Line-emitting regions of CO₂ (top), HCN (middle), and C₂H₂ (bottom). On the upper panels of each sub-plot, the 20 μm dust continuum and gas line optical depths and the line flux are plotted. The optical depths and line fluxes are calculated using the vertical escape probability. The noisy appearance of the C₂H₂ line flux is an artefact of the low vertical resolution in comparison to the very thin line-emitting region. On the lower panels, the plotted colour map is the molecular abundance (relative to the total hydrogen abundance ⟨H⟩), with contourincrements every 0.5 dex. The over-plotted contours are the main line-emitting area (black-and-white line), the visual extinction (dashed white line at A_V = 1, and solid white line at A_V = 1), and gas temperature. The gas temperature is plotted in red at 100, 200, 300, and 1000 K (solid, dashed, dotted, and dash-dotted lines respectively).