(Sub)millimetre dust polarization of protoplanetary discs from scattering by large millimetre-sized irregular grains

• Autores: Lin, Zhe Yu Daniel; Li, Zhi Yun; Yang, Haifeng; Muñoz, Olga; Looney, Leslie; Stephens, Ian; Hull, Charles L. H.; Fernandez Lopez, Manuel; Harrison, Rachel
• Año de publicación: 2023
• Idioma: inglés
• Tipo de recurso: artículo
• Materia: polarization; protoplanetary discs; circumstellar matter
• Institución: Consejo Nacional de Investigaciones Científicas y Técnicas

The size of dust grains, a, is key to the physical and chemical processes in circumstellar discs, but observational constraints of grain size remain challenging. (Sub)millimetre continuum observations often show a per cent-level polarization parallel to the disc minor axis, which is generally attributed to scattering by ∼100μm-sized spherical grains (with a size parameter x ≡ 2πa/λ < 1, where λ is the wavelength). Larger spherical grains (with x greater than unity) would produce opposite polarization direction. However, the inferred size is in tension with the opacity index β that points to larger mm/cm-sized grains. We investigate the scattering-produced polarization by large irregular grains with a range of x greater than unity with optical properties obtained from laboratory experiments. Using the radiation transfer code, RADMC-3D, we find that large irregular grains still produce polarization parallel to the disc minor axis. If the original forsterite refractive index in the optical is adopted, then all samples can produce the typically observed level of polarization. Accounting for the more commonly adopted refractive index using the DSHARP dust model, only grains with x of several (corresponding to ~mm-sized grains) can reach the same polarization level. Our results suggest that grains in discs can have sizes in the millimetre regime, which may alleviate the tension between the grain sizes inferred from scattering and other means. Additionally, if large irregular grains are not settled to the mid-plane, their strong forward scattering can produce asymmetries between the near and far side of an inclined disc, which can be used to infer their presence.

Fil: Lin, Zhe Yu Daniel. University of Virginia; Estados Unidos
Fil: Li, Zhi Yun. University of Virginia; Estados Unidos
Fil: Yang, Haifeng. Peking University; China
Fil: Muñoz, Olga. Instituto de Astrofísica de Andalucía; España
Fil: Looney, Leslie. University of Illinois. Department of Astronomy; Estados Unidos
Fil: Stephens, Ian. Worcester State University; Estados Unidos
Fil: Hull, Charles L. H.. National Astronomical Observatory of Japan; Japón
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
Fil: Harrison, Rachel. University of Illinois. Department of Astronomy; Estados Unidos