Numerical simulations. Each box shows a cut of the density map (φ = 0) before or after the ionisation of a slab of interstellar gas. a) A clump with a density of 3  ×  10⁵ cm^-3 is embedded in the slab of interstellar gas. Radiation from the O9.5 star comes from the position (0, 0). b) After 0.6 Myr, the ionisation has led to the formation of a pillar. The base of the pillar is located within the compressed shell, in between the ionisation and shock fronts. The head of the pillar remains nearly at the position of the initial clump. c) A larger clump with a lower density is ionised by the O9.5 photon flux. d) A clump is formed within the compressed shell with a hole around it.