DUST EVOLUTION IN PROTOPLANETARY ACCRETION DISKS

The time evolution of dust particles in circumstellar disk-like struct ures around protostars and young stellar objects was investigated. For the first time, we coupled the dust evolution directly to the evoluti on of the disk and followed the influence of opacity changes due to co llisional aggregation on the dynamics of the disk. For that purpose, w e numerically simulated the dynamical evolution of a turbulent protopl anetary accretion disk described by a time-dependent one-dimensional ( radial) ''alpha'' model. Within this model, the growth of dust grains due to coagulation was calculated by solving numerically the non-linea r Smoluchowski equation. As physical processes leading to relative vel ocities between the grains, Brownian motion, turbulence, and drift mot ion were taken into account. In contrast to other studies, we especial ly considered particle-cluster agglomeration (PCA) as growth mode but also included cluster-cluster agglomeration (CCA) into our considerati ons. For time periods of 100 years and disk radii up to 100 AU, the ma ss distributions of coagulated dust grains were calculated. From these mass spectra, we determined the corresponding Rosseland mean dust opa cities. The variations of the dust grain opacity drive changes in the energetic structure of the protoplanetary disk which again influences the accretion process itself. Our results show three evolutionary stag es of the PCA process. For CCA particles, there is no dust growth afte r the disappearance of the smallest grains. The different characterist ic timescales for the coagulation at different radii result in the res tructuring of the dust region of the protoplanetary disks. Significant changes in the thermal and optical structure of the disk occur.