Critical metallicity of large aggregates necessary to trigger planetesimal formation as a function of total metallicity. The points present data obtained in direct numerical simulations by Bai & Stone (2010c) for three different values of the Π parameter. We note that Bai & Stone (2010c) use the symbol Z_crit for the critical total metallicity they find for each run, whereas we use it for the metallicity of grains larger than St_crit = 10^-2. The lines show our fit to the data (Eq. (12)). The shaded region is where the metallicity of large grains would be higher than the total one, which is not possible.