Probing the Turbulence Dissipation Range and Magnetic Field Strengths in Molecular Clouds. II. Directly Probing the Ion-neutral Decoupling Scale

TL;DR

This study provides observational evidence of ion-neutral decoupling in molecular clouds by analyzing turbulence spectra, revealing scale-dependent differences in ion and neutral turbulence that relate to magnetic field influence.

Contribution

It presents direct observational evidence of ion-neutral decoupling at specific scales in molecular clouds using velocity spectrum analysis, extending previous theoretical and observational work.

Findings

Neutral turbulence follows Kolmogorov spectrum (k^-5/3).

Ion turbulence spectrum is steeper (k^-2) below 0.404 pc scale.

Spectrum differences are not due to optical depth or hyperfine effects.

Abstract

The linewidth of ions has been observed to be systematically narrower than that of the coexisting neutrals in molecular clouds (Houde et al. 2000) and been interpreted as the signature of the decoupling of the neutral turbulence from magnetic fields in partially ionized medium (Li & Houde 2008; Paper I, hereafter). As a sequel of Paper I, here we present further observational evidence that lend support to these earlier proposals with the velocity coordinate spectrum analysis (Lazarian & Pogosyan 2006). We recover the turbulent energy spectra of HCN and HCO+(4-3) in a starless molecular filament in NGC 6334 where magnetic fields play a dynamically important role (Li et al. 2015). Our analysis showed that the neutral spectrum is consistent with Kolmogorov-type (k^-5/3, where k is the wave number), while that of the ions is the same on the large scale but steeper (k^-2) for scales smaller than 0.404pc. We carefully ruled out the possibilities that the spectrum difference can stem from the differences of ion and neutral optical depth and hyper-fine structures.