Effect of model parameter variations on the line fluxes of carbon species in the disk atmosphere. Two central stars are considered: a 4000 K star with UV excess (red) and a pure 10 000 K photosphere (blue). Horizontal bars show the range of variation resulting from the parameter values given in Table 4. The fiducial model (black vertical line) is a disk with R_c = 50 au and with [C]/[H]_gas = 1.35 × 10^-4, and the reference fluxes for the 4000 K models are, from left to right, 9.6 × 10^-19, 9.0 × 10^-20, 2.2 × 10^-19 and 3.4 × 10^-18W m^-2; for 10 000 K they are 4.6 × 10^-18, 2.6 × 10^-19, 1.1 × 10^-18 and 3.1 × 10^-17W m^-2.