CO excitation conditions from RADEX models versus observations. The plots present models at a different N(CO) of 10¹⁷ and 10¹⁸ cm⁻². The models are presented with empty circles, and their colours correspond to a different hydrogen volume density, , between 10³ and 10⁷ cm⁻³. At each volume density level, five temperatures – 150, 250, 500, 1000, and 3000 K – were sampled. Observations assuming a beam filling factor of 1 are shown in crosses, while observations assuming a tiny source of 2″, which correspond to an extreme case of a small beam filling factor, are shown with triangles.