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.