Continuum-subtracted spectra (black line) of the selected regions (see Sect. 3.3 and bottom right panel of Fig. 1) where the CO v = 1−0 band is detected in emission. The red line is the best-fit model. The ¹³CO and C¹⁸O emission lines are fitted and subtracted (see Sect. 3.3.2) before fitting the remaining spectral features. The green, blue, and orange lines, which are also included in the best fit, represent the atomic and molecular transitions, the D_n-PAH feature, and the ¹³CO v = 1−0 band, respectively. The wavelengths of the ¹²CO v = 1−0 transitions are indicated in purple. All the spectral features present in the best-fit model are listed in Table 1.