Disc model CO v = 1 population diagrams: LTE and NLTE with and without UV pumping in the optically thin case (solid lines) and optically thick case (symbols). All the data have been normalized to the J = 0 population in LTE. The blue diamonds correspond to the column densities after line transfer in models with UV pumping whereas the red triangles correspond to column densities in line-transfer model results without UV pumping. The LTE populations are fitted by straight lines (black plus signs). The separations between P- and R-branches are clearly seen in the NLTE optically thick cases.