Results from a two-layer, multi-component fit to the observed CO(2–1) isotopes at the northern Musca position. Upper two panels and the lower left panel: fit with two velocity components derived from the optically thin C¹⁸O(2–1) emission line, lower right panel: best model fit for the observed ¹²CO(2–1) line. In each panel, upper left sub-panel: observed data as a red line and the model fit as a green dashed line. Upper rightsub-panel: individual components, lower left sub-panel: residuals and lower right sub-panel: information on the number and contribution of the foreground. It becomes obvious that two components can be fitted for C¹⁸O(2–1) and ¹³CO(2–1), but are insufficient for the ¹²CO(2–1) line.