A MODEL FOR THE AMORPHOUS-CARBON GRAINS AROUND C-RICH OBJECTS

By taking into account both observational and laboratory results, we present here a model of carbon grains whose extinction properties can be easily computed and are able to reproduce the experimental data obtained for the various kinds of carbon grains condensed and thermally processed in our laboratories. The results obtained support the idea that variations in the spectral properties of carbon grains, observed as the annealing temperature increases, may be attributed to the increasing degree of graphitization subsequent to the progressive hydrogen loss undergone by the annealed particles. In a previous paper, using homogeneous data relative to freshly formed and annealed amorphous carbon grains produced in our laboratory, we fitted the ultraviolet extinction observed toward carbon-rich sources having different circumstellar ambient conditions and exhibiting a broad absorption peak at wavelengths ranging from 230 to 250 nm. The model discussed in this paper gives us a useful tool to evaluate and theoretically fit the observed extinction of astrophysical sources and allows us to have physical insight into the possible structure and evolution of circumstellar and laboratory carbon dust.