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

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 $μ$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 $μ$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.

Figures (18)

• Figure 1: (a) Map of gas column density $N(\mathrm{H_2})$ of the whole observed field of G16 region with the white rectangle indicating the region of the G16 filament; (b) Zoomed-in map of $N(\mathrm{H_2})$ for the rectangular region; (c) Map of gas volume density $n(\mathrm{H_2})$; and (d) Map of dust temperature $T_\mathrm{d}$. The black circles overlaid on (a) represent the dense cores identified on the whole observed field of G16 region, and those in (b) and (c) with black colors and (d) with magenta color represent the dense cores MM1, MM2, MM3, MM4, MM5, MM6, MM7 and MM8 identified on the G16 filament. The black contours are drawn at JCMT/POL-2 850 $\mu$m total emission intensity $I$ values of 50, 100, 150, 200, 250, 280, 300, 350 mJy/beam. The "+" symbols in (b), (c) and (d) indicate the pixels of outermost and innermost regions having polarization measurements.
• Figure 2: Map of total emission intensity $I$ of the G16 filament observed by JCMT/POL-2 at 850 $\mu$m with a resolution of $14".4$ indicated with a solid black circle. The black circles overlaid on the map represent the dense cores MM1, MM2, MM3, MM4, MM5, MM6, MM7 and MM8 identified on the filament and are labelled with black colors. The black contours are drawn at $I$ values of 50, 100, 150, 200, 250, 280, 300, 350 mJy/beam.
• Figure 3: (Left) Same map as in Figure \ref{['Figure:Intensity_map']} but overlaid with polarization vectors shown with blue color. The lengths of the vectors are proportional to the polarization fractions $P$, and the directions of the vectors indicate the magnetic field orientations. A reference scale of $P$ of 20% is also shown. (Right) Histogram of $P$ with the vertical dotted green line and shaded region denoting the median value of $P$ and its uncertainty.
• Figure 4: Variation of total emission intensity with the gas column density for the pixels having polarization measurements. The magenta dashed line represents the running mean.
• Figure 5: Variations of dust temperature with (a) the total emission intensity and (b) the gas column density for the pixels having polarization measurements. The magenta dashed lines represent the running means.