Contour diagrams corresponding to τ₀ = 1 and τ_eff = 1 limits for the resonance w and intercombination y line of O vii (top panel) and Fe xxv (bottom panel). The green vertical lines in each panel indicate N_crit, which is the critical Li-like ion column density limit corresponding to continuum optical depth τ_cont = 1 at the He-like triplet. The dashed lines or curves are for the CIE plasma case, and the solid lines or curves are for the PIE plasma case as described in Sect. 3.1.