Aerodynamical sticking of dust aggregates

We present results of collision experiments of a dense beam of aggregated 1.2 microm SiO2 particles entrained in a gas flow with metal targets of different widths. Depending on the target width (d=25.4, 50.8, and 127 microm) and the ambient gas pressure (p=0.5-2.0 mbar), the growth of a dust pile on the target begins at a threshold impact speed (v(imp)=6-12.5 m/s). These threshold velocities for sticking exceed the limit for total disruption of aggregates by more than a factor of 5-10 for the given parameters. We found that a significant number of fragments (single particles) from the collisions had a very low coefficient of restitution c(r) at least down to c(r)<0.05 that is much lower than the value c(r)>0.5 that one of the single solid micron-sized particles would have while impinging a rigid target. Due to the drag of the gas flow these slow fragments are forced back to the target a second time resulting in sticking that eventually leads to the formation of the dust pile in spite of the high impact velocities. Together, the fragmentation, the low coefficients of restitution of a significant number of fragments, and the gas flow provide an efficient growth mechanism for bodies that would otherwise lose mass. We consider this an important mechanism for the formation of planetesimals in the solar nebula.