Variation of the predicted excitation temperature (top) and opacity (bottom) for the two lowest lines of p-H₂CO. The top panel shows the 1_0,1 − 0_0,0 line at 72 GHz, and the bottom panel shows the 2_0,2 − 1_0,1 line at 145 GHz. The p-H₂CO column density is set to 7.3 × 10¹¹ 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 varies 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 1_0,1 − 0_0,0 line at 72 GHz has not been observed in this work.