Channel-by-channel optical depth of the CO (3–2) line (right-hand y-axis), obtained under two assumption for the [¹²CO]/[¹³CO] relative abundance: 50 and 60 (empty and full red circles), for all channels with at least a 3σ detection in ¹³CO (3–2). Also shown the ¹²CO and ¹³CO (3–2) spectra (in black and pink respectively, left-hand y-axis), obtained in knot D with the APEX telescope.