Contour plot showing the opacity (in cm² g⁻² ) from OPAL tables (colour-coded, see scale at right) as a function of temperature (in K) and of density (in g cm⁻³ ) for a chemical composition of [X, Y, Z] = [0, 0.98, 0.02], with X hydrogen mass fraction, Y helium mass fraction, and Z metallicity. The black lines correspond to the outer layers of the hot, more compact 15 M_⊙ stellar models with Ṁ ≥ 8 × 10⁻⁶ M_⊙ yr⁻¹ from Fig. 1 (see also Table 1), while the pink line indicates the cooler model with Ṁ = 4 × 10⁻⁶ M_⊙ yr⁻¹, and the orange dot-dashed line the plane parallel grey atmosphere model with Ṁ = ×10⁻⁵ M_⊙ yr⁻¹. The yellow circles indicate the locations of the sonic points of this set of models, while the white lines indicate the iso-contours of the Eddington opacity for the 10 M_⊙ (dotted), 15 M_⊙ (continuous), and 20 M_⊙ (dashed) stellar models. Grey areas indicate regions in the log(ρ) − log(T) diagramwhere the sonic point of a radiation-driven wind cannot be located (d κ∕d r ≤ 0).