CO and HCO⁺ column densities. Shown are results from this work (larger green filled squares) and the earlier observations of Liszt & Lucas (1998) (small orange circles). Integrated CO J = 1− 0 optical depths ϒ_CO have been converted to CO column density using the relationship between ϒ_CO and N(CO) shown in Fig. 3, N(CO) = 1.861 × 10¹⁵ cm⁻² ϒ_CO^1.131. Grey dashed shaded lines show fiducial values N(CO)/N(H₂) = 10⁻⁶ and 10⁻⁵ assuming N(HCO⁺)/N(H₂) = 3 × 10⁻⁹. The broad grey line at N(CO)/N(H₂) = 5 × 10⁻⁶ divides the plot area into regions where X_CO is larger or smaller than 2 × 10²⁰H₂ (K km s⁻¹)⁻¹. The red dash-dotted line at N(CO) = 10¹⁵cm⁻² shows the CO column density at which the integrated J = 1−0 emission brightness W_CO = 1 K km s⁻¹.