CO vibrational diagram with and without UV pumping for the four models. We used the v = 1−6 population after line radiative transfer. The blue diamonds correspond to models with UV pumping whereas the red triangles correspond to models without UV pumping. The purple stars are models without UV pumping and 5 vibrational levels. Straight lines have been fitted to each dataset. The derived vibrational temperatures are indicated in each panel. The blue lines are for models with UV pumping whereas the red dashed-lines are for models without UV pumping. In the upper left panel, massive disc including UV pumping show the same vibrational temperature (T_vib = 1413 K) with either 5 or 9 electronic ground-state vibrational levels included in the model. On the other hand, the vibrational temperatures of a low-mass disc depend on the number of assumed vibrational levels in the ground electronic state with higher vibrational temperatures as more levels are included (upper-right panel). The UV pumping is the most efficient for the lowest disc mass model with the largest inner hole (model 4a in the lowest-right panel with M_disc = 10^-4M_⊙, R_disc = 20 AU).