(a) HCO+ column density vs. CO column density in a log–log scale. (b) ﹩N({\mathrm{HCO}}^{+})/N(\mathrm{CO})﹩ ratio vs. CO column density in a log–log scale. The red dots are from our work and black dots are from Liszt & Lucas (1998, CO column density in Table 5) and Lucas & Liszt (1996, HCO+ column density in Table 2). The black dotted lines show the fit slope to the data points from all components (upper limits are not included).