λC_abs/V (left) and (right) plotted versus the wavelengths in μm. Aggregates consist of BGs of astronomical silicate a), of BGs of amorphous carbon b) and of a mixture of both c). Results are shown for 1 BG only and for aggregates consisting of 4 BGs and of 16 BGs. We consider bare BGs and BGs with 1000 VSGs per BG (40% of the volume of a BG); “car” indicates amorphous carbon, “sil” astronomical silicate and “mixed” aggregates consisting of 4 BGs, where 1 BG is of amorphous carbon and 3 BGs of astronomical silicate, and of 16 BGs, where 4 BGs are of amorphous carbon and 12 BGs of astronomical silicate. For case c) the black curves show the result for isolated grains of 1 BG of amorphous carbon and 3 BGs of astronomical silicate. The bend occurring at 300 μm is due to the optical constants of amorphous carbon that are assumed to have a power-law at wavelengths longward 300 μm. The results at wavelengths longer than 300 μm are therefore uncertain and shown as dotted lines here.