Azimuthal Dust Polarization from Aerodynamically Aligned Grains as Evidence for the Streaming Instability in Protoplanetary Disks

TL;DR

This paper demonstrates that azimuthal dust polarization in protoplanetary disks, caused by aerodynamically aligned grains, provides evidence for the streaming instability, supported by 3D simulations and polarized radiation transfer.

Contribution

It introduces the Badminton Birdie-like Aerodynamic Alignment model and links polarization patterns to streaming instability regions in disks.

Findings

Polarization angle follows azimuthal direction in high dust density regions.

A-field is radial in low dust-to-gas ratio regions and azimuthal in high ratio regions.

Azimuthal polarization pattern indicates ongoing streaming instability.

Abstract

(Sub)millimeter dust polarization in protoplanetary disks has revealed the presence of large (~ 100 um) dust grains that are aligned along their long axis following the azimuthal direction of the disk. The novel Badminton Birdie-like Aerodynamic Alignment predicts large grains to align with their long axes following the direction of gas flow experienced by the dust, denoted as the A-field. With 3D streaming instability (SI) simulations, we find that the A-field is predominantly in the radial direction in regions of low dust-to-gas ratio, but in the azimuthal direction in regions of high dust-to-gas ratio. Through polarized radiation transfer, we find that the resulting polarization angle indeed follows the disk azimuthal direction in the high dust density regions. Therefore, the azimuthal dust polarization pattern, as observed in an increasing number of disks, especially at relatively long millimeter wavelengths, offers evidence of ongoing SI in protoplanetary disks.