Investigation of effect of CO envelope contamination showing time evolution of [CO] (left), [HCO⁺] (middle) and [N₂H⁺] (right) for the overall best fitting model (black line, ζ = 2.2 × 10⁻¹⁸ s⁻¹). Here the observed CO abundance has been reduced by a factor of 10 and the total uncertainty set to be a factor of 3. The effects of varying the CRIR by a factor of 10 to higher (magenta) and lower (blue) values compared to the XR best model value are shown. The best timescales for each of the models are marked with corresponding ‘+’ symbols.