B-fields And dust in interstelLar fiLAments using Dust POLarization (BALLAD-POL): VI. Grain alignment mechanisms in the massive quiescent filament G16.96+0.27 using dust polarization observations from JCMT/POL-2

TL;DR

This study investigates grain alignment mechanisms in a massive filamentary cloud using dust polarization data, confirming the RAT theory and revealing the need for magnetically-enhanced RAT effects to explain high polarization fractions.

Contribution

It provides observational evidence supporting the RAT mechanism and introduces the magnetically-enhanced RAT (M-RAT) theory to explain high polarization in dense regions.

Findings

Polarization hole attributed to reduced grain alignment efficiency, not B-field tangling.

RAT theory explains observed grain alignment properties and polarization fractions.

Modeling with M-RAT reproduces observational data with specific grain size and shape parameters.

Abstract

Dust polarization induced by aligned non-spherical grains acts as an important tool to trace the magnetic field (B-field) morphologies and strengths in molecular clouds and constrain grain properties and their alignment mechanisms. The widely accepted grain alignment theory is the alignment induced by RAdiative Torques (RATs). In this work, we investigate grain alignment mechanisms in a massive, quiescent and filamentary Infrared Dark Cloud G16.96+0.27 using thermal dust polarization observation with JCMT/POL-2 at 850 $\mu$m. We observe the so-called phenomenon of polarization hole attributed to the decrease in polarization fraction in denser regions of higher total intensity and gas density. Our study finds that B-field tangling effect is minimal to cause the polarization hole, and the dominant factor is the reduction in grain alignment efficiency in denser regions, consistent with RAT mechanism. To test RAT theory, we calculate various quantities describing grain alignment, including minimum size of aligned grains, magnetic and magnetic relaxation parameter, and show that RAT mechanism can explain observational data. Our study also reveals evidence for magnetically-enhanced RAT (M-RAT) mechanism required to explain the observed high polarization fractions of above 10 % in the outer regions of the filament. Finally, we perform detailed modeling of thermal dust polarization using $\mathrm{DustPOL\_py}$ based on M-RAT theory and find that the modeling could successfully reproduce the observational data when maximum grain size is around 0.45 $\mu$m accompanied by an increase in grain axial ratio, along with the consideration of variations in the magnetic field's inclination angle with the line of sight.