POROSITIES OF PROTOPLANETARY DUST AGGLOMERATES FROM COLLISION EXPERIMENTS

The aggregation of dust through sticking collisions is the first step of planet formation. The basic physical properties of the evolving dust aggregates strongly depend on the porosity of the aggregates; e.g., mechanical strength, thermal conductivity, and the gas-grain coupling time. Also, the outcome of further collisions depends on the porosity of the colliding aggregates. In laboratory experiments we study the growth of large aggregates of similar to 3 mm to 3 cm through continuous impacts of small dust agglomerates of 100 mu m in size, consisting of mu m grains at different impact velocities. The experiments show that agglomerates grow by direct sticking as well as through gravitational reaccretion. The latter can be regarded as a suitable analog to the reaccretion of fragments by gas drag in protoplanetary disks. Experiments were carried out in the velocity range between 1.5 m s(-1) and 7 m s(-1). With increasing impact velocities the volume filling factor of the resulting agglomerates increases from phi = 0.2 for 1.5 m s(-1) to phi = 0.32 for 7 m s(-1). These values are independent of the target size. Extrapolation of the measured velocity dependence of the volume filling factor implies that higher collision velocities will not lead to more compact aggregates. Therefore, phi = 0.32 marks a degree of compaction suitable for describing structures forming at nu > 6 m s(-1). At small collision velocities below 1 m s(-1), highly porous structures with phi approximate to 0.10 will form. For intermediate collision velocities porosities vary. Depending on the disk model and resulting relative velocities, objects in protoplanetary disks up to decimeters in size might evolve from highly porous (phi approximate to 0.10) to compact (phi = 0.32) with a more complex intermediate size range of varying porosity.