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Fig. 9

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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).

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