Non-LTE radiative transfer models for different gas physical conditions at the PDR. Left: expected CO J = 3−2 peak brightness temperature as a function of the gas temperature for N(CO) = (1.0−5.0) × 10¹⁷ cm⁻² and different gas densities (colored lines). Right: expected HCO+ J = 4−3 integrated line intensity as a function of the gas density for N(HCO+) = (0.8−1.2) × 10¹³ cm⁻² and different gas temperatures (colored lines). The solid lines display the model predictions for N(CO) = 3 × 10¹⁷ cm⁻² and N(HCO+) = 1 × 10¹³ cm⁻², and the colored areas represent the possible range of values for the assumed range of column densities. In both panels, the gray shaded areas represent the observations at the PDR position, with a ±1σ uncertainty.