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

• Autores: Lin, Zhe Yu Daniel; Li, Zhi Yun; Yang, Haifeng; Stephens, Ian; Looney, Leslie; Harrison, Rachel; Fernandez Lopez, Manuel
• Año de publicación: 2022
• Idioma: inglés
• Tipo de recurso: artículo
• Materia: POLARIZATION; PROTOPLANETARY DISCS
• Institución: Consejo Nacional de Investigaciones Científicas y Técnicas

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 ∼ 870 μm. 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.
Fil: Lin, Zhe Yu Daniel. University of Virginia; Estados Unidos
Fil: Li, Zhi Yun. University of Virginia; Estados Unidos
Fil: Yang, Haifeng. Tsinghua University; China
Fil: Stephens, Ian. Wocester State University; Estados Unidos
Fil: Looney, Leslie. University of Illinois; Estados Unidos. University of Illinois. Urbana - Champaign; Estados Unidos
Fil: Harrison, Rachel. University of Illinois. Urbana - Champaign; Estados Unidos
Fil: Fernandez Lopez, Manuel. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Instituto Argentino de Radioastronomía. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto Argentino de Radioastronomía; Argentina