Spectra (in T_MB) of ¹²CO (3–2) (grey, left ordinate) and ¹³CO (3–2) (green, left ordinate) together with the line temperature ratio of ¹²CO (3–2)/¹³CO (3–2) (dots, right ordinate). The ratio is shown for ¹³CO (3–2) line temperatures higher than 2σ (orange dots) and 3σ (red dots). Grey dotted lines indicate values of the temperature ratio as displayed on the left-hand side of these lines. The positions displayed are W44E1 and W44E2, and W44F1, W44F2, and W44F3 (top to bottom). The corresponding values of the optical thickness in the line wings are in the range of 3–7.