Opacity as a function of column density (dots). Here, we assume that the radiative transfer model underestimates the opacity effects, and a factor of f = 2.3 is applied (see text), so that the values correspond to the red dots plotted in Fig. 11. Colour-coding is according to the LOS mean temperature , and the colour scale is given at the right. A polynomial fitting of degree 2 is shown for various temperatures (35, 55, 70, 80, 100, 120, and 140 K). The dashed line corresponds to the two-line fit of values from dataset S1 shown in Fig. 11.