The evidence of radio polarization induced by the radiative grain alignment and self-scattering of dust grains in a protoplanetary disk

TL;DR

This study uses ALMA observations to distinguish polarization mechanisms in a protoplanetary disk, confirming dust grain alignment with radiative flux at 3.1 mm and self-scattering at 1.3 mm, and constraining grain size to 100 μm.

Contribution

It provides observational evidence for radiative grain alignment and self-scattering as polarization mechanisms in a protoplanetary disk, and constrains dust grain size.

Findings

Azimuthal polarized emission detected at 3.1 mm.

Different polarization morphologies at 3.1 mm and 1.3 mm.

Maximum grain size constrained to 100 μm.

Abstract

The mechanisms causing millimeter-wave polarization in protoplanetary disks are under debate. To disentangle the polarization mechanisms, we observe the protoplanetary disk around HL Tau at 3.1 mm with the Atacama Large Millimeter/submillimeter Array (ALMA), which had polarization detected with CARMA at 1.3 mm. We successfully detect the ring-like azimuthal polarized emission at 3.1 mm. This indicates that dust grains are aligned with the major axis being in the azimuthal direction, which is consistent with the theory of radiative alignment of elongated dust grains, where the major axis of dust grains is perpendicular to the radiation flux. Furthermore, the morphology of the polarization vectors at 3.1 mm is completely different from those at 1.3 mm. We interpret that the polarization at 3.1 mm to be dominated by the grain alignment with the radiative flux producing azimuthal polarization vectors, while the self-scattering dominates at 1.3 mm and produces the polarization vectors parallel to the minor axis of the disk. By modeling the total polarization fraction with a single grain population model, the maximum grain size is constrained to be $100{\rm~\mu m}$, which is smaller than the previous predictions based on the spectral index between ALMA at 3 mm and VLA at 7 mm.