The origin of a universal filament width in molecular clouds

TL;DR

This study explains the near-universal filament width in molecular clouds as a consequence of converging flows and accretion shocks, with simulations matching observed widths around 0.1 pc.

Contribution

It demonstrates that filament widths arise naturally from hydrodynamic converging flows and accretion shocks, providing a physical explanation for the observed universal filament width.

Findings

Filament widths depend on inflow Mach number, with higher Mach numbers producing narrower filaments.

Simulated filament width distribution matches observed peaked distribution around 0.1 pc.

Flow Mach numbers greater than 3 lead to widths narrower than observed.

Abstract

Filamentary structures identified in far-infrared observations of molecular clouds are typically found to have full-widths at half-maximum $\sim\!0.1$ pc. However, the physical explanation for this phenomenon is currently uncertain. We use hydrodynamic simulations of cylindrically-symmetric converging flows to show that the full-width at half-maximum of the resulting filament's surface density profile, FWHM$_{\Sigma}$, is closely related to the location of the accretion shock, where the inflow meets the boundary of the filament. For inflow Mach Number, ${\cal M}$, between 1 and 5, filament FWHM$_{\Sigma}$s fall in the range $0.03$ pc $\lesssim$ FWHM$_{\Sigma} \lesssim 0.3$ pc, with higher ${\cal M}$ resulting in narrower filaments. A large sample of filaments, seen at different evolutionary stages and with different values of ${\cal M}$, naturally results in a peaked distribution of FWHM$_{\Sigma}$s similar in shape to that obtained from far-infrared observations of molecular clouds. However, unless the converging flows are limited to ${\cal M} \lesssim 3$, the peak of the distribution of FWHM$_{\Sigma}$s is below the observed $\sim 0.1$ pc.