Exponentially fast growth of dust aggregates during the sedimentation/drift stage

In a novel experimental setup, we studied the growth of dust aggregates for an extended period of time under realistic protoplanetary conditions, i.e. in a rarefied gas and for relative sedimentation of the grains with velocities on the order of 1 cm/s. We observed a rapid growth of fractal aggregates consisting of µm-sized silica grains and measured their mass spectra as a function of time. Sampling of the aggregates allowed a better characterization of the fractal structure than in-situ observations. We also performed Monte Carlo simulations of the growth process, including explicit collisions, and could achieve a good match to our laboratory observations. Our mean-field model of the growth process is in agreement with the experimental findings and predicts an exponentially fast growth for fractal aggregates (whose collision cross sections are proportional to their mass and whose gas-grain response times are mass-independent), as long as mass loss of the system is negligible. For systems with mass loss, a maximum aggregate mass can be predicted. Our results are directly applicable to the study of dust-aggregate growth in protoplanetary disks, in particular for (but not restricted to) the sedimentation/drift stage before the onset of restructuring.