EXPERIMENTS ON PREPLANETARY DUST AGGREGATION

For the study of low-velocity dust interactions in the early solar neb ula, we have performed two sets of experiments. In the first type of e xperiments, we studied the grain-mass evolution of a dust cloud embedd ed in a rarefied turbulent gas environment in which the initially mono disperse spherical SiO2 grains (1.9 mu m diameter) rapidly aggregate. The analysis of the resulting aggregate structures revealed that the c lusters were formed by ballistic cluster-cluster aggregation without r estructuring and follow a mass-size relation of the form m alpha a(g)( D)f, where D-f approximate to 1.91 is the fractal dimension and a(g) i s the radius of gyration of an aggregate with the mass m. A comparison with model calculations shows that the mean collisional velocity v(c) falls into the interval 0.07 m s(-1) less than or similar to v(c) les s than or similar to 0.5 m s(-1). By extraction of the fractal aggrega tes from the turbomolecular pump and injection into a levitation tube in the second set of experiments, we were able to observe individual c ollisions between the aggregates. These types of simulations were perf ormed in the laboratory so that the dominant source of the collisions was relative sedimentation. We investigated 28 collisions between aggr egates with monomer numbers between i = 1 and i approximate to 100 in the collision velocity range 0.001 m s(-1) less than or similar to v(c ) less than or similar to 0.01 m s(-1). Our observations show a sticki ng efficiency of beta(c) = 1 for the above-mentioned aggregate masses and collision velocities with no signs of grain restructuring. As we h ave attached importance to the similarity between our laboratory exper iments and the situation in the solar nebula, e.g., grain size and com position, collision velocities, and friction regime, the results of ou r investigations are directly applicable to the solar nebula modeling and may be used for time-scale estimations of the aggregate growth in the early Solar System. Our experiments suggest that an ensemble of du st grains which is collisionally self-interacting caused by gas drag e ffects, such as sedimentation, radial drift, or gas turbulence, will a dopt a bell-shaped mass distribution. This evolution results in fracta l aggregates with fractal dimensions below or close to D-f = 2. (C) 19 98 Academic Press.