Maps of the chemical properties for ¹²CO, ¹³CO, C¹⁸O, and HCO⁺, from top to bottom. The three first columns show the column densities for the inner, outer (foreground or background), and total (inner plus twice outer) column densities, while the last column shows the mean abundance, namely, the total column density divided by the H₂ column density deduced from the visual extinction. The last column is the ratio of the inner by the outer column density. Contours are drawn at visual extinctions of 3, 7, and 16 mag.