Fractional abundance of C₂H as a function of time in the UCLCHEM models. Each line shows the median value of the abundance for a given cosmic-ray ionization rate as temperature is varied and the shaded regions show the minimum and maximum abundances over that temperature variation. The cosmic-ray ionization rate is given as a multiple of ζ₀ = 1.3 × 10⁻¹⁷ s⁻¹ and a sample of the lines are given in the legend. The grey shaded area shows the observed range.