Variation of the predicted excitation temperature (top) and opacity (bottom) for the three lowest lines of o-H₂CO. The left panel shows the 1_1,0 − 1_1,1 line at 4.8 GHz, the middle panel shows the 2_1,2 − 1_1,1 line at 140 GHz, and the right panel the 2_1.1 − 1_1,1 line at 150 GHz. The o-H₂CO column density is set to 2.2 × 10¹² cm⁻², the line FWHM is 0.5 km s⁻¹. The kinetic temperature varies from 10 K to 60 K. The H₂ ortho-to-para ratio increases with the kinetic temperature. The symbol sizes and shapes indicate the electron fraction : empty triangles for 2 × 10⁻⁶, starred triangles for 10⁻⁵, filled triangles for 3 × 10⁻⁵, empty squares for 6 × 10⁻⁵, starred squares for 10⁻⁴, and filled squares for 2 × 10⁻⁴. Larger symbols are used for larger values of the electron fraction. The 2_1,2 − 1_1,1 line at 140 GHz has not been observed in this work.