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S. Kempf (Max Planck Institute for Nuclear Physics, Heidelberg, Germany), J. Blum (Astrophysical Institute, University of Jena, Germany), G. Wurm (LASP, University of Colorado, Boulder, USA)
It is now widely believed that the kilometer-size planetesimal grew from microscopic dust particles embedded in the dilute solar nebula gas. The relevant process for forming larger aggregates is the growth due to collisional sticking. For particles to collide and stick, there must be a relative velocity component between the grains. In the onset of dust growth, Brownian motion dominates other velocity sources. The most important quantity for the dust evolution is the structure of the growing aggregates, described by the fractal dimension. Much of the interest in the dynamics of the dust growth arises because the typical formation time scale of km-sized bodies as predicted by numerical studies is much too large for explaining the formation of planets within the lifetime of a proto-planetary disk. The recent Cosmic Dust Aggregation Experiment CODAG allows for the first time to verify the basic assumptions of the Brownian stage of dust growth. Perhaps the most important finding of the CODAG experiment was that, at least in the onset of the dust, growth needle-like fractal aggregates rather than symmetric fractals are formed. This suggests that the thermal rotation of the colliding aggregates causes significantly anisotropic cross sections. Here we discuss the implications of this effect for the theoretical description of the dust growth.