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Fig. 8


Predicted brightness temperature ratio of 12CO(4–3)/12CO(2–1) by RADEX as a function of temperature for different densities. This demonstrates the need for a warm and dense CO layer to at least reach the observed brightness ratios in the blueshifted component. For each density we plot the ratio for a column density such that the predicted brightness of 12CO(4–3) by RADEX is roughly similar to the observed brightness towards Musca (Tmb = 1.5–3 K) at the temperatures that obtain sufficiently high brightness ratios. The lowest three densities ( = 103, 3 × 103 and 5 × 103 cm−3) use = 1.1 × 1015 cm−2, and = 7 × 103 cm−3 uses = 9 × 1014 cm−2. The black horizontal line indicates the average observed ratio towards Musca, and the grey area indicates the standard deviation.

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