Predicted intensities for model C (I_model) normalised by their observed value (I_obs) for all ionic lines considered, as a function of n_H and r_in for a given starburst age (of 3.7 Myr). The vertical dashed line shows the best fit model density n_H = 10^2.8 cm^-3, and the vertical dotted line the critical density of the ionic specie, if within the range displayed. The best fit inner radius is r_in = 10^21.4 cm (green symbols). (This figure is available in color in electronic form.)