Observed (histograms) and modelled (red lines) spectra for a) N₂H⁺ (J = 1 − 0) (top panel), and N₂H⁺ (J = 3 − 2) (bottom panel). b) N₂D⁺ (J = 1 − 0) (top panel), and N₂D⁺ (J = 3 − 2) (bottom panel). c) N¹⁵NH⁺ (J = 1 − 0) (top panel), and ¹⁵NNH⁺ (J = 1 − 0) (bottom panel). The shaded areas correspond to the variations expected from the error bars set on the abundance profiles and represented in Fig. 5.