Left panel: observational residuals (observed spectrum minus best-fit model with the ¹³CO abundance set to zero) in black and ¹³CO model spectrum given the best-fit ¹²CO/¹³CO ratio (~97) in orange. Right panel: cross-correlation function (CCF) between the ¹³CO model and residuals. The CCF is normalised by its standard deviation within the velocity ranges [−2200, −800] and [800, 2200] km s⁻¹, so that the y-axis represents the signal-to-noise of the CCF peak.