Contours (in white) of the brightness distribution of the C₂H N = 1−0 transition measured by Guélin et al. (1999) with PdBI, overlayed on the normalised predicted brightness distribution, assuming LTE conditions. The three panels represent the results obtained with three different chemical models. For the map in panel a), we assumed a constant mass loss, and the interstellar UV field of Draine (1978). The map in panel b) shows the brightness distribution when assuming density enhancements in the CSE as described by Cordiner & Millar (2009) and as discussed in Sect. 4.3.2. For panel c), we assumed again a constant mass loss, but an interstellar UV field that is only 25% as strong as that presented by Draine (1978).