Abstract:
Dust polarization induced by aligned nonspherical 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 μm. We observe the
so-called phenomenon of a polarization hole attributed to the decrease in polarization fraction in denser regions of
higher total intensity and gas density. Our study finds that the B-field tangling effect is a minor cause of the
polarization hole, and the dominant factor is the reduction in grain alignment efficiency in denser regions,
consistent with the RAT mechanism. To test RAT theory, we calculate various quantities describing grain
alignment, including the minimum size of aligned grains, magnetic and magnetic relaxation parameters, and show
that the RAT mechanism can explain observational data. Our study also reveals evidence for a 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
DUSTPOL_PY based on M-RAT theory and find that the modeling could successfully reproduce the observational
data when the maximum grain size is around 0.45 μm 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
