Giant grains around protostars

Turbulent-driven coagulation of interstellar grains is examined for the case of a dense circumstellar shell produced by a powerful protostellar wind. It is shown that a wind driving into a cloud of similar mass to the central star produces a cold shell of sufficient density for frequent grain-grain collisions. Assuming subsonic turbulence and a temperature of around 20 K for the molecular gas, the relative velocities of grains are estimated to be about 10 m/s. It is therefore argued that grains with icy mantles will stick together, leading to the aggregation of large particles. An approximate description of this process indicates that 'giant' grains with radii in the range 1-100 microns can be formed within the evolutionary time available.