Modelled optical depth contours (τ_1.3mm, τ_2.9mm, τ_8.1mm), overlaid on the observed intensity maps at 1.3, 2.9, and 8.1 mm, respectively. The observed intensity maps are identical to those shown in the leftmost panels of Fig. 3, and show the dust emission convolved with the indicated beam sizes, while the optical depth contours are not convolved. The optical depth value is indicated at each corresponding contour. The zoomed-in optical depth distributions are displayed in Fig. 9.