Time evolution of our fiducial run: a) total planetesimals and dust mass; b) ratio of the surface density of particles larger than St = 10^-2 to the gas density compared to the threshold value Z_crit; and c) dust-to-gas ratio in the midplane compared to the threshold value of unity. The dashed vertical line corresponds to the time when the first planetesimals are formed. It can be seen that although the metallicity of large grains was already higher than Z_crit at t ~ 700 years b), the planetesimal formation only starts at t ~ 990 years a) because the large grains have not settled to the midplane yet c).