Spectra extracted at position 6 in Table 2. The grey filled histogram shows the spectrum around the ¹³C¹⁸O transition and the velocity scale refers to its rest frequency. The thick curve shows the overall fitted profile to the observations, while the thinner curves show the profiles of ¹³C¹⁸O, as well as the identified emission of the hyperfine structure of C₄H (see Sect. 3.2.1 for details) and the line of H₂CS. The empty histogram shows the ¹³CO profile (divided by 100) at the same position as a reference.