Upper panel: time dependence of N/¹⁵N isotopic ratios in atomic and molecular nitrogen and N₂H⁺ ions. a) and b) correspond to the models defined in Table 4; the black heavy dotted line represents the elemental N/¹⁵N. Lower panel: time dependence of the fractional abundances relative to H₂ of N, N₂, and N₂H⁺ for models (a) and (b). The value reported for N₂H⁺ towards TMC1 (Ohishi et al. 1992) is displayed as a horizontal green dashed line.