THE PHYSICS OF PROTOPLANETESIMAL DUST AGGLOMERATES. III. COMPACTION IN MULTIPLE COLLISIONS

To study the evolution of protoplanetary dust aggregates, we performed experiments with up to 2600 collisions between single, highly porous dust aggregates and a solid plate. The dust aggregates consisted of spherical SiO(2) grains with 1.5 mu m diameter and had an initial volume filling factor ( the volume fraction of material) of phi(0) = 0.15. The aggregates were put onto a vibrating baseplate and, thus, performed multiple collisions with the plate at a mean velocity of 0.2 m s(-1). The dust aggregates were observed by a high-speed camera to measure their size which apparently decreased over time as a measure for their compaction. After 1000 collisions the volume filling factor was increased by a factor of 2, while after similar to 2000 collisions it converged to an equilibrium of phi approximate to 0.36. In few experiments the aggregate fragmented, although the collision velocity was well below the canonical fragmentation threshold of similar to 1 m s(-1). The compaction of the aggregate has an influence on the surface-to-mass ratio and thereby the dynamic behavior and relative velocities of dust aggregates in the protoplanetary nebula. Moreover, macroscopic material parameters, namely, the tensile strength, shear strength, and compressive strength, are altered by the compaction of the aggregates, which has an influence on their further collisional behavior. The occurrence of fragmentation requires a reassessment of the fragmentation threshold velocity.