Line intensity ratio of H₂ 1–0 S(1) to Brγ for the ionized region as a function of T _⋆ . Curves for different a) L _⋆ , b) n_H, and c) M_d/M_g are shown. The line styles are the same as in Fig. 2. Observed ratios are represented by dots. Boxes with up arrows are lower limits and with down arrows are upper limits. References for the observations are given in the text. The standard PN values are adopted for the parameters not mentioned.