Author(s): , ,
Institution(s): 1. CEA/ Saclay
The filamentary structure of molecular clouds and its potential link to star formation have been brought into focus by Herschel's high resolution observations of the local Interstellar Medium. An especially puzzling result from the same surveys is that local interstellar filaments have a preferred thickness of 0.1 pc, independent of their column density. What can be the origin of this apparently universal property?
Filamentary structure is characteristic of MHD turbulence, appearing as a result of shear, magnetic tension and shocks. If the observed filaments are indeed the dissipative structures of interstellar turbulence, then ambipolar diffusion is the best candidate for setting a characteristic thickness by damping MHD waves. We test this hyporthesis with high-resolution, 3D MHD simulations performed with the AMR code RAMSES. To avoid confusion with grid effects, our simulations reach a physical resolution of 200 AU, resolving the observed 0.1 pc with about 100 cells.
These simulations of both driven and decaying MHD turbulence show that the fluid assumes a different morphology when ambipolar diffusion is included in the models: ion-neutral friction acts on a characteristic scale to cut off the cascade, broadening the dense structures and flattening their mass spectra with respect to the corresponding ideal MHD situation. Altough the peak in the thickness distribution of filaments is not as dramatic in this series of simulations as in the observations, the comparison between ideal and non-ideal MHD points to ion-neutral friction as a very good candidate for setting a characteristic scale for interstellar filaments.