3D simulations of pillar formation around HII regions: the importance of shock curvature

Aims. Radiative feedback from massive stars is a key process for understanding how HII regions enhance or inhibit star formation in pillars and globules at the interface with molecular clouds. We aim to contribute by modelling the interactions between ionization and gas clouds to better understand the processes at work. We study in detail the impact of modulations on the cloud-HII region interface and density modulations inside the cloud. Methods. We ran 3D hydrodynamical simulations based on Euler equations coupled with gravity using the HERACLES code. We implemented a method to solve ionization/recombination equations and took into account typical heating and cooling processes at work in the interstellar medium that are caused by ionization/recombination physics. Results. UV radiation creates a dense shell compressed between an ionization front and a shock ahead. Interface modulations produce a curved shock that collapses on itself, which leads to stable growing pillar-like structures. The narrower the initial interface modulation, the longer the resulting pillar. We interpret pillars resulting from density modulations in terms of the ability of these density modulations to curve the shock ahead of the ionization front. Conclusions. The shock curvature is a key process for understanding the formation of structures at the edge of HII regions. Interface and density modulations at the edge of the cloud have a direct impact on the morphology of the dense shell during its formation. Deeper inside the cloud, structures have less influence because of the high densities reached by the shell during its expansion.