(Sub)millimeter Dust Polarization of Protoplanetary Disks from Scattering by Large Millimeter-Sized Irregular Grains

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 ≡ 2π a/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. © 2023 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society.
YDL acknowledges support from the Jefferson Scholars Foundation, NASA 80NSSC18K1095, and also support from the ALMA Student Observing Support (SOS). ZYL is supported in part by NASA 80NSSC20K0533 and National Science Foundation (NSF) AST-1910106. LWL acknowledges support from NSF AST-1910364. Research by OM is supported by grant PID2021-123370OB-I00 funded MCIN/AEI/FEDER, UE.
With funding from the Spanish government through the "Severo Ochoa Centre of Excellence" accreditation (CEX2021-001131-S).