[HCO⁺]/[N₂H⁺] abundance ratio as a function of the CR ionization rate computed for the inner (blue line) and outer (red line) envelope components using the Nahoon code. The shaded areas represent the results obtained within the uncertainties listed in Table 3. The ζ^H₂ values in the upper-left and lower-right corners of the shaded areas correspond to the maximum and minimum values of the [HCO⁺]/[N₂H⁺] ratio, respectively (see text for discussion).