Deprojected and azimuthally-averaged radial profiles of the HCO⁺ and H¹³CO⁺ line emissionsmoothed to a common beam and the subsequent HCO⁺/H¹³CO⁺ ratio (from left to right). The coloured shaded region on each profile represents the errors (see text for details). The vertical dashed grey lines highlight the rings and gaps in the mm-dust continuum profile and the dashed grey profile is the best-fit model of the continuum uv data from Walsh et al. (2016) and is normalised to the peak value in each plot.