Contribution of the shell to the column density profile of the central UCH ii region. Its density radial extension is given by the two dashed black lines. The column density resulting from the line-of-sight accumulation is illustrated by the black solid line. The shell column density is then convolved with the Herschel beams (coloured dotted lines) to simulate the flux or column density of the shell (coloured solid lines) observable from 6″ (70 μm) to 36″ (500 μm) resolutions.