Gas-phase CO abundance as a function of ξ (external radiation field in units of the standard interstellar radiation field), ζ (cosmic-ray flux expressed in terms of 1.3 × 10⁻¹⁷ s⁻¹ ), and A_v (visual extinction) at 10⁵ years (dotted lines), 5 × 10⁵ years (dashed lines) and 10⁶ years (solid lines).