APEX observations (circles, triangles) and DALI full disk models (diamonds) of the CO 6−5 (top), [C i] 1−0, [C i] 2−1, and [C ii] 158 μm (bottom) transitions. All values are colour-coded for the stellar T_eff; T Tauri stars are red and Herbig Ae/Be stars blue. The fluxes are plotted with stellar T_eff values in the upper subpanels (with [C]/[H]_gas= 1.35 × 10^-4 and R_c = 50 au fixed for the fiducial model), and with the gas-phase total elemental carbon abundance, [C]/[H]_gas, in the lower subpanels (solid lines connect models with R_c = 50 au, dashed lines R_c = 10 au). Horizontal lines in the lower subpanels show the flux changes from varying the gas-to-dust ratio by a factor of ten around the fiducial T Tauri and Herbig Ae/Be models.