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 (T_mb = 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.