Blend-emitting regions from evolutions models from Greenwood et al. (2019a) for o-H₂O blend at 17.12 µm and HCN blend at 14.59 µm with epochs 1.78·10⁴ yr (top), 1.78·10⁵ yr (middle), and 1.78·10⁶ yr (bottom). In each figure, the top panel shows the continuum optical depth (black line) and the blend optical depth (blue line). The middle panel shows the cumulative blend flux from vertical escape probability as a function of the radial distance from the star. The bottom panel shows the H₂O or HCN density in color scale with the region from which the vertical × radial integrated flux, amounting then to 49% of the total blend flux, is emitted. This is reported as numbers in the middle slice of each plot (black contours, 15–85% of the vertical and radial integrated blend flux, dashed lines).