The evolution of dust particles in circumstellar disk-like structures around protostars and young stellar objects is discussed. We especially consider the coagulation of grains due to collisional aggregation and the influence of this process on the optical properties of the particles. These dust opacities are important for both the derivation of the circumstellar dust mass from submillimetre continuum observations and the dynamical behaviour of the disks.

We present first results of a numerical study of the coagulation of dust grains in a turbulent protoplanetary accretion disk described by a time-dependent one-dimensional (radial) “alpha” model. The influence of grain opacity changes due to dust coagulation on the dynamical evolution of a protostellar disk is investigated. In addition, we consider the grain motion in two-dimensional disks.

A nonequilibrium calculation for the chemistry in a classical protoplanetary accretion disk is presented. Slow radial particle transport moves grains from the cold outer regions of a protoplanetary accretion disk into its warm central part where grains are destroyed. We consider the destruction processes for the silicate and carbon dust component and follow the chemical composition of the gas as a function of the radial distance from the protostar. The main result of this calculation is the presence of huge amounts of methane at a distance of ~ 1 AU from the protostar as product of the carbon dust destruction. It is very likely that more complex organics also are present' in this region.