Top panels: abundance profiles for the four analysed species (from left to right: N₂H⁺, N₂D⁺, HC¹⁸O⁺, DCO⁺) at t = 10⁶ yr. The colours refer to different models used for the CR ionisation rate: model (blue), model (red), and model (green). Bottom panels: the green and red solid curves show the ratio of model and model to model , respectively. The dashed, horizontal lines represent the inverse ratios of the corrective factors f_corr used to reproduce the observed lines, as reported in Sect. 4.1 and Table 2. To give an example, the radiative transfer of DCO⁺ lines requires f_corr = 0.9 in model , and f_corr = 1.5 in model ; therefore thered solid curve / in the rightmost panel has to be compared with the red dashed line at .