Upper panel: comparison of the data (dots for July 26, diamonds for August 13) with spectral models (solid black line for July 26, dotted black line for August 13), showing the increased C₂H₆ emission on July 26. The data are an average over the 4.1′′-long slit for the cross-dispersed echelle mode of VLT/VISIR. Lower panel: residual between the data and the model for July 26 (solid black line) and August 13 (dotted black line). Models were calculated for the C₂H₆-only case, fixing T(p) to the August 13 values. Note that the vertical scales for a) and b) differ from one another.