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Appendix A: Gaussian fit of spectral lines

The line profiles of the observed ¹²CO and ¹³CO lines are the result of several components overlapping along the line of sight and in the velocity space, which are affected by self-absorption, mainly in the lower-J transitions. The lower- and mid-J¹²CO lines can be fit with several Gaussian components, but the higher J (J_up ≥ 11) can be fit with just one or two components. We selected the Gaussian component of the lower- and mid-J lines based on the central velocity and line width obtained for the single Gaussian used to fit the ¹²CO J = 16 → 15 line.

With the exception of the J = 1 → 0 transitions, the higher-J HCN and HCO⁺ lines seem to consist of a single

component that can be fit with one Gaussian. Because the main HCN and HCO⁺ line profiles show more structure than that of their rare isotopologues, the single component of ¹³C bearing lines is indicative of self-absorption in the main lines, at least at the rms level of our HCN and HCO⁺ spectra. Therefore, we fit a single Gaussian component to the main HCN and HCO⁺ lines masking the line centers and using the less self-absorbed line wings to reproduce the missing flux.

The Gaussian parameters of the fit is summarized in Tables A.1–A.3, for all the observed transitions at the four selected positions.

Appendix B: Alternative solution for the LSED fit

The HCN and HCO⁺ LSEDs at the position of the peak HCN J = 8 → 7 emission, at offset position (−65′′, +31′′), can only be fit using the ambient conditions found for the warm component of the ¹²CO LSED. The parameters of the model are presented in Table B.1 and the LSED fit is shown in Fig. B.1. Here we used the same isotope ratio of 50, but lower ratios of 30 and 20 would lead to a better fit of the HCN and HCO⁺ lines, respectively.

The LSED of the HCN and HCO⁺ lines observed deeper into the PDR at offset position (−60′′, −30′′) can also be fit using just the temperature and density of the cold component found for the ¹²CO LSED, but it requires a larger isotope ratio of 75 to fit the HCN and HCO⁺ lines. The parameters of the model are presented in Table B.2 and the LSED fit is shown in Fig. B.2.

	Fig. B.1 Two-component fit of the line spectral energy distribution of the CO, HCN, and HCO+ species for the spectra at position (−65′′, +31′′) toward the HCN peak. The fit of the 13C bearing isotopologues is shown in the insets. The cold and warm components are shown in dashed and dotted lines, respectively. The error bars are as in Fig. 9.
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	Fig. B.2 Two-component fit of the line spectral energy distribution of the CO, HCN, and HCO+ species for the spectra at position (−60′′, −30′′) toward the southern PDR. The fit of the 13C bearing isotopologues is shown in the insets. The cold and warm components are shown in dashed and dotted lines, respectively. The error bars are as in Fig. 9.
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© ESO, 2015