The effect of ambipolar diffusion on low-density molecular ISM filaments

Abstract

Context. The filamentary structure of the molecular interstellar medium and the potential link of this morphology to star formation have been brought into focus recently by high resolution observational surveys. An especially puzzling matter is that local interstellar filaments appear to have the same thickness, independent of their column density. This requires a theoretical understanding of their formation process and the physics that governs their evolution. Aims. In this work we explore a scenario in which filaments are dissipative structures of the large-scale interstellar turbulence cascade and ion-neutral friction (also called ambipolar diffusion) is affecting their sizes by preventing small-scale compressions. Methods. We employ high-resolution (512 3 and 1024 3 ), 3D magnetohydrodynamic (MHD) simulations, performed with the grid code RAMSES, to investigate non-ideal MHD turbulence as a filament formation mechanism. We focus the analysis on the mass and thickness distributions of the resulting filamentary structures. Results. Simulations of both driven and decaying MHD turbulence show that the morphologies of the density and the magnetic field are different when ambipolar diffusion is included in the models. In particular, the densest structures are broader and more massive as an effect of ion-neutral friction and the power spectra of both the velocity and the density steepen at a smaller wavenumber. Conclusions. The comparison between ideal and non-ideal MHD simulations shows that ambipolar diffusion causes a shift of the filament thickness distribution towards higher values. However, none of the distributions exhibit the pronounced peak found in the observed local filaments. Limitations in dynamical range and the absence of self-gravity in these numerical experiments do not allow us to conclude at this time whether this is due to the different filament selection or due to the physics inherent of the filament formation.

The Role of Magnetic Field in Molecular Cloud Formation and Evolution

“…They conclude that contrary to the simple analytical estimate, the simulations do not reveal a clear sign of a specific smoothing or dissipative scale. Other simulations like the ones performed by Li et al (2008), Downes & O'Sullivan (2011) and Ntormousi et al (2016) found that ion-neutral friction affects the turbulent fluctuations at a scale below the ambipolar diffusion one leading to a smoother structure. Left-panel of Fig.…”

“…However, so far this characteristic width has not been observed in numerical simulations. Ntormousi et al (2016) have performed a detailed analysis of the filament width distribution (in simulations that do not include self-gravity) and found that while ion-neutral friction affects the density structure and reduces the numbers of small scale filaments, it does not produce a characteristic width near 0.1 pc as can be seen in Fig. 5.…”

Probing interstellar turbulence in cirrus with deep optical imaging: no sign of energy dissipation at 0.01 pc scale

“…Because density and velocity fluctuations are related, it is expected that the power spectrum of the density field (and therefore of the column density field) will show a similar loss of power at small scales. This effect is clearly seen by Ntormousi et al (2016) in their analysis of numerical simulations dedicated to the study of the imprint of ambipolar diffusion on the statistical properties of the diffuse ISM.…”

A census of dense cores in the Aquila cloud complex: SPIRE/PACS observations from theHerschelGould Belt survey

“…Smith et al (2014) explored filament properties in a set of numerical hydrodynamic simulations and found a range of filament widths rather than a constant value. Recent magneto-hydrodynamic (MHD) simulations by Ntormousi et al (2015), however, suggest that non-ideal MHD turbulence can account for the properties of observed filaments much better than hydrodynamic turbulence does (see also Hennebelle 2013).…”