On the nature of interstellar carbonaceous dust

The average galactic interstellar extinction curve From 1000 to 8000 Å can be synthesized using the measured optical properties of standard demineralized coals with different degrees of graphitization (structural order). A highly-graphitized polycrystalline carbon simulates the 2175 Å hump and a weakly graphitized carbon simulates the underlying continuum: both contribute to the far-UV rise. An acceptable fit is obtained with a grain radius distribution in a−3.5 and amax ≈ 0.25 μm. Surface roughness is not critical. No spurious feature is introduced anywhere in the spectrum. The feature width increases with structural disorder and maximum grain size. These results are understandable in the light of the known universal properties of coal.