Planetesimal formation by sweep-up: how the bouncing barrier can be beneficial to growth

Context. The formation of planetesimals is often accredited to the collisional sticking of dust grains. The exact process is unknown, as collisions between larger aggregates tend to lead to fragmentation or bouncing rather than sticking. Recent laboratory experiments have however made great progress in the understanding and mapping of the complex physics involved in dust collisions. Aims. We study the possibility of planetesimal formation using the results of the latest laboratory experiments, particularly by including the fragmentation with mass transfer effect, which might lead to growth even at high impact velocities. Methods. We present a new experimentally and physically motivated dust collision model capable of predicting the outcome of a collision between two particles of arbitrary mass and velocity. The new model includes a natural description of cratering and mass transfer, and provides a smooth transition from equal-to different-sized collisions. It is used together with a continuum dust-size evolution code, which is both fast in terms of execution time and able to resolve the dust at all sizes, allowing for all types of interactions to be studied without biases. Results. For the general dust population, we find that bouncing collisions prevent any growth above millimeter-sizes. However, if a small number of cm-sized particles are introduced, for example by either vertical mixing or radial drift, they can act as a catalyst and start to sweep up the smaller particles. At a distance of 3 AU, 100-m-sized bodies are formed on a timescale of 1 Myr. Conclusions. Direct growth of planetesimals might be a possibility thanks to a combination of the bouncing barrier and the fragmentation with mass transfer effect. The bouncing barrier is here even beneficial, as it prevents the growth of too many large particles that would otherwise only fragment among each other, and creates a reservoir of small particles that can be swept up by larger bodies. However, for this process to work, a few seeds of cm-size or larger have to be introduced.