstr. 36, D-10623 Berlin, Germany
² DEMIRM, Observatoire de Paris, 61, av. de l'Observatoire, F-75014 Paris, France

Received 22 November 1996 / Accepted 29 April 1997

Abstract

We present a consistent time-dependent model for the extreme carbon star AFGL 3068. The model includes a consistent treatment of the interactions among hydrodynamics, thermodynamics, dust formation, and chemistry. It is determined by the stellar parameters of the initial hydrostatic model: stellar mass , stellar luminosity , stellar temperature K, and carbon-to-oxygen abundance ratio . The pulsation of the star is simulated by a piston approximation at the inner boundary where the velocity varies sinusoidally with a period of d and an amplitude of km s^-1. This model yields a time averaged outflow velocity of km s^-1 in good agreement with the rather well determined observed value and an average mass loss rate of yr^-1 which is about a factor of 4 larger than the value usually derived from the fitting of CO rotational lines profiles. Based on the atmospheric structure resulting from this hydrodynamic calculation, we have performed angle- and frequency-dependent continuum radiative transfer calculations, which yield the spectral energy distributions at different phases of the pulsation cycle, synthetic light curves at different wavelengths and synthetic brightness profiles of the dust shell model. The computed energy distributions and the synthetic light curves are in good agreement with the observations of AFGL 3068. We derive a distance to AFGL 3068 of 1200 parsec. Furthermore, properties of the resulting grain size distribution function are discussed. The dominant size of the grains formed in the circumstellar shell is found to be m but grains with radii up to 0.7µm are also formed in such a quantity that they may contribute significantly to light scattering in the near-infrared.

We propose to perform high-resolution spatial observations of the circumstellar dust shell around AFGL 3068 which could be compared with the predictions of the model calculation.

Key words: hydrodynamics radiative transfer stars: carbon circumstellar matter stars: late type stars: individual: AFGL 3068

Send offprint requests to: J.M. Winters
e-mail: Winters@physik.TU-Berlin.DE

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Contents

• 1. Introduction
• 2. The observational data
• 3. The modeling method
• 4. Results
  • 4.1. Radial structure of the dust shell model
  • 4.2. Spectral energy distribution
  • 4.3. Spatial spectra
  • 4.4. Light curves
• 5. Discussion
  • 5.1. Remarks on the light curves
  • 5.2. Comparison with the model by Le Bertre et al. (1995 )
  • 5.3. The assumption of the small particle limit
  • 5.4. Remarks on the assumption of spherical symmetry
• 6. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 1997

Online publication: April 20, 1998
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