Comparison between observations and model results for HCN (top) and CS (bottom). Each row presents the intensity of the different transitions observed for the species (main and rare isotopologs) as a function of H₂ column density (blue symbols) together with the predictions from the model (dashed red lines). The CS(3–2) dataset has fewer points due to the limited observations carried out in the 2 mm-wavelength band. Several H¹³ CN(1–0) points can be seen above the plot lower limit at very low column densities. A visual inspection of their spectra suggests that they represent noise or baseline residuals and not true molecular emission.