Contour maps of the integrated intensity ratio of ¹³CO to C¹⁸O in the parameter plane of the optical depth of ¹³CO and the abundance ratio of ¹³CO to C¹⁸O. In the left panel, φ_¹³CO = 1.0, φ_C¹⁸O = 1.0, T_ex ¹³CO = 30.0 K, and T_{ex  C¹⁸CO} = 30.0 K are assumed. In the right panel, φ_¹³CO = 1.0, φ_C¹⁸O = 0.8, T_ex ¹³CO = 30.0 K, and T_{ex  C¹⁸CO} = 20.0 K are assumed. The yellow dashed lines indicate /X_C¹⁸O = 5.5, which is the solar system value. The black dashed lines indicate R_13,18 = 5, 10, 15, 20, and 25. The red lines correspond to R_13,18 = 17.0 and 14.5, which are the values for the Orion Bar and DLSF, respectively. The white lines correspond to R_13,18 = 12.15, 11.7, 11.4, and 10.1, which are the values for the OMC-1, OMC-2/3, OMC-4, and the bending structure, respectively.