Radiative transfer predictions of the CO intensity line ratios as a function of the temperature and density. The blue, red, and green areas show the observed CO(2-1)/CO(1-0), CO(3-2)/CO(1-0), and CO(4-3)/CO(1-0) ratio. The darkest area corresponds to the models which fit the observations. From left to right, the columns correspond to the blueshifted velocity component, the redshifted velocity component and all the emission, respectively.