Temperature-density structure of a suite of BH* models, for varying ionization parameter. Same as Fig. 9, but for a fixed n_H = 10⁹ cm⁻³, for different choices of the ionization parameter log₁₀U. The solid lines indicate temperature (left axis) and the dashed lines indicate electron density (right axis). The ionization parameter mainly influences the temperature of the gas before the ionization front. All the models present a thermalized outer layer with T_e ≈ 6000–7500 K, independently of log₁₀U.