Telescopes are now able to resolve dust polarization across circumstellar discs at multiple wavelengths, allowing the study of the polarization spectrum. Most discs show clear evidence of dust scattering through their unidirectional polarization pattern typically at the shorter wavelength of $\sim 870 \, \mu$m. However, certain discs show an elliptical pattern at ∼3 mm, which is likely due to aligned grains. With HL Tau, its polarization pattern at ∼1.3 mm shows a transition between the two patterns making it the first example to reveal such transition. We use the T-matrix method to model elongated dust grains and properly treat scattering of aligned non-spherical grains with a plane-parallel slab model. We demonstrate that a change in optical depth can naturally explain the polarization transition of HL Tau. At low optical depths, the thermal polarization dominates, while at high optical depths, dichroic extinction effectively takes out the thermal polarization and scattering polarization dominates. Motivated by results from the plane-parallel slab, we develop a simple technique to disentangle thermal polarization of the aligned grains T0 and polarization due to scattering S using the azimuthal variation of the polarization fraction. We find that, with increasing wavelength, the fractional polarization spectrum of the scattering component S decreases, while the thermal component T0 increases, which is expected since the optical depth decreases. We find several other sources similar to HL Tau that can be explained by azimuthally aligned scattering prolate grains when including optical depth effects. In addition, we explore how spirally aligned grains with scattering can appear in polarization images.
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Polarization is a unique tool to study the dust grains of protoplanetary discs. Polarization around HL Tau was previously imaged using the Atacama Large Millimeter/submillimeter Array (ALMA) at Bands 3 (3.1 mm), 6 (1.3 mm), and 7 (0.87 mm), showing that the polarization orientation changes across wavelength λ. Polarization at Band 7 is predominantly parallel to the disc minor axis but appears azimuthally oriented at Band 3, with the morphology at Band 6 in between the two. We present new ∼0.2 arcsec (29 au) polarization observations at Q-Band (7.0 mm) using the Karl G. Jansky Very Large Array (VLA) and at Bands 4 (2.1 mm), 5 (1.5 mm), and 7 using ALMA, consolidating HL Tau’s position as the protoplanetary disc with the most complete wavelength coverage in dust polarization. The polarization patterns at Bands 4 and 5 follow the previously identified morphological transition with wavelength. From the azimuthal variation, we decompose the polarization into contributions from scattering (s) and thermal emission (t). s decreases slowly with increasing λ, and t increases more rapidly which are expected from optical depth effects of toroidally aligned scattering prolate grains. The weak λ dependence of s is inconsistent with the simplest case of Rayleigh scattering by small grains in the optically thin limit but can be affected by factors such as optical depth, disc substructure, and dust porosity. The sparse polarization detections from the Q-band image are also consistent with toroidally aligned prolate grains.
more » « less- PAR ID:
- 10532801
- Publisher / Repository:
- MNRAS
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 528
- Issue:
- 1
- ISSN:
- 0035-8711
- Page Range / eLocation ID:
- 843 to 862
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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