Top panels: ¹³CO 6–5 spectra of IRAS 4A and 4B at the source positions. The green line is the fit to the narrow plus broad components. Bottom panels: the same lines after subtracting the broad component. The red line indicates the ¹³CO envelope model emission using the CO drop abundance profile derived from the C¹⁸O data. The figure indicates that a substantial fraction of the on-source emission comes from the passively-heated envelope. For IRAS 4A, however, there is also significant extended emission that is not due to the envelope (see Fig. 19).