Upper panel is the ¹²CO J = 6−5 to C¹⁸O J = 6−5 ratio channel maps (θ_HPBW = 10″) at different velocities from 6 to 11 km s^-1 in the OMC-1 core. The edge cut and empty area are due to different image sizes and data blanking (2 σ level of the C¹⁸O J = 6−5 temperature). Lower panel is the corresponding C¹⁸O J = 6−5 to J = 3−2 ratio channel maps (θ_HPBW = 20″). The black arrow in the 10 km s^-1 image indicates the ratio gradient across the Orion Bar (see text). The Orion Hot Core (HC) position is marked, and Orion BN is located at the offset (0′′, 0′′). The positions of the five Trapezium stars are marked in each map.