Relative abundances (absolute molecular density with respect to the hydrogen density) of gaseous C¹⁸O (black solid line), frozen-out C¹⁸O (dashed black line) and gaseous N₂H⁺ (gray solid line) in the final models. At the outer radii of the emission profiles derived from the observations, the lines turn from a solid into a dotted line shape, indicating that these parts of the profiles are not constrained by observations.