Structure and Stability of Filamentary Clouds Supported by Lateral Magnetic Field

TL;DR

This paper develops analytical models for isothermal filamentary clouds supported by magnetic tension, analyzing their equilibrium, stability, and evolution, and finds critical mass-to-flux ratios and instability conditions relevant to molecular cloud fragmentation.

Contribution

It introduces two new analytical models for filamentary clouds supported by magnetic tension, extending previous work and analyzing their stability and evolution.

Findings

Critical mass-to-flux ratio is (2π√G)^(-1).

Models reproduce features of previous 2D equilibrium models.

Clouds are unstable against fragmentation with long-wavelength modes.

Abstract

We have constructed two types of analytical models for an isothermal filamentary cloud supported mainly by magnetic tension. The first one describes an isolated cloud while the second considers filamentary clouds spaced periodically. Both the models assume that the filamentary clouds are highly flattened. The former is proved to be the asymptotic limit of the latter in which each filamentary cloud is much thinner than the distance to the neighboring filaments. We show that these models reproduce main features of the 2D equilibrium model of Tomisaka (2014) for filamentary cloud threaded by perpendicular magnetic field. It is also shown that the critical mass to flux ratio is $ M /\Phi = (2 \pi \sqrt{G}) ^{-1} $, where $ M $, $ \Phi $ and $ G $ denote the cloud mass, the total magnetic flux of the cloud, and the gravitational constant, respectively. This upper bound coincides with that for an axisymmetric cloud supported by poloidal magnetic fields. We applied the variational principle for studying the Jeans instability of the first model. Our model cloud is unstable against fragmentation as well as the filamentary clouds threaded by longitudinal magnetic field. The fastest growing mode has a wavelength several times longer than the cloud diameter. The second model describes quasi-static evolution of filamentary molecular cloud by ambipolar diffusion.