Recombination temperature as a function of T_O   iii for various models. Along each sequence, only the metallicity varies. Solid and dashed lines are used to represent . A dotted line of the same color depicts , but it is plotted only for cases where differs from by more than 3%. The ionization parameter is always U = 0.01. In the case of the 50 km   s^-1 combined photoionization-shock model sequence (light green), Z assumes the values of 0.01 Z_⊙, 0.10 Z_⊙, 0.25 Z_⊙  0.49 Z_⊙  1.0 Z_⊙, and 1.6 Z_⊙ (the higher the metallicity, lower is T_rec ). All other sequences cover the same metallicity range of 0.01 Z_⊙ to 2.5 Z_⊙, with a fixed increment of 0.2 dex between successive models. Three sequences represent κ-distributions (20, 10, or 5, see legend) and two sequences represent the case of temperature inhomogeneities (t² = 0.05 or 0.10, see legend). The homogeneous pure photoionization sequence is represented by the magenta line (same as in Fig. 2).