JCMT data. Comparison of spectra at the location of the YSO (coinciding with the centre of the outflow and the sub-mm continuum peak). Panel a: ¹²CO(3–2) spectrum (broad emission line; taken from the mapping data). The weaker, narrow blue line is the average of 45 spectra in the ¹²CO(3–2)-map, with α > 25′′, well away from the outflow and hence representing the quiescent gas. The dashed line indicates its central velocity (2.75 km s⁻¹), which corresponds with that of the dip in the line profile at the centre of the outflow, while the width of the dip is virtually identical to the fwhm of the quiescent gas profile. Panel b: deep J=3–2 spectra of three CO-isotopologues. Only C¹⁸O (red) does not show a self-absorption dip. The velocity of its peak nearly coincides with the central velocity of the quiescent gas in panel a.