Steady state C₂H abundance shown in colour scale as a function of gas density and cosmic-ray ionization rate in units of ζ₀ = 1.3 × 10⁻¹⁷ s⁻¹. The values given are a median over different gas temperatures. Each point represents a GMC with the vertical lines showing the upper and lower limits on the density for that cloud. The horizontal lines span the cosmic-ray ionization rate values that give abundances within the measured range for those density values. GMC 7 does not appear on this diagram as no model produces a large enough abundance of C₂H to match even its observed lower-limit.