Revised critical solid abundance as a function of the dimensionless stopping time τ_s from Carrera et al. (2015). The filled and open circles show our models with and without significant radial concentration of solids, respectively. The solid black line for τ_s ≳ 10^-1 and the dashed blue line for τ_s ≲ 10^-1 are the unmodified critical curve from Carrera et al. (2015). The black solid line for τ_s ≲ 10^-1 shows the revised part of the critical curve, where the black dashed line is the extrapolation of it. For solid abundances Z above the black line (green region), spontaneous concentration of solid particles by the streaming instability can occur.