Column density of carbon monoxide vs. that of H₂. Data for CO were taken from Sonnentrucker et al. (2007), Burgh et al. (2007), Sheffer et al. (2007), Sheffer et al. (2008) and Burgh et al. (2010) and sources with measured N(C I) are labelled as in Fig. 1. Sources labelled “radio” are taken from Liszt & Lucas (1998) assuming X(HCO ⁺ ) = 3 × 10^-9; sources with measured N(C I) are called out as in Fig. 1. Steady-state, self-consistent model results (solid curves) are shown for n(H) = 256   cm^-3, G₀ = 1 and 1.7 under the assumption that HCO ⁺  recombines with electrons in thermal equilibrium at the local kinetic temperature for this value of X(HCO ⁺ ), see Sect. 3 of the text. The dashed (orange) curve is for G₀ = 1 and n(H) = 32   cm^-3, illustrating the extreme sensitivity of X(CO) to the radiation field.