The physics of protoplanetesimal dust agglomerates. II. Low-velocity collision properties

For the investigation of collisions among protoplanetesimal dust aggregates, we performed microgravity experiments in which the impacts of high-porosity millimeter-sized dust aggregates into 2.5 cm high-porosity dust aggregates can be studied. The dust aggregates consisted either of monodisperse spherical, quasi-monodisperse irregular, or polydisperse irregular micrometer-sized dust grains and were produced by random ballistic deposition with porosities between 85% and 93%. Impact velocities ranged from similar to 0.1 to similar to 3 m s(-1), and impact angles were almost randomly distributed. In addition to the smooth surfaces of the target aggregates formed in our experiments, we molded'' target aggregates such that the radii of the local surface curvatures corresponded to the projectile radii, decreasing the targets' porosities to 80%-85%. The experiments showed that impacts into the highest porosity targets almost always led to sticking, whereas for the less porous dust aggregates, consisting of monodisperse spherical dust grains, the collisions with intermediate velocities and high impact angles resulted in the bouncing of the projectile with a mass transfer from the target to the projectile aggregate. Sticking probabilities for the impacts into the molded'' target aggregates were considerably decreased. For the impacts into smooth targets, we measured the depth of intrusion and the crater volume, and were able to derive some interesting dynamical properties which can help to derive a collision model for protoplanetesimal dust aggregates. Future models of the aggregate growth in protoplanetary disks should take into account noncentral impacts, impact compression, the influence of the local radius of curvature on the collisional outcome, and the possible mass transfer between the target and projectile agglomerates in nonsticking collisions.