Stokes number of CAIs, a dimensionless coupling constant that indicates how well the dust grains are coupled to the gas. Lower values of the Stokes number imply better coupling to the gas and a shorter stopping time τ_stop, which evens out velocity differences between the dust and gas. For a Stokes number that is zero, the radial drift velocity of the gas υ_d equals the radial velocity of the gas itself υ_R, as shown by Eq. (25).