How to build planetesimals from a size distribution of particles

In planet formation, growing from cm size to km size is traditionally one of the most difficult hurdles to overcome. Radial drift of dust particles is extremely efficient, threatening to throw all building blocks into the central star, while collisions between particles tend to be too violent to allow for fast growth beyond cm size. Resonant drag instabilities such as the streaming instability provide an elegant way out: they use the free energy associated with the problem (dust drift) to very quickly grow dust overdensities that can collapse gravitationally. This way, km-sized planetesimals can be formed in a very efficient way. However, most models assume a monodisperse size distribution: all dust particles have the same size. I will discuss how having a size distribution makes planetesimal formation through the streaming instability more difficult.