Results of T Tauri disk models with carbon abundances reduced by factors of three and ten. Panel a) shows C¹⁸O J = 1−0 and J = 6−5 line luminosities as a function of the disk mass (respectively in red and blue), and panel c) the C¹⁸O J = 2−1 and J = 6−5 line luminosities. The models with ISM-like carbon abundances are shown by the solid lines, those with a factor of 3 of carbon depletion by the dashed lines and those with a factor of 10 depletion by the dotted lines. Panel b) shows C¹⁸O J = 1−0 /J = 6−5 line luminosity ratios as a function of the disk mass, and panel d) the J = 2−1 /J = 6−5 ratios. The models with ISM-like carbon abundances are shown by the red circles, those with a factor of 3 of carbon depletion by yellow squares and those with a factor of 10 depletion by the blue hexagons. Model disk parameters: R_c = 60 au, γ = 0.8, h_c = 0.1, ψ = 0.1, and M_disk = 10^-5, 10^-4, 10^-3, 10^-2, 10^-1 M_⊙.