Growth of dust aggregates in protoplanetary disks beyond the "bouncing barrier"
Final Report Abstract
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 caused by van der Waals forces. Experiments have shown that small dust agglomerates stick in mutual collisions, whereas large aggregates are supposed to rebound or, for 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. These simulations showed that the growth is stopped at aggregate masses of at most a few grams, due to bouncing collisions. However, experiments and simulations prior to our project indicated that growth was still possible via the so-called mass-transfer collisions between small and large aggregates. Within the scope of this proposal, two prospective growth processes were studied experimentally. In this project, we performed a set of dust-agglomerate collisions under microgravity conditions in the drop tower and in parabolic flights to investigate as yet unexplored regions of the parameter space. In the first series of experiments, we showed that initially ~100 µm-sized dust aggregates can indeed evolve into cm-sized aggregates for realistic collision velocities. The second series of experiment concentrated on the evolution of compact (due to bouncing collisions) cm-sized dust aggregates in the bouncing regime. We showed that these sub-catastrophic collisions are abrasive, i.e., they lead to the gradual decrease of the mass of the aggregates and act as a source of small dust aggregates. To apply these data and other new published data to protoplanetary disk growth simulations, we updated our dust-aggregate collision model. Among others, we added the effects of abrasion and erosion, which were not included in the previous version of the model. From the revised model we learned that the classical bouncing is not as pronounced in the collisional parameter space as assumed. Abrasion (which is also bouncing, but with an additional small mass loss of the aggregates) in collisions between approx. cm-sized agglomerates produces small fragments, which can lead to either growth by mass transfer or to erosion. For µm-sized monomer silica grains, erosion dominates over mass transfer so that planetesimal formation by sticking collisions seems impossible for these materials. To adapt the model to smaller dust particles, we performed tensile-strength measurements on dust aggregates with µm-sized and sub-µm-sized silica monomers. These experiments confirmed an earlier model that predicted that the aggregate strength increases towards smaller monomer sizes. We accordingly adapted our dust-aggregate collision model to silica monomer sizes of 0.1 µm. In this case, growth of large dust aggregates or even planetesimals due to sticking collisions by mass transfer seems still possible.
Publications
- (2014). Laboratory drop towers for the experimental simulation of dust-aggregate collisions in the early solar system, Journal of Visualized Experiments 88, e51541
Blum, J., Beitz, E., Bukhari, M., Gundlach, B., Hagemann, J.-H., Heißelmann, D., Kothe, S., Schräpler, R., von Borstel, I., Weidling, R.
(See online at https://doi.org/10.3791/51541) - (2016). Mikrogravitationsexperimente zur Entwicklung eines empirischen Stoßmodells für protoplanetare Staubagglomerate, Ph.D. Thesis, Technische Universität Braunschweig
Kothe, S.
- (2016). Sub-millimetre-sized dust aggregate collision and growth properties. Experimental study of a multi-particle system on a suborbital rocket, Astronomy & Astrophysics 593, A3
Brisset, J., Heißelmann, D., Kothe, S., Weidling, R., Blum, J.
(See online at https://doi.org/10.1051/0004-6361/201527288) - (2017). Low-velocity collision behaviour of clusters composed of sub-mm sized dust aggregates, Astronomy & Astrophysics 603, A66
Brisset, J., Heißelmann, D., Kothe, S., Weidling, R., Blum, J.
(See online at https://doi.org/10.1051/0004-6361/201630345) - (2018). Dust evolution in protoplanetary discs and the formation of planetesimals – What have we learned from laboratory experiments? Space Science Reviews 214, 52
Blum, J.
(See online at https://doi.org/10.1007/s11214-018-0486-5)
