Anchoring Magnetic Fields in Turbulent Molecular Clouds II - from 0.1 to 0.01 parsec

TL;DR

This study investigates magnetic field orientations in molecular clouds at different scales, revealing that small-scale cores can be slightly super-Alfvenic, which aligns with simulations and observations, challenging previous sub-Alfvenic assumptions.

Contribution

The paper extends previous work by analyzing high-resolution interferometric data and demonstrates that cores can be slightly super-Alfvenic, reconciling observations with MHD simulations.

Findings

Cores with high density are slightly super-Alfvenic.

Magnetic field orientations deviate at small scales.

Simulations reproduce observed magnetic and velocity relations.

Abstract

We (Li et al. 2009; Paper-I) compared the magnetic field directions inferred from polarimetry data obtained from 100-pc scale inter-cloud media (ICM) and from sub-pc scale molecular cloud cores. The highly correlated result led us to conclude that cloud turbulence must be sub-Alfvenic. Here we extend the study with 0.01-pc cores observed by interferometers. The inferred field directions at this scale significantly deviate from that of the surrounding ICM. An obvious question to ask is whether this high-resolution result contradicts the sub-Alfvenic picture concluded earlier. We performed MHD simulations of a slightly super-critical (magnetic criticality = 2) clouds with Alfvenic Mach number $M_A = 0.63$, which can reproduce the Paper-I results, and observed the development towards smaller scales. Interestingly, all subregions hosting cores with $n_H$$_2$ > $10^{5}$/cc (the typical density observed by interferometers) possess $M_A = 2-3$. Not too surprisingly, these slightly super-Alfvenic cores result in B-field orientation offsets comparable to the interferometer observations. The result suggests that gravity can concentrate (and maybe also contribute to, which takes more study to confirm) turbulent energy and create slightly super-Alfvenic cores out from sub-Alfvenic clouds. The results of our simulations also agree with the observed velocity-scale (Kauffmann et al. 2013), mass-scale (Lombardi et al. 2010) and field strength-density (Li et al. 2015; Crutcher et al. 2010) relations.