Two Different Grain Size Distributions within the Protoplanetary Disk around HD 142527 Revealed by ALMA Polarization Observation

TL;DR

This study uses ALMA polarization observations to differentiate polarization mechanisms in the HD 142527 protoplanetary disk, revealing grain size segregation and magnetic field alignment in distinct disk regions.

Contribution

It provides observational evidence for different polarization mechanisms in a protoplanetary disk and introduces a simple grain size segregation model based on polarization data.

Findings

Northern region polarization consistent with self-scattering

Southern region polarization aligned with magnetic fields

Evidence of grain size segregation within the disk

Abstract

The origin of polarized emission from protoplanetary disks is uncertain. Three mechanisms have been proposed for such polarized emission: grain alignment with magnetic fields, grain alignment with radiation gradients, and self-scattering of thermal dust emission. Aiming to observationally identify the polarization mechanisms, we present ALMA polarization observations of the 0.87 mm dust continuum emission toward the circumstellar disk around HD 142527 with high spatial resolution. We confirm that the polarization vectors in the northern region are consistent with self-scattering. Furthermore, we show that the polarization vectors in the southern region are consistent with grain alignment by magnetic fields, although self-scattering cannot be ruled out. To understand the differences between the polarization mechanisms, we propose a simple grain size segregation model: small dust grains ($\lesssim$ 100 microns) are dominant and aligned with magnetic fields in the southern region, and middle-sized ($\sim100$ microns) grains in the upper layer emit self-scattered polarized emission in the northern region. The grain size near the middle plane in the northern region cannot be measured because the emission at 0.87 mm is optically thick. However, it can be speculated that larger dust grains ($\gtrsim$ cm) may accumulate near this plane. These results are consistent with those of a previous analysis of the disk, in which large grain accumulation and optically thick emission from the northern region were found. This model is also consistent with theories where smaller dust grains are aligned with magnetic fields. The magnetic fields are toroidal, at least in the southern region.