CO isothermal models carried out with MADEX (filled squares) and RADEX (empty squares) non-LTE radiative transfer codes. Two gas densities are considered: n(H₂) = 3 × 10³ cm^-3 (upper panels) and n(H₂) = 3 × 10⁵ cm^-3 (lower panels). Excitation temperatures and line centre opacities (left and right panels, respectively) are shown for several rotational transitions in the millimetre domain as a function of CO column density. Two gas temperatures are considered, 10 K (blue points) and 150 K (red points).