Velocity profile as a function of radius for a set of 10 M_⊙ stellar models with different mass-loss rates (as in Fig. 1). The black continuous lines indicate models with Ṁ ≥ 2 × 10⁻⁵ M_⊙ yr⁻¹ (2, 3, 5 × 10⁻⁵), while the pink dashed lines indicate models with Ṁ ≤ 1 × 10⁻⁵ M_⊙ yr⁻¹ (5, 8, 10 × 10⁻⁶). The green dashed line and the orange dot-dashed line indicate the local isothermal sound speed of the model with Ṁ = 5 × 10⁻⁶ M_⊙ yr⁻¹ and the plane parallel grey atmosphere model with Ṁ = 8 × 10⁻⁶ M_⊙ yr⁻¹, respectively.