Same as Fig. 6, but with combined photoionization and shock models. The photoionization models use the energy distribution of PopStar at an age of 1 Myr. The shock models are extracted from the shock-model library of Allen et al. (2008) for a magnetic field of 0.5 μG and have a velocity of 300 km s^-1. They are added in such a proportion to obtain He ii/Hβ= 0.01 for the combined model (see text for more details).