Statistical analysis of the interplay between interstellar magnetic fields and filaments hostingPlanckGalactic cold clumps

D Alina,J-Ph Bernard,E R Micelotta,L Montier,I Ristorcelli,E Abdikamalov,M Juvela,K Ferrière

doi:10.1093/mnras/stz508

Abstract

We present a statistical study of the relative orientation in the plane of the sky between interstellar magnetic fields and filaments hosting cold clumps. For the first time, we consider both the density of the environment and the density contrast between the filaments and their environment. Moreover, we geometrically distinguish between the clumps and the remaining portions of the filaments. We infer the magnetic field orientations in the filaments and in their environment from the Stokes parameters, assuming optically thin conditions. Thus, we analyze the relative orientations between filaments, embedded clumps, and internal and background magnetic fields, depending on the filament environment and evolutionary stages. We recover the previously observed trend for filaments in low column density environments to be aligned parallel to the background magnetic field; however, we find that this trend is significant only for low contrast filaments, whereas high contrast filaments tend to be randomly orientated with respect to the background magnetic field. Filaments in high column density environments do not globally show any preferential orientation, although low contrast filaments alone tend to have perpendicular relative orientation with respect to the background magnetic field. For a subsample of nearby filaments, for which volume densities can be derived, we find a clear transition in the relative orientation with increasing density, at $n_{\rm H} \sim 10^{3}~{\rm cm}^{-3}$, changing from mostly parallel to mostly perpendicular in the off-clump portions of filaments and from even to bimodal in the clumps. Our results confirm a strong interplay between interstellar magnetic fields and filaments during their formation and evolution.

Highlights

First observed in extinction by interstellar dust particles (Schneider & Elmergreen 1979) and in dust and CO emission (Abergel et al 1994; Falgarone et al 2001), filamentary structures in molecular clouds recently became central to many studies
Our goal was to investigate whether the relative orientations in the POS between interstellar magnetic fields and filaments hosting Planck cold clumps may vary with the density of the environment and may depend on the filaments evolutionary stage
The filaments embedded in low-density environment, i.e. below NH,bkg = 1.2 × 1021 cm−2 show a clear parallel relative orientation with the background magnetic field

Summary

Introduction

First observed in extinction by interstellar dust particles (Schneider & Elmergreen 1979) and in dust and CO emission (Abergel et al 1994; Falgarone et al 2001), filamentary structures in molecular clouds recently became central to many studies. It was shown that prestellar cores and protostars form primarily in the densest and gravitationally-bound filaments (Andre et al 2010; Polychroni et al 2013; Konyves et al 2015). Investigating the origin and evolution of filaments is crucial to better understand the early stages of star formation. MHD simulations show that a hierarchy of sheets and filaments can form as a result of shock-compression and shear flows in supersonic turbulence, at the same time as the collapse or fragmentation of gravitationally unstable structures (Andre et al 2014; Li et al 2014). Magnetic fields are believed to play a key role in the formation of structures, but their interplay with turbulence and self-gravity still needs to be better understood at different spatial scales and evolutionary stages

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