Illustration of the proposed scenario. Observations: a) Schematic view of the initial ridge as derived in Sect. 3.2. b) Modelled ring superimposed on the Herschel 3-colour image. Numerical simulations: c) Cut through the plane of the initial ridge. The radius of the yellow circular section is 1 pc. The density is given in H i cm^-3. d) Cut through the plane of the ridge after ionisation reshaped it and formed a ring. e) Cut of d) from above. The bipolar shape clearly appears. f) Cut of d) from the backside. The ridge/filament projected shape clearly appears. The orientation of each cut is given by x, y, z axes on each image.