Contour plots of the χ² values of the 3D grid (log scale). The parameters are given in logarithmic values. Units are cm^-3 for the density, yr for the burst age, and cm for the inner radius. The minimum reduced χ² found for C_HII is . Contour levels at 1-σ, 3-σ, and 5-σ from the minimum χ² are displayed in orange. There are 5 other models within 3-σ of . The red color on the plots correspond to  >1000-σ level. Top: χ² contours in the n_H-r_in plane, for A_burst = 3.7 Myr (or 6.57 in log). Bottom: χ² contours in the n_H-A_burst plane, for r_in = 10^21.4 cm. (This figure is available in color in electronic form.)