Relative contribution of selected elements to the line radiative force as a function of the effective temperature of a model star, plotted at the wind critical point for models with M = 40 M_⊙. Different line styles denote metallicity, with the solid line corresponding to Z_⊙, dashed line to 0.5 Z_⊙, and dotted line to 0.2 Z_⊙. The thick blue line (with corresponding axis on the right) plots the mean ionization state driving the wind for metallicity 0.5 Z_⊙. This is defined as ∑f_iz_i/(∑f_i), where f_i is the contribution of ion i with charge z_i to the line radiative force. For simplicity, the values were obtained within the Sobolev approximation.