Spectral line profiles of CO, 13CO, C18O, and C17O. The 1–0 and 2–1 spectra are from the IRAM 30 m telescope, the 3–2 lines are from JCMT/HARP-B. All higher-J lines are from Herschel/HIFI. For clarity, consecutive spectra, each labeled by their upper and lower J-level, are offset in the vertical direction. Corresponding rest frequencies and energy levels are listed in Table 2. The systemic velocity ( − 5.8 km s-1) of AFGL 2591 is indicated by the dashed vertical line.
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