1995A&A...293..463G

The spectral energy distributions (SEDs) and LRS spectra of 21 infrared carbon stars are fitted using a dust radiative transfer model. The parameters derived are the temperature of the dust at the inner radius (T_inn), the mass loss rate and the ratio of silicon carbide (SiC) to amorphous carbon (AMC) dust. Mass loss rates between a few 10^–6 and 1.3 10^–4M_☉/yr are found. The SiC/AMC ratio and T_inn are found to decrease with increasing S₂₅/S₁₂ ratio. The former correlation may be due to an increasing C/O ratio. The latter correlation may be due to the fact that dust growth continues until the density is too low. For increasing mass loss rates (i.e. larger S₂₅/S₁₂ ratios) this leads to larger effective dust radii and hence to a decrease of T_inn. The standard model with a constant mass loss rate and amorphous carbon dust (with Q_λ~λ^–β; β≃1 for λ>30µm) predicts too much flux at 60 and 100µm compared to the observations. The discrepancy increases with the S₂₅/S₁₂ ratio. This indicates that either β>1 and/or that the mass loss rate has been lower in the past. Mass loss histories as proposed by Bedijn (1987A&A...186..136B) and related to thermal pulses are considered. An increase in the mass loss rate by a factor of 3-30 over the past 10⁴yrs or β's in the range 1.2-1.9 both fit the observed IRAS 60 and 100µm flux-densities. Theoretically one expects that β decreases or remains constant as the dust continuum temperature decreases, contrary to the β's needed to fit the observed IRAS 60 and 100µm flux-densities. This points to the mass loss histories rather than a steeper slope of the absorption coefficient to explain the observed 60 and 100µm flux-densities, at least for the reddest stars in the sample. Both mass loss histories predict sub-mm fluxes in better agreement with observations than a large value for β. From the SEDs the precise form of the mass loss rate history can not be reconstructed. It is suggested that with mapping the circumstellar shell with 2" resolution in the far-infrared and sub-mm region this may become possible. An excess emission above the dust emission and photospheric component is found at sub-mm wavelengths for all seven stars where sub-mm data is available. This is possibly due to free-free emission.