Peak T_b ratio between the CO(4–3) and CO(2–1) lines versus H₂ density, and the CO column density per unit velocity width (N_CO/ΔV) for two values of the kinetic temperature: T_k = 60 K, the dust temperature (left), and T_k = 30 K (right). The black contours are underlining the values obtained in the data. In both panels, the vertical line corresponding to (N_CO/ΔV) = 7 × 10¹⁶ cm^-2/(kms^-1), value chosen in Table 4, has been emphasized with blue dashes. The predictions come from the LVG models that are part of the Radex code (van der Tak 2007).