On the interstellar extinction hump and laboratory carbonaceous grains

We present a model of the average interstellar extinction curve obtained using a mixture of composite carbon particles and astronomical silicate grains. The carbon component, which is responsible for 80% of the extinction at the visible wavelength V = 550 nm, consists of small spherical particles with a Mathis, Rumpl, & Nordsieck (MRN)-type size distribution with a radius a in the range 20-100 Angstrom and is composed of a matrix of amorphous carbon with small inclusions of glassy carbon. The silicate grains contribute to the remaining 20% of visual extinction. The same model of composite carbon particles, whose optical properties can be calculated using the effective medium theory of Maxwell-Garnet, has already been successfully proposed to explain both the circumstellar extinction observed toward carbon-rich sources and the measured ultraviolet properties of various kinds of carbon grains which have been condensed and thermally processed in our laboratory. It is shown that, while the interstellar extinction feature is well matched at 217.5 nm, the width of the band remains to be explained. An analysis of the results obtained by other authors and concerning various laboratory carbonaceous candidates indicates that in all cases the mismatch of the width is a common problem. An investigation of the optical properties of bulk graphite and two different types of glassy carbon seems to indicate the existence of a correlation between increasing degree of structural order (i.e., size of crystallites) and width and wavelength position of the ultraviolet absorption peak. The same result from independent and different methods has been obtained also by other authors. We conclude that certainly some carbonaceous grains have the right attributes to be considered laboratory analogs of interstellar dust. However, in order to clear this point definitely, future experiments aimed at measuring the optical properties of carbon particles have to be designed to deal with isolated dust grains or at least to take into account the unwanted effects of neighboring systems.