1994A&A...288..929K

We present the absorption and extinction cross section κ as a function of wavelength for dust that is thought to exist in cold dense clouds as well as in stellar disks. The grains are fluffy and composed of subparticles of astronomical silicate and amorphous carbon with an admixture of frozen ice. We assume a grain size distribution n(a)∝a^–3.5 with fixed lower limit a_–=300A and variable upper limit a₊. Particular attention is given to the cross sections at 2.2µm and 1.3mm, as these are the wavelengths for detecting embedded young stars and deriving masses from dust emission. As long as the grains are smaller than 100µm, which must apply to the cores of cold protostellar clouds, the absorption coefficient at 1.3mm is about 0.02cm² per g of interstellar matter, an enhancement by a factor of eight relative to the diffuse interstellar medium; its variation with frequency is κ∝ν² in the submm/mm region. At 2.2µm, the optical depth increases by a factor of 1.5 if the grains are small (a₊<1µm). Should coagulation have increased their size the outcome depends sensitively on the precise value of a₊. We show in detail how variations in grain size, fluffiness and ice mantle affect the cross section and also the temperature that grains acquire in a far IR radiation field. We also discuss the dust around Vega-type stars where the largest grains are known to be several millimeters big.