Astron. Astrophys. 322, 924-932 (1997)

Mid-infrared imaging of AGB star envelopes

II. Modelling of observed sources

M. Marengo ¹, G. Canil ², G. Silvestro ², L. Origlia ³, M. Busso ³ and P. Persi <sup>4

1</sup> International School for Advanced Studies, Trieste, Italy
² Dipartimento di Fisica Generale, Università di Torino, Italy
³ Osservatorio Astronomico di Torino, Pino Torinese, Italy
⁴ Istituto di Astrofisica Spaziale, CNR, C.P. 67, I-00044 Frascati, Italy

Received 23 July 1996 / Accepted 10 December 1996

Abstract

Radiative transfer modelling of AGB circumstellar envelopes is applied to a sample of AGB stars previously observed with the mid-IR imaging camera TIRCAM (Busso et al. 1996: Paper I). We present the results of our simulations, aimed at deriving the physical parameters of the envelope, such as its optical depth, radial thermal structure, mass loss and dust-to-gas mass ratio. The chemical composition of the dust in the observed envelopes is discussed. The ability of different sets of dust opacities to fit the mid-infrared spectra is evaluated. The hypothesis of dust grain aging and annealing in O-rich envelopes is considered in order to explain an apparent inadequacy of the availabe opacities to describe the variety of observed spectra, as previously noted by other authors. Various possible origins of the discrepancies are discussed, together with their consequences on the dust grain formation processes.

Key words: infrared: stars stars: circumstellar matter stars: AGB radiative transfer

Send offprint requests to: M. Marengo

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Contents

• 1. Introduction
• 2. Spectra of AGB circumstellar envelopes
• 3. Our model
  • 3.1. Dust opacities
• 4. Model results
  • 4.1. Spectral distribution
  • 4.2. Temperature profiles
• 5. Discussion
• 6. Conclusions
• References

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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