Beam smearing effect for C₂H. Panel a: azimuthally averaged integrated intensity profile of the C₂H line. Emission peaks are marked with dashed red lines and the emission gap is highlighted with a dashed grey line. Panel b: rotation curve of C₂H (purple line) compared to that of ¹²CO (green line). The rotation velocity of C₂H is super-Keplerian just before the gap and sub-Keplerian after. It shows the opposite behavior in correspondence of the emission peaks. Panel c: anticorrelation between the C₂H integrated intensity gradient and its difference in rotation velocity compared to ¹²CO. The two quantities show an opposite modulation, and they cross at the radii corresponding to the emission gap or peaks.