Thermal emission and scattering by aligned grains: Plane-parallel model and application to multiwavelength polarization of the HL Tau disc

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 similar to 870 mu m. However, certain discs show an elliptical pattern at similar to 3 mm, which is likely due to aligned grains. With HL Tau, its polarization pattern at similar to 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 T-0 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 T-0 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.

Automated summary

Telescopes are now capable of studying the polarization spectrum of circumstellar discs by resolving dust polarization at multiple wavelengths. Most discs show evidence of dust scattering through unidirectional polarization at shorter wavelengths, while some discs exhibit an elliptical pattern at longer wavelengths due to aligned grains. The polarization pattern of HL Tau at 1.3 mm reveals a transition between these two patterns, making it the first example to demonstrate such a transition. By using the T-matrix method and a plane-parallel slab model, researchers were able to model elongated dust grains and properly account for scattering of non-spherical aligned grains. The study found that a change in optical depth can explain the polarization transition of HL Tau, where thermal polarization dominates at low optical depths and scattering polarization dominates at high optical depths. A simple technique was developed to separate the thermal polarization and scattering-induced polarization by analyzing the azimuthal variation of the polarization fraction. The researchers also identified other sources similar to HL Tau that could be explained by azimuthally aligned scattering prolate grains when considering optical depth effects. Additionally, the study explores how spirally aligned grains with scattering can appear in polarization images.