Line flux ratios plotted against the ionization parameter U for radiation temperature of T_eff = 40   000 K (orange models) and 45   000 K (blue models). The horizontal lines show the observed values toward specific regions illustrated in previous figures, while the associated vertical bars show the range of values within each region. The light gray horizontal rectangle shows the range of values observed across the entire N 11B nebula. The dark gray vertical rectangle indicates the range of U values agreeing the best with the observed line ratios (see text). The vertical arrow shows the effect of dust extinction on the observations for A_V = 1. The W1 region was not covered by the [O iii]_opt HST/WFPC2 observation (Fig. 6), and [N ii]₁₂₂ was only detected by the Herschel/PACS follow-up pointed observations (Fig. 4).