Recent papers, Episode 1

From now on, each time something gets published with my name on it, I will try to post some (hopefully easy to understand) explanation about what we have been doing. So here's my first post about Fredriks recent paper:

Planetesimal formation by sweep-up: How the bouncing barrier can be beneficial to growth
F. Windmark et al. A&A (2012) vol. 540, A148

Dust grains in protoplanetary disks grow due to sticking collisions and at some point are supposed to become the precursors of planets. However there are a couple of issues: growth barriers and radial drift. Ignoring the drift issue one for now, particles grow until at some specific particle size, they do not stick upon colliding, but instead they just bounce off or fragment each other.

In this work, we investigated how recent laboratory work on dust collision physics can be implemented in our numerical models to possibly circumvent at least some of these problems. Laboratory work has shown that high-velocity impacts of small particles can cause two effects: erosion (like sandblasting) or mass-transfer fragmentation (i.e. the impactor is shattered but still deposits some mass on the target). So if there is enough small dust hitting the larger particles at the right velocity (so that they add mass instead of eroding the target), the target could continue to grow.

Fredriks results have shown that with the current collision model, particles still get stuck at small sizes, just bouncing off each other. These particles are however of the right size to be swept up by larger ones. So if, for some reason, particles of ~ 1 cm in size were present, they would continue to grow through this sweep up process. Only a few "seeds" are needed because once these seeds have grown to larger bodies, they would produce fragments which are also able to grow through the same process. Bodies of around 100 m in size could be formed by this process, the origin of the seeds, however, is yet to be explained.