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

TL;DR

This study uses MHD simulations to investigate the formation of interstellar filaments, revealing their alignment with turbulence strain, magnetic confinement, and dissipation processes that set their typical size.

Contribution

It demonstrates that turbulence-induced strain and magnetic forces are key to filament formation and survival, and links filament size to energy dissipation scales in the ISM.

Findings

Filaments are more prevalent in magnetized turbulence.

Filaments align with turbulent strain and are confined by magnetic forces.

Typical filament thickness is a few grid cells, related to dissipation length.

Abstract

{Filaments are ubiquitous in the interstellar medium as recently emphasized by Herschel, yet their physical origin remains elusive} {It is therefore important to understand the physics of molecular clouds to investigate how filaments form and what is the role played by various processes such as turbulence and magnetic field.} {We use ideal MHD simulations to study the formation of clumps in various conditions including different magnetization and Mach numbers as well as two completely different setup. We then perform several analysis to compute the shape of the clumps and their link to velocities and forces using various approaches.} {We find that on average, clumps in MHD simulations are more filamentary that 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 strech induced by turbulence. Moreover filaments tend to be confined by the Lorentz force which therefore lead them to survive longer in magnetized flows. We show that they have in all simulations 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 of the order of 0.1 pc and therefore could possibly set the typical size of non self-gravitating filaments.} {Filaments are ubiquitous because they are the results of the very generic turbulent strain and because magnetic field help to keep them coherent. We suggest that energy dissipation is playing a determinant role in their formation.}