Integrated line intensity ratios and upper and lower limits for CN 2→1/HCN 3→2 (left) and HCN 3→2/HCO⁺ 3→2 and H₂CO 3₀₃ → 2₀₂/HCO⁺ 3→2 (right) transitions, as a function of the stellar luminosity. Horizontal dashed lines indicated the corresponding values for the Orion Bar (Goicoechea, priv. comm.). The letters correspond to each of the sources. (A) DM Tau; (B) V4046 Sgr; (C) AA Tau; (D) GM Aur; (E) LkCa 15; (F) IM Lup; (G) AS 209; (I) SAO 206462; (J) CQ Tau; (K) MWC 480; (L) AB Aur; (M) HD 142527.