Value of the Stokes number of particles with zero radial velocity, as a function of the radial distance beyond the planet, normalized to the planet’s R_H. The parameters used for the calculations are the same as those of the nominal simulation in Weber et al. (2018): q = 10⁻³, α = 3 × 10⁻³, and H/r = 0.05, so that the curve depicted here can be compared with the red curve in the bottom panel of Fig. 4 of Weber et al.. The critical Stokes number is the minimal value along the curve.