Variation of the opacity and excitation temperature of the four detected lines of ortho c-C₃H₃, as a function of the H₂ density and kinetic temperature. The calculations used collision cross sections with He from Ben Khalifa et al. (2019), and their scaled version for para H₂. Stars show the values for the 1_1,0 − 1_0,1 line at 18 GHz, filled squares for the 2_1,2 − 1_0,1 line at 85 GHz, filled triangles for the 3_1,2 − 2_2,1 line at 145 GHz, and empty hexagons for the 4_1,4 − 3_0,3 line at 150 GHz. The left panel presents the line opacities and the right panel the difference between the excitation temperature and the CMB. The adopted column density is 10¹² cm⁻² and the line FWHM is 0.6 km s⁻¹.