Schematic visualisation of the functions describing the dust-to-gas mass ratio in the MREF and MREF* models. For illustrative purposes, the density was held constant, representative profiles for the temperature and radial optical depth have been chosen and the models are shown as functions of a dimensionless radius. The vertical dashed black lines mark locations where the temperature crosses the dust sublimation temperature. $r_{\text{S}}^{-}$ and $r_{\text{S}}^{+}$ denote crossings with a negative and positive temperature gradient, respectively. The dashed green line represents the radius at which the radial optical depth is equal to three.