C₂O/C₃O ratios as a function of time (in years) for ζ = 1 × 10⁻¹⁶ s⁻¹ (top panels, A, B, and C) and for ζ = 1 × 10⁻¹⁷ s⁻¹ (bottom panels, D, E, and F). Panels A and D: 10 K; panels B and E: 20 K; panels C and F: 30 K. The modelwas run for different C/O initial ratios and hydrogen number density. The green boxes represent the lower range of observed ratios (including the errors, see Table 1). Black line: C/O = 0.4, n_H = 2 × 10⁴ cm⁻³; red line: C/O = 0.8, n_H = 2 × 10⁴ cm⁻³; green line: C/O = 0.65, n_H = 2 × 10⁴ cm⁻³; blue line: C/O = 0.65, n_H = 2 × 10⁵ cm⁻³; cyan line: C/O = 0.8, n_H = 2 × 10⁵ cm⁻³; magenta line: C/O = 0.9, n_H = 2 × 10⁵ cm⁻³.