Contour plots of HCO⁺/H₂ (blue line) and HCS⁺/H₂ (red line) abundance ratios as a function of cosmic-ray ionisation rate and S/H gaseous elemental abundance. The blue and red zones mark the observational abundances. N(HCO⁺) = N(HC¹⁸O⁺) × ¹⁶O/¹⁸, where ¹⁶O/¹⁸O = 557. The models were obtained for a gas temperature of 12 K, an evolutionary time of 10 Myr, and an H₂ density of 2 × 10⁴ cm⁻³ (869), 6× 10⁴ cm⁻³ (1390), and 2.6 × 10⁴ cm⁻³ (4149).