RADEX simulations. We consider a standard CO abundance N_CO = 10⁻⁴⟨N_H2⟩. We measure a mean molecular hydrogen column density of 3 × 10²⁰ cm⁻² that we correct for the surface beam filling factor (η_bf = 0.052) assuming the CO(2-1) is optically thick and computed for T_ex = 5 K. Last, we estimate a mean ΔV of 30 km s⁻¹. Left panel: the different line types represent the simulations for 3 different kinetic temperature T_kin. Left and right panels: expected CO(2-1) to CO(1-0) line ratio (in temperature) and excitation temperature of the 2 lines, respectively, as a function of the hydrogen density. Right panel: the black (resp. grey) lines correspond to the CO(1-0) (resp. CO(2-1)) line. The green, blue, and magenta ticks correspond to the line ratios measured in the 3 positions. This shows that this gas is subthermally excited and well below the expected critical density for a kinetic temperature of 10 K, cm⁻³ and cm⁻³.