Six sequences of combined shock-photoionization models along which the shock velocity V_s is increased in locked steps. The preshock conditions are set by photoionization equilibrium, using the parameter U = 0.01 and the M03 SED, and assuming thermal and ionization equilibria (see Sect. 3.5). Each sequence is characterized by a different metallicity Z value, which is expressed in Z_⊙ units in the line color legend. For each model, the preshock temperature, the postshock density, the postshock temperature, and the resulting shock velocity are given in Table 1. The first two models of the Z = 0.5 Z_⊙ and 1.0 Z_⊙ sequences overlap too closely to be distinguished.