Keplerian disk: selected line emission ratio (see Sect. 3.2.1 for details). (a) CO /C¹⁸O, J = 4− 3. The highlighted velocity channels at 0.3 and 2.8 km s⁻¹ indicate thefeatures caused by self obscuring and differences in radiating areas between the isotopologs, respectively. (b) C¹⁸O, J = 4− 3/J = 3−2. The highlighted velocity channels at 0.3 and 2.8 km s⁻¹ indicate thefeatures caused by self-obscuring and differences in radiating areas between the isotopologs, respectively. (c) CO /C¹⁸O J = 4− 3. The blue highlighted channel at 4.2 km s⁻¹ indicates thevelocity at the cavity outer edge. (d) C¹⁸O J = 4− 3/J = 3−2 line emissionratio. The blue highlighted channel at 4.2 km s⁻¹ indicates the velocity at the cavity’s outer edge.