Observations and modelling of spectral energy distributions of carbon stars with optically thin envelopes

We present broad-band photometry in the optical, near-infrared and submillimetre, and mid infrared spectrophotometry of a selection of carbon stars with optically thin envelopes. Most of the observations were carried out simultaneously. Beside the emission feature at 11.3 mu m due to silicon carbide grains in the circumstellar environment, many of our mid-infrared spectra show an emission feature at 8.6 mu m. All the observed spectral energy distributions exhibit a very large far-infrared flux excess. Both these features are indeed common to many carbon stars surrounded by optically thin envelopes. We have modelled the observed spectral energy distributions by means of a full radiative transfer treatment, paying particular attention to the features quoted above. The peak at 8.6 mu m is usually ascribed to the presence of hydrogenated amorphous carbon grains. We find also that the feature at 8.6 mu m might be reproduced by assuming that the stars have a circumstellar environment formed of both carbon-and oxygen-rich dust grains, although this is in contrast with what one should expect in a carbon-rich environment. The far-infrared flux excess is usually explained by the presence of a cool detached dust shell. Following this hypothesis, our models suggest a time-scale for the modulation of the mass-loss rate of the order of some 10(3) yr.