Growing into and out of the bouncing barrier in planetesimal formation

In recent laboratory studies the robustness of a bouncing barrier in planetesimal formation was studied with an ensemble of preformed compact mm-sized aggregates. Here we show that a bouncing barrier indeed evolves self-consistently by hit-and-stick from an ensemble of smaller dust aggregates. In addition, we feed small aggregates to an ensemble of larger bouncing aggregates. The stickiness temporarily increases, but the final number of aggregates still bouncing remains the same. However, feeding on the small particle supply, the size of the bouncing aggregates increases. This suggests that in the presence of a dust reservoir aggregates grow into but also out of a bouncing barrier at larger size

Photophoresis boosts giant planet formation

In the core accretion model of giant planet formation, a solid protoplanetary core begins to accrete gas directly from the nebula when its mass reaches about 5 earth masses. The protoplanet has at most a few million years to reach runaway gas accretion, as young stars lose their gas disks after 10 million years at the latest. Yet gas accretion also brings small dust grains entrained in the gas into the planetary atmosphere. Dust accretion creates an optically thick protoplanetary atmosphere that cannot efficiently radiate away the kinetic energy deposited by incoming planetesimals. A dust-rich atmosphere severely slows down atmospheric cooling, contraction, and inflow of new gas, in contradiction to the observed timescales of planet formation. Here we show that photophoresis is a strong mechanism for pushing dust out of the planetary atmosphere due to the momentum exchange between gas and dust grains. The thermal radiation from the heated inner atmosphere and core is sufficient to levitate dust grains and to push them outward. Photophoresis can significantly accelerate the formation of giant planets.Comment: accepted in Astronomy and Astrophysics, 201

Crossing barriers in planetesimal formation: The growth of mm-dust aggregates with large constituent grains

Collisions of mm-size dust aggregates play a crucial role in the early phases of planet formation. We developed a laboratory setup to observe collisions of dust aggregates levitating at mbar pressures and elevated temperatures of 800 K. We report on collisions between basalt dust aggregates of from 0.3 to 5 mm in size at velocities between 0.1 and 15 cm/s. Individual grains are smaller than 25 \mum in size. We find that for all impact energies in the studied range sticking occurs at a probability of 32.1 \pm 2.5% on average. In general, the sticking probability decreases with increasing impact parameter. The sticking probability increases with energy density (impact energy per contact area). We also observe collisions of aggregates that were formed by a previous sticking of two larger aggregates. Partners of these aggregates can be detached by a second collision with a probability of on average 19.8 \pm 4.0%. The measured accretion efficiencies are remarkably high compared to other experimental results. We attribute this to the rel. large dust grains used in our experiments, which make aggregates more susceptible to restructuring and energy dissipation. Collisional hardening by compaction might not occur as the aggregates are already very compact with only 54 \pm 1% porosity. The disassembly of previously grown aggregates in collisions might stall further aggregate growth. However, owing to the levitation technique and the limited data statistics, no conclusive statement about this aspect can yet be given. We find that the detachment efficiency decreases with increasing velocities and accretion dominates in the higher velocity range. For high accretion efficiencies, our experiments suggest that continued growth in the mm-range with larger constituent grains would be a viable way to produce larger aggregates, which might in turn form the seeds to proceed to growing planetesimals.Comment: 9 pages, 20 figure

Preplanetary scavengers: Growing tall in dust collisions

Dust collisions in protoplanetary disks are one means to grow planetesimals, but the destructive or constructive nature of high speed collisions is still unsettled. In laboratory experiments, we study the self-consistent evolution of a target upon continuous impacts of submm dust aggregates at collision velocities of up to 71m/s. Earlier studies analyzed individual collisions, which were more speculative for high velocities and low projectile masses. Here, we confirm earlier findings that high speed collisions result in mass gain of the target. We also quantify the accretion efficiency for the used SiO2 (quartz) dust sample. For two different average masses of dust aggregates (0.29g and 2.67g) accretion efficiencies are decreasing with velocity from 58% to 18% and from 25% to 7% at 27m/s to 71m/s, respectively. The accretion efficiency decreases approximately as logarithmic with impact energy. At the impact velocity of 49m/s the target acquires a volume filling factor of 38%. These data extend earlier work that pointed to the filling factor leveling off at 8m/s to a value of 33%. Our results imply that high speed collisions are an important mode of particle evolution. It especially allows existing large bodies to grow further by scavenging smaller aggregates with high efficiency.Comment: This paper has been replaced by the author due to a transmission error of references. Now the citations and references are give