Radiative transfer results (in red) overlaid to the observations (in black) for all the targeted transition obtained with the chemical model with constant ζ₂ = 3 × 10⁻¹⁷ s⁻¹, using the same evolutionary time steps and f_corr factors that provide the best fit for the fiducial model . The top row shown the N₂D⁺ line, the middle row shows the DCO⁺ lines, and the bottom row presents the two N₂H⁺ transitions and the HC¹⁸O⁺ (1-0) line, from left to right. The used time step and f_corr value are shown in the title of each panel.