Comparison between the HC¹⁷O⁺ and C¹⁷O radial abundance profiles. The dotted curve shows the C¹⁷O fractional abundance (abundance with respect to H₂) as a function of radius. The solid curve shows the fractional abundance of HC¹⁷O⁺, scaled by 10⁴ computed with the chemical code from Sipilä et al. (2015, 2019) by scaling the CO and HCO⁺ fractional abundance profiles by the isotope ratio [¹⁶O/¹⁷O] = 2044 (Penzias 1981; Wilson & Rood 1994).