1999A&A...343..466L

We have developed a new method to determine the physical properties and the local circumstances of dust shells surrounding Carbon- and Oxygen-rich stars for a given pulsation phase. The observed mid-IR dust emission feature(s), in conjunction with IRAS BB photometry and coeval optical and near-IR BB photometry, are modelled from radiative transport calculations through the dust shell using a grid of detailed synthetic model input spectra for M-S-C giants. From its application to the optical Carbon Mira R For we find that the temperature of the inner shell boundary exceeds 1000K, ranging between 1200K and 1400K. The optical depth of the shell at 11.3µm is determined at τ_11µm=0.105 with T_eff=3200±200K for the central star in the considered phase of variability. By-products of the analysis are the shell composition of 90% amorphous carbon and only 10% SiC grains with rather small average radii of 0.05±0.02µm. The dust density distribution assumes a power law of r^–2 for a steady-state wind with a geometrical thickness ranging between 10⁴ and 5x10⁴ times the inner boundary shell radius and with a high gas mass-loss rate of 3-4x10^–6 M_☉/yr derived by radiation pressure onto the dust. We show that the optical and near-IR light curves are strongly affected by small changes of T_eff and of the shell optical depth with pulsation. A comparison of high resolution optical spectra of R For and medium/low resolution spectra of other carbon stars with the selected model input spectrum is also provided.