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Fig. 6


Observed 12CO(4–3)/12CO(2–1) and 12CO(4–3)/12CO(3–2) ratios compared with predictions by the PDR Toolbox. The shaded areas, constructed by logarithmic interpolation between the points in the PDR Toolbox density grid, show the evolution of the 12CO(4–3)/12CO(2–1) and 12CO(4–3)/12CO(3–2) brightness temperature ratio as a function of the FUV field in the allowed density range of the Musca cloud. At a typical density = 4 × 102 cm−3 of the ambient cloud (dashed line) for the allowed FUV field strength (<1 G0), the predicted line ratios are only a fraction of the observed ones in the blueshifted component.

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