Upper panel: time dependence of C/¹³C isotopic ratios in C, CH, CO, and HCO⁺. The black heavy dotted line represents the elemental ¹²C/¹³C. Lower panel: time dependence of the fractional abundances relative to H₂ of C, ¹²C⁺, CO, CH, and HCO⁺, for models (a) and (b). The observational values towards TMC1 (Ohishi et al. 1992) are displayed as horizontal dashed lines with corresponding colors in the left panel. a) and b) correspond to the models defined in Table 4.