Crossing barriers in planetesimal formation: The growth of mm-dust aggregates with large constituent grains

摘要

Collisions of mm-size dust aggregates play a crucial role in the early phasesof planet formation. We developed a laboratory setup to observe collisions ofdust aggregates levitating at mbar pressures and elevated temperatures of 800K. We report on collisions between basalt dust aggregates of from 0.3 to 5 mmin size at velocities between 0.1 and 15 cm/s. Individual grains are smallerthan 25 \mum in size. We find that for all impact energies in the studied rangesticking occurs at a probability of 32.1 \pm 2.5% on average. In general, thesticking probability decreases with increasing impact parameter. The stickingprobability increases with energy density (impact energy per contact area). Wealso observe collisions of aggregates that were formed by a previous stickingof two larger aggregates. Partners of these aggregates can be detached by asecond collision with a probability of on average 19.8 \pm 4.0%. The measuredaccretion efficiencies are remarkably high compared to other experimentalresults. We attribute this to the rel. large dust grains used in ourexperiments, which make aggregates more susceptible to restructuring and energydissipation. Collisional hardening by compaction might not occur as theaggregates are already very compact with only 54 \pm 1% porosity. Thedisassembly of previously grown aggregates in collisions might stall furtheraggregate growth. However, owing to the levitation technique and the limiteddata statistics, no conclusive statement about this aspect can yet be given. Wefind that the detachment efficiency decreases with increasing velocities andaccretion dominates in the higher velocity range. For high accretionefficiencies, our experiments suggest that continued growth in the mm-rangewith larger constituent grains would be a viable way to produce largeraggregates, which might in turn form the seeds to proceed to growingplanetesimals.