Calculated Stokes number. The blue lines indicate the cases where CO abundance is assumed to be 10⁻⁴, 10⁻⁵, and 10⁻⁶ from top to bottom. The red lines show values when assuming the gas-to-dust ratio is always 100. The green lines mark the drifting barriers Eq. (12) or St = 1, depending on which one is smaller. As we assume the gas-to-dust ratio is equal to 100, Eq. (12) can be simply written as St_drift = 0.005/η.