Isotopic ratios predicted by GCE models. Left panel: comparison between the (¹⁴N/¹⁵N)×(¹³C/¹²C) ratios derived from observations and predicted by GCE models. The red line and light orange regions are the same as in Fig. 1. Right panel: same as the left panel, but for the ¹⁴N/¹⁵N ratio. In both panels grey points correspond to the observed star-forming regions. The dark green, light green, magenta, and pink lines are the predictions from GCE models (see Sect. 4 and Table 2). Note that in the left panel the dark green model is below the light green one.