RADEX models. Optical depth of the SO₂ 14_0,14−13_1,13 transition for three different excitation temperatures: 50, 100, and 150 K. The cyan contour represents τ = 1, the two dashed white vertical lines indicate the range of values obtained for for most of the sources, and the dashed white horizontal line shows an H₂ number density of 10⁷ cm⁻³ (lower limit for n_H used in the calculations).