Abundance ratios of (¹⁴N/¹⁵N) × (¹³C/¹²C) (left) and ¹⁴N/¹⁵N with D_GC (right). The filled black circles are sources with peak T_mb of H¹³CN 2–1 lower than 4 K, and the orange stars sources with peak T_mb higher than 4 K. The blue lines are linear fitting results. The cyan parabola was obtained by multiplying the linear regression fit of (¹⁴N/¹⁵N) × (¹³C/¹²C) given in Eq. (3) by the ¹²C/¹³C ratio (Eq. (2)). The green, dark green, pink, and magenta lines are the models from Colzi et al. (2022).