Temperature exponent q as a function of the surface density exponent p_dust. Large bullets show the median values, smaller bullets the 1st and 2nd quartiles, and vertical bars the complete distribution of models; i.e., the horizontal ticks on each bar represent the minimum and maximum values reached in the whole set of models. Models with maximum grain sizes ranging from 1 μm to 10 μm are shown in red, while models with a_max ranging from 100 μm to 10 mm are shown in blue. The different models are slightly shifted along the p axis for clarity. The plane is divided by the stability criteria for dust grains: q = 1 with growth, −p_gas + q + 1/2 = 0 (assuming p_gas = p_dust, black dashed line) without growth and q = 2 /3 for planetesimals (black dot-dashed lines). The thick gray line indicates the location of steady-state, constant α viscous disc models p_gas = 3/2 − q.