1995A&A...299..791L

An analysis of the (1-1000µm) broad-band spectrum of the variable carbon-rich infrared source, GL 3068, is presented. This spectrum is interpreted with a radiative transfer model consisting of a central star surrounded by a circumstellar dust shell. It is found that a dust opacity law ∝λ^–1.3 in the range 1-500µm is adequate for reproducing the general characteristics of the spectrum. The temperature of the hottest grains is adjusted around 930K. The optical depth at 1 µm is found to vary from 26 to 36 between maximum and minimum and the bolometric luminosity, from ∼12 to 5x10³L_☉ (for a distance of 1kpc). These results are consistent with the models obtained for other sources of the same type, like IRC +10216. However, the fluxes observed at wavelengths longer than 500µm are not explained in the framework of this model. On the basis of this model, an investigation of the heating of circumstellar dust by the interstellar radiation field is presented. The dust temperature is increased in the external layers of the shell where it reaches values in the range 18 to 30K. The intensity profiles at λ>100µm are significantly affected. Nevertheless, the effect cannot account for the fluxes at λ>500µm. More generally, we find that dust emission cannot explain the observed excess. Other possibilities are assessed. An important contribution of the circumstellar molecular lines is likely. A chromospheric contribution to the sub-millimeter fluxes might have also to be considered.