Observations and modelling of HD 100546. The panels show, from top left to bottom, the spectral energy distribution; spectrally resolved line profiles; spatially resolved integrated CO 3–2 emission; and the full set of line fluxes used in the analysis. CO isotopolog lines for a given rotational transition are plotted at the same x-axis location. Two well-fitting models, with a gas to dust ratio Δ_{gas / dust} = 10 and [C]/[H]_gas = 10^-4 (solid blue) and Δ_{gas / dust} = 100 and [C]/[H]_gas = 10^-5 (dashed/open blue) are plotted on the observations (black) to illustrate the range of allowed parameters. The data and model values for C₂H have been multiplied by a factor of 10³ for display purposes.