  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.
