DUST IN PROTOSTELLAR CORES AND STELLAR DISKS

We present the absorption and extinction cross section kappa as a func tion of wavelength for dust that is thought to exist in cold dense clo uds as well as in stellar disks. The grains are fluffy and composed of subparticles of astronomical silicate and amorphous carbon with an ad mixture of frozen ice. We assume a grain size distribution n(a) is-pro portional-to a-3.5 with fixed lower limit a- = 300 angstrom and variab le upper limit a+. Particular attention is given to the cross sections at 2.2 mum and 1.3mm, as these are the wavelengths for detecting embe dded young stars and deriving masses from dust emission. As long as th e grains are smaller than 100 mum, which must apply to the cores of co ld protostellar clouds, the absorption coefficient at 1.3mm is about 0 .02 cm2 per g of interstellar matter, an enhancement by a factor of ei ght relative to the diffuse interstellar medium; its variation with fr equency is kappa is-proportional-to nu2 in the submm/mm region. At 2.2 mum, the optical depth increases by a factor of 1.5 if the grains are small (a+ < 1 mum). Should coagulation have increased their size the outcome depends sensitively on the precise value of a+. We show in det ail how variations in grain size, fluffiness and ice mantle affect the cross section and also the temperature that grains acquire in a far I R radiation field. We also discuss the dust around Vega-type stars whe re the largest grains are known to be several millimeters big.