Abundance ratios of HCN over HC¹⁵N at 1 Myr in the fiducial disk model for three variations of the chemical network: with low-temperature isotope exchange reactions disabled (left), with self-shielding of N₂ disabled (center), or with both fractionation mechanisms disabled (right). The ratios obtained with both mechanisms active are shown in the top center panel of Fig. 6. Contours are drawn at ratios of 100, 200, 300, 400 and 500.