2016A&A...593A...3B

Context. In the very first steps of the formation of a new planetary system, dust agglomerates grow inside the protoplanetary disk that rotates around the newly formed star. In this disk, collisions between the dust particles, induced by interactions with the surrounding gas, lead to sticking. Aggregates start growing until their sizes and relative velocities are high enough for collisions to result in bouncing or fragmentation. With the aim of investigating the transitions between sticking and bouncing regimes for colliding dust aggregates and the formation of clusters from multiple aggregates, the Suborbital Particle and Aggregation Experiment (SPACE) was flown on the REXUS 12 suborbital rocket.
Aims. The collisional and sticking properties of sub-mm-sized aggregates composed of protoplanetary dust analogue material are measured, including the statistical threshold velocity between sticking and bouncing, their surface energy and tensile strength within aggregate clusters.
Methods. We performed an experiment on the REXUS 12 suborbital rocket. The protoplanetary dust analogue materials were micrometre-sized monodisperse and polydisperse SiO₂ particles prepared into aggregates with sizes around 120µm and 330µm, respectively and volume filling factors around 0.37. During the experimental run of 150s under reduced gravity conditions, the sticking of aggregates and the formation and fragmentation of clusters of up to a few millimetres in size was observed.
Results. The sticking probability of the sub-mm-sized dust aggregates could be derived for velocities decreasing from ∼22 to 3cm/s. The transition from bouncing to sticking collisions happened at 12.7^+2.1_–1.4cm/s for the smaller aggregates composed of monodisperse particles and at 11.5^+1.9_–1.3 and 11.7^+1.9_–1.3cm/s for the larger aggregates composed of mono- and polydisperse dust particles, respectively. Using the pull-off force of sub-mm-sized dust aggregates from the clusters, the surface energy of the aggregates composed of monodisperse dust was derived to be 1.6x10^–5J/m², which can be scaled down to 1.7x10^–2J/m² for the micrometre-sized monomer particles and is in good agreement with previous measurements for silica particles. The tensile strengths of these aggregates within the clusters were derived to be 1.9^+2.2_–1.2 Pa and 1.6^+0.7_–0.6 Pa for the small and large dust aggregates, respectively. These values are in good agreement with recent tensile strength measurements for ∼mm-sized silica aggregates.
Conclusions. Using our data on the sticking-bouncing threshold, estimates of the maximum aggregate size can be given. For a minimum mass solar nebula model, aggregates can reach sizes of ∼1cm.