Growth of dust aggregates in protoplanetary disks beyond the "bouncing barrier"

• Applicant: Professor Dr. Jürgen Blum
• Subject Area: Astrophysics and Astronomy
• Term: from 2014 to 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 255668493

Today we know that planets form around young stars in so-called protoplanetary disks. While the disks cool down, dust particles progressively condense and start to collide with each other, due to their interaction with the surrounding gas. At low velocities, these collisions lead to sticking, due to surface forces. Thus, a number of growing dust agglomerates emerge. Experiments also show that large agglomerates rebound in collisions or, for even higher collision energies, fragment. To study the evolution from individual dust grains to km-sized planetesimals, several growth simulations of protoplanetary dust, based on the experimental data, were performed within the last years. The simulations show that the growth is stopped at aggregate masses of a few grams, due to bouncing collisions. However, recent experiments and simulations indicate that growth is still possible. Within the scope of this proposal, two prospective growth processes shall be studied experimentally.First, collisions in which one of the collision partners fragments can lead to a mass transfer to the surviving object. This mass transfer is well known from high-velocity impacts between very different-sized agglomerates. Recent experiments showed that growth is also possible in collisions between similar-sized dust aggregates. Since only the extreme cases of identical and very-different-sized collision partners have been studied so far, we intend to address in this proposal the influence of the mass ratio on the mass-transfer efficiency.Second, the common assumption of nearly spherical and homogenous dust agglomerates shall be investigated. Our preliminary experiments showed that sticking collisions of approx. 0.1 mm-sized dust agglomerates lead to the formation of very open and sometimes chain-like clusters. Collisions between these clusters still lead to sticking at velocities where compact agglomerates of the same mass rebound. The formation and evolution of such clusters, as well as their collision behavior and, thus, their growth conditions, shall be studied within this proposal by means of laboratory and drop-tower experiments.

• DFG Programme: Research Grants