Macroscopic Dust in Protoplanetary Disks - From Growth to Destruction

Abstract: The collision dynamics of dusty bodies are crucial for planetesimal formation. Especially decimeter agglomerates are important in the different formation models. Therefore, in continuation of our experiments on mutual decimeter collisions, we investigate collisions of centimeter onto decimeter dust agglomerates in a small drop tower under vacuum conditions (p<5*$10^{-1}$ mbar) at a mean collision velocity of 6.68 +- 0.67m/s. We use quartz dust with irregularly shaped micrometer grains. Centimeter projectiles with different diameters, masses and heights are used, their typical volume filling factor is $\Phi_{p,m}$=0.466+-0.02. The decimeter agglomerates have a mass of about 1.5kg, a diameter and height of 12cm and a mean filling factor of $\Phi_{t,m}$=0.44+-0.004. At lower collision energies only the projectile gets destroyed and mass is transferred to the target. The accretion efficiency decreases with increasing obliquity and increasing difference in filling factor, if the projectile is more compact than the target. The accretion efficiency increases with increasing collision energy for collision energies under a certain threshold. Beyond this threshold at 298+-25 mJ catastrophic disruption of the target can be observed. This corresponds to a critical fragmentation strength Q*=190+-16 mJ/kg, which is a factor of four larger than expected. Analyses of the projectile fragments show a power law size distribution with average exponent of -3.8+-0.3. The mass distributions suggest that the fraction of smallest fragments increases for higher collision energies. This is interesting for impacts of small particle on large target bodies within protoplanetary disks, as smaller fragments couple better to the surrounding gas and re-accretion by gas drag is more likely.