Laboratory experiments on preplanetary dust aggregation

For a better understanding of the processes which lead to the formation of planetesimals in the early solar nebula, we performed an extensive series of laboratory experiments. We find that the capture velocities in collisions between spherical grains are more than one order of magnitude higher than predicted by Chokshi et al (1993). In contrast, irregular grains have no capture threshold and can be better described by a sticking probability which is typically a few 10%, even for velocities exceeding 10 m/s. However, adhesion forces between spherical, micron-sized particles match the theoretical predictions very well, although contact areas and deformations are of the order of inter-atomic distances only. Aggregation experiments in rarefied turbulent gases reveal the fractal nature of dust aggregates. Mass distribution functions are bell-shaped. Similar behaviour can be found in aggregation experiments with sedimenting particles. Experiments on collision-induced aggregate compaction and fragmentation match the numerical simulations by Dominik and Tielens (1997) very well if revised experimental values of the break-up energy (from our impact experiments) and the rolling-friction force (from our AFM measurements on particle chains) are used.