Azimuthally averaged radial profile of the ²⁸SiO(ν= 2, J = 8−7) line emission (green curve) and best fit model (red curve). It is computed from the sum along the frequency axis of the continuum-subtracted data cube, scaled to the flux level measured at the sky frequency of the line. The shaded green area is the standard deviation of the shell over the considered ring radius. The equivalent uniform disk radius of the star is shown with a solid orange line, and the radius of the shell is represented with a dashed blue line.