Line-emitting regions for the model TT highres for C₂H₂, HCN, CO₂, NH₃, OH, and o-H₂O. On the upper panel, 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. On the lower panel, the plotted colour map is the molecular abundance (relative to the total elemental hydrogen abundance, across all species), with contour increments 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).