Number density (top panels) and region of the disk included in the parameter study (bottom panels) for a typical T Tauri (L= 0.3 L_⊙; left panels) and Herbig (L = 20 L_⊙; right panels) disk model (see Appendix A for details). The part of the disk that is included in the physical models is shown in dark orange and is bound by the highest temperature included (40 K) and the restriction that the UV is fully shielded. Light orange denotes the region with temperatures and densities probed by our models but with some low level of UV. White contours show the regions contributing to 25% and 75% of the emission from the C¹⁸O 3–2 line. The green crosses numbered #1, #2, #3 and #4 are approximate locations of the representative models of Sect. 3.1.3.