Logarithm of R_{H, CO,i,j} ratio with the corresponding R_ref = R_{H, CO} calculated with the single binding energy approach, for the (r, t) combinations indicated with circular markers in Figs. 2 and 3, namely cases A, B, and C (left to right panels), for t = 10² yr and t = 2.7 × 10⁴ yr (upper and lower panels). In the box, we report the dust temperature T_d(r, t), t, r, and R_ref in units of cm⁻³ s⁻¹. The cross indicates the position of the single-case binding energy. For the sake of clarity, the colour palette lower limit is set to − 5. Compare with the sketch in Fig. 5.