# Radiative grain alignment in protoplanetary disks: Implications for polarimetric observations

## TL;DR

This paper models radiative torque alignment in protoplanetary disks to predict polarization patterns, revealing that large grains do not align with magnetic fields, but smaller grains with magnetic inclusions can, affecting observational interpretations.

## Contribution

It applies 3D radiative transfer and RAT theory to protoplanetary disks, providing new predictions for polarization maps and grain alignment mechanisms.

## Key findings

- Large mm-sized grains at midplane do not align with magnetic fields.
- Polarization in face-on disks is expected to be azimuthal.
- Magnetic inclusions in dust grains enable magnetic alignment at the surface layer.

## Abstract

We apply the theory of radiative torque (RAT) alignment for studying protoplanetary disks around a T-Tauri star and perform 3D radiative transfer calculations to provide the expected maps of polarized radiation to be compared with observations, such as with ALMA. We revisit the issue of grain alignment for large grains expected in the protoplanetary disks and find that mm-sized grains at midplane do not align with the magnetic field as the Larmor precession timescale for such large grains becomes longer than the gaseous damping timescale. Hence, for these grains the RAT theory predicts that the alignment axis is determined by the grain precession with respect to the radiative flux. As a result, we expect that the polarization will be in the azimuthal direction for a face-on disk. It is also shown that if dust grains have superparamagnetic inclusions, magnetic field alignment is possible for (sub-)micron grains at the surface layer of disks, and this can be tested by mid-infrared polarimetric observations.