Comparison between the mean flux of the CO and SiO emission in a circular aperture of 1″, centre on the continuum peak. Black dashed lines have been added to the plot to indicate the zero flux level and the υ_*. The SiO is intrinsically much brighter, and as a consequence it exhibits much broader high-velocity wings than CO. This is illustrated by the red dashed lines, which mark the velocities at which the CO line subsides below the noise.