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

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1. Introduction

The IRAS satellite has produced the largest (spectro)photometric data base for AGB stars in the mid- and far-infrared and these data have been the starting point for many investigations on the properties of these stars (see the recent review of Habing 1996). A main tool for analyzing the dust envelopes around carbon stars has been the modelling of the spectral energy distributions starting with the work of Rowan-Robinson & Harris (1983). The large majority of these modelling efforts used a number of simplifying assumptions like stationary winds, an instantaneous condensation of dust grains with given particle sizes at a certain temperature or distance from the star. Winters et al. (1994a) have applied the description of grain formation and growth developed by Gail & Sedlmayr (1988) and Gauger et al. (1990) to stationary winds and have calculated synthetic IR spectra and IRAS colours.

Time-dependent hydrodynamical simulations of the atmospheres of carbon-rich long-period variables (LPVs) have demonstrated the strong interaction of dust formation and gas dynamics in these stars (e.g. Fleischer et al. 1991, 1992, 1995, Höfner et al. 1995, 1996, Höfner & Dorfi 1997) and Winters et al. (1994b, 1995) have calculated the corresponding theoretical IR light curves as well as brightness profiles. These investigations reveal the complex structure of dust shells caused by shock waves and study the time-dependence of the inner parts of the circumstellar envelope. Since the dust forming zones of LPVs never develop into a stationary state the synthesis of spectra of carbon-rich LPVs also demands a careful treatment of the hydrodynamics together with the time-dependent dust formation process.

Our current paper deals with the mid- and far-infrared properties of a number of dynamical models of carbon-rich LPVs. The IRAS colours, the corresponding IRAS LRS classes as well as a relation between the mass loss rate and the [FORMULA] m flux are presented for dynamical models of circumstellar shells. These results are based on accurate numerical solutions of radiation hydrodynamics interacting with time-dependent dust formation and growth processes where the optical constants of the amorphous carbon grains (and SiC particles) are taken from laboratory data.

In Sect. 2 we briefly describe the physical and numerical input, Sect. 3 deals with the synthetic spectral energy distributions as well as the IRAS two colour diagram and is followed by the LRS classes of our models in Sect. 4 and by a discussion in Sect. 5.

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© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998