Dependence of the direct drag force and Coulomb drag force on drift velocity and the structure of the photo-evaporation flow with the inclusion of Coulomb drag. Left: direct drag (solid line) and the Coulomb drag on a 300 Å dust grain with charge Z_d = 195 (upper dashed line) and a 3000 Å dust grain with charge Z_d = 1809 (lower dashed line). Plotted is the dependence of the drag forces on the relative velocity between the gas and the dust, expressed in the Mach number ℳ = v_drift/c_s. The dotted horizontal line represents the radiation pressure force at r = 0.1 pc. Right: calculated trajectories of a dust grain with κ_rp = 6.7 × 10³ including the Coulomb drag force. The grains only decouple from the gas very close to the star (i.e., d < 0.025 pc).