Abundances ϵ(X) relative to hydrogen for CO, HCO⁺ and N₂H⁺ for the reference model CI_XN. The white solid contour line shows N_{⟨ H ⟩ ,rad} = 10²⁵ cm^-2, the white dashed line shows the CO ice line. We call the regions above and below the CO ice line the warm and cold molecular layer, respectively. The dotted iso-contours show where the X-ray ionization rate is equal to the ISM CR (ζ_CR = 2 × 10^-17 s^-1) and equal to the low CR (ζ_CR = 2 × 10^-19 s^-1) ionization rate, respectively.