H¹²CN/H¹³CN ratio as a function of n_H, for different temperatures, at t₁ (top right panel), t₂ (bottom left panel), and t₃ (bottom right panel). The large pink and brown circles in the bottom panels represent the result of the model that simulates the physical conditions in the centre of the pre-stellar cores B1b observed by Daniel et al. (2013) and L1498 observed by Magalhães et al. (2018), respectively. The H¹²CN/H¹³CN ratios observed by Daniel et al. (2013) and Magalhães et al. (2018) are represented by the dark green and orange stars, respectively. We note that the error bars are within the symbols. In all panels, the two vertical purple dashed lines represent the range of densities of the sample of high-mass star-forming region described in Sect. 3.2.2, and the black horizontal dashed lines represent the initial ¹²C/¹³C ratio of 68.