Direct Evidence for Two-Fluid Effects in Molecular Clouds

TL;DR

This paper combines theoretical analysis and simulations to demonstrate how two-fluid ambipolar drift affects molecular line widths and turbulence in molecular clouds, providing observable signatures of ion-neutral interactions.

Contribution

It offers the first combined theoretical and simulation-based evidence of two-fluid effects influencing turbulence and line widths in molecular clouds.

Findings

Ion-neutral line width differences are significant and observable.

Wave damping occurs at scales comparable to the ambipolar diffusion scale.

Synthetic line width-size relations show detectable two-fluid effects.

Abstract

We present a combination of theoretical and simulation-based examinations of the role of two-fluid ambipolar drift on molecular line widths. The dissipation provided by ion-neutral interactions can produce a significant difference between the widths of neutral molecules and the widths of ionic species, comparable to the sound speed. We demonstrate that Alfven waves and certain families of magnetosonic waves become strongly damped on scales comparable to the ambipolar diffusion scale. Using the RIEMANN code, we simulate two-fluid turbulence with ionization fractions ranging from 10^{-2} to 10^{-6}. We show that the wave damping causes the power spectrum of the ion velocity to drop below that of the neutral velocity when measured on a relative basis. Following a set of motivational observations by Li & Houde (2008), we produce synthetic line width-size relations that shows a difference between the ion and neutral line widths, illustrating that two-fluid effects can have an observationally detectable role in modifying the MHD turbulence in the clouds.