Illustration of the method of detecting gaps. The blue line indicates a radial cut through the HCO⁺ (4−3) velocity-channel map (Fig. 14) at the central frequency (v = 0.0 m/s). For the simulated ALMA observations, the gap is considered as being detected if the depth of the gap is deeper than three times the noise level σ of the selected transition. In this particular case, the gap is clearly detectable, resulting in a gap contrast of ~7σ.