How to form asteroids from mm-sized grains
  The size distribution of asteroids in the solar system suggests that they formed top-down, with 100-1000 km bodies forming from the gravitational collapse of dense clumps of small solid particles. We investigate the conditions under which solid particles can form dense clumps in a protoplanetary disc. We used a hydrodynamic code to model the solid-gas interaction in disc. We found that particles down to millimeter size can form dense clumps, but only in regions where solids make $\sim$ 8% of the local surface density. More generally, we mapped the range of particle sizes and concentrations that is consistent with the formation of particle clumps. 
Daniel Carrera, Anders Johansen, Melvyn B. Davies
astro-ph.EP