Spectrum of IRAS4B around the 215 GHz ${\text{H}}_2^{33}\text{S}$ line. In black, the integrated emission in a 1.5″ circle around the protostar’s position. In red, the fit to the ${\text{H}}_2^{33}\text{S}$ emission. In yellow, the emission of CH₃CHO, assuming its column density is three times greater than that of ${\text{H}}_2^{33}\text{S}$. The purple dashed line represents the sum of the emission of both species. The different lines from Table A.1 appear marked in the bottom side of the figure with small vertical lines. For reference, the most intense ${\text{H}}_2^{33}\text{S}$ line (the fourth starting from the left) is at a velocity of −3.1 km/s.