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

TL;DR

This study demonstrates that large irregular dust grains in protoplanetary disks can produce observed polarization levels through scattering, suggesting millimeter-sized grains are present and resolving previous size estimation conflicts.

Contribution

It shows that irregular grains larger than a few millimeters can generate polarization consistent with observations, challenging the assumption of spherical grains and refining grain size estimates.

Findings

Large irregular grains produce polarization parallel to the disk minor axis.

Millimeter-sized grains can account for observed polarization levels.

Irregular grains can cause asymmetries in inclined disks due to forward scattering.

Abstract

The size of dust grains, $a$, is key to the physical and chemical processes in circumstellar disks, but observational constraints of grain size remain challenging. (Sub)millimeter continuum observations often show a percent-level polarization parallel to the disk minor axis, which is generally attributed to scattering by $\sim 100\mu$m-sized spherical grains (with a size parameter $x \equiv 2\pi a / \lambda < 1$, where $\lambda$ 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 $\beta$ 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 disk 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 $\sim$mm-sized grains) can reach the same polarization level. Our results suggest that grains in disks can have sizes in the millimeter 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 midplane, their strong forward scattering can produce asymmetries between the near and far side of an inclined disk, which can be used to infer their presence.