DUST SHELLS AROUND INFRARED CARBON STARS

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(-4) M. yr(-1) are found. The SiC/AMC ratio and T-inn are found to decrease with increasing S-25/S-12 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 S-25/S-12 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(lambda) similar to lambda(-beta); beta approximate to 1 for lambda > 30 mu m) predicts too much flux at 60 and 100 mu m compared to the observations. The discrepancy increases with the S-25/S-12 ratio. This indicates that either beta > 1 and/or that the mass loss rate has been lower in the past. Mass loss histories as proposed by Bedijn (1987) and related to thermal pulses are considered. An increase in the mass loss rate by a factor of 3-30 over the past 10(4) yrs or beta's in the range 1.2-1.9 both fit the observed IRAS 60 and 100 mu m flux-densities. Theoretically one expects that beta decreases or remains constant as the dust continuum temperature decreases, contrary to the beta's needed to fit the observed IRAS 60 and 100 mu m flux-densities. This points to the mass loss histories rather than a steeper slope of the absorption coefficient to explain the observed 60 and 100 mu m 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 beta. 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.