Δχ² contours of the fits to the observed 6.2/8.1, 6.2/11.3, and 8.1/11.3 μm AIB ratios in N_C,max − ϕ parameter space (for N_C,min = 10 and f_PAH = 20% corresponding to the minimum χ² model, see text in Sect. 6.2.2). The σ levels are indicated with dotted contour lines. The 1σ contour helps in predicting the fiducial model values: N_C = 10–26 and ϕ = 0.15, thereby estimating the final PAH-to-dust mass ratio as f_PAH = 17% for the best fiducial model (Fig. 7 and Sect. 6.2.2).