Variation of the excitation temperature and opacity with the electron fraction, kinetic temperature, and H₂ ortho-to-para ratio for the 1_1,0−1_1,1 line at 4.8 GHz (top left), and zoom on the region with excitation temperature lower than the CMB indicated by a dotted box (top right). The 2_1,2−1_1,1 line at 140 GHz is shown in the bottom left panel and the 2_1,1−1_1,0 line at 150 GHz in the bottom right panel. The difference between the excitation temperature and the CMB is plotted to illustrate the change of regime of the 1_1,0−1_1,1 transition at 4.8 GHz, and a different scale is used for the 140 GHz and 150 GHz lines. Different symbols show the values of the H₂ ortho-to-para ratio between 3 × 10^-5 and 3, their color and size indicate the kinetic temperature between 10 and 60 K. The column density of o-H₂CO is 2.2 × 10¹² cm^-2, the H₂ density is set to 10³ cm^-3, and the FWHM is 0.5 km s^-1. The 2_1,2−1_1,1 line at 140 GHz has not been observed in this work.