Fractional abundances, relative to H₂, of N₂ and N (top panel) and C⁺, C, and CO (bottom panel) as a function of radius. Solid lines: from the SS model (Li et al. 2014), full shielding (dust + self- + H₂ + H) are considered using an updated N₂ photodissociation rate and self-shielding function (Li et al. 2013). Dashed lines: calculated from a model similar to that of McElroy et al. (2013), but adopting assumptions specifically for IK Tau. In this model only dust shielding is considered when calculating N₂ photodissociation, and CO self-shielding is treated only approximately.