¹³CO (J = 3−2) luminosity as a function of dust mass. Light blue circles present the simulated line luminosities with different levels of carbon depletion: no depletion (δ_C = 1, black contoured circles), depletion by a factor of 10 (δ_C = 0.1, filled light blue circles), and depletion by two orders of magnitude (δ_C = 0.01, light blue empty circles). Purple circles present the simulated line luminosities obtained without carbon depletion, fixing the gas mass M_gas = 10^-4M_⊙ and increasing the dust mass to M_dust = 10^-5,10^-4M_⊙, accordingly with gas-to-dust ratios Δ_gd = 10, 1. A subsample of six Lupus sources is shown with gray stars. Models run with R_c = 30 au and with R_c = 60 au are shown in the left and right panels, respectively.