Model spectrum, in colour, over the observed data, in black, of AFGL 2591 for transition lines of (a) CH₃CCH on the brightest pixel of the CH₃CCH 5₀−4₀ (E_up = 12 K), line, and (b) c-C₃H₂ on the brightest pixel of the c-C₃H₂ 2_1,2−1_0,1 (E_up = 6 K), line. One component is used to fit the two carbon chain molecules together. The dashed grey line represents the observed transition frequency, with the energy level of this transition labelled.