On the origin of non-self-gravitating filaments in the ISM

Context. Filaments are ubiquitous in the interstellar medium, as recently emphasized by Herschel, yet their physical origin remains elusive Aims. It is therefore important to understand the physics of molecular clouds to investigate how filaments form and which role play various processes, such as turbulence and magnetic field. Methods. We used ideal magnetohydrodynamic (MHD) simulations to study the formation of clumps in various conditions, including different magnetization and Mach numbers as well as two completely different setups. We then performed several analyses to compute the shape of the clumps and their link to velocities and forces using various approaches. Results. We found that on average, clumps in MHD simulations are more filamentary than clumps in hydrodynamical simulations. Detailed analyses reveal that the filaments are in general preferentially aligned with the strain, which means that these structures simply result from the stretch induced by turbulence. Moreover, filaments tend to be confined by the Lorentz force, which therefore leads them to survive longer in magnetized flows. We show that in all simulations they have a typical thickness equal to a few grid cells, suggesting that they are primarily associated to the energy dissipation within the flow. We estimate the order of magnitude of the dissipation length associated to the ion-neutral friction and conclude that in well UV shielded regions it is about 0.1 pc and therefore could possibly set the typical size of non-self-gravitating filaments. Conclusions. Filaments are ubiquitous because they are the results of the very generic turbulent strain and because the magnetic field helps to keep them coherent. We suggest that energy dissipation is playing a determining role in their formation.