Submillimeter polarization observation of the protoplanetary disk around HD 142527 (1610.06318v2)

Abstract: We present the polarization observations toward the circumstellar disk around HD 142527 by using Atacama Large Millimeter/submillimeter Array (ALMA) at the frequency of 343 GHz. The beam size is $0.51 " \times 0.44 "$, which corresponds to the spatial resolution of $\sim$ 71 $\times$ 62 AU. The polarized intensity displays a ring-like structure with a peak located on the east side with a polarization fraction of $P= 3.26 \pm 0.02$ %, which is different from the peak of the continuum emission from the northeast region. The polarized intensity is significantly weaker at the peak of the continuum where $P= 0.220 \pm 0.010$ %. The polarization vectors are in the radial direction in the main ring of the polarized intensity, while there are two regions outside at the northwest and northeast areas where the vectors are in the azimuthal direction. If the polarization vectors represent the magnetic field morphology, the polarization vectors indicate the toroidal magnetic field configuration on the main ring and the poloidal fields outside. On the other hand, the flip of the polarization vectors is predicted by the self-scattering of thermal dust emission due to the change of the direction of thermal radiation flux. Therefore, we conclude that self-scattering of thermal dust emission plays a major role in producing polarization at millimeter wavelengths in this protoplanetary disk. Also, this puts a constraint on the maximum grain size to be approximately 150 ${\rm \mu m}$ if we assume compact spherical dust grains.

Submillimeter Polarization Observation of the Protoplanetary Disk around HD 142527

This paper presents groundbreaking observations concerning the polarized emissions from the circumstellar disk surrounding HD 142527, utilizing the capabilities of the Atacama Large Millimeter/submillimeter Array (ALMA). Focusing on a frequency of 343 GHz, the paper achieves a spatial resolution of approximately 71 × 62 au.

Results Overview

The observed polarized intensity manifests a distinctive ring-like structure, peaking on the east side of the disk, and demonstrating a polarization fraction of $P = 3.26 \pm 0.02\%$. This contrasts sharply with the peak location of the continuum emission at the northeastern part of the disk, where the polarization fraction is notably lower ($P = 0.220 \pm 0.010 \%$). The polarization vectors predominantly exhibit a radial orientation within the main ring. Contrastingly, in the northwest and northeast peripheral regions, the vectors feature an azimuthal alignment.

Mechanism Exploration

Two competing mechanisms are discussed regarding polarization origins in protoplanetary disks:

1. Grain Alignment with Magnetic Fields: Traditional explanations involve dust grain alignment, facilitated by radiative torques, responding to magnetic field morphologies assumed to be toroidal. The observed radial polarization vector orientation on the main ring might suggest such alignment.
2. Self-Scattering of Thermal Dust Emission: This mechanism attributes polarization to anisotropic scattering by dust grains, effective when grain sizes approach the radiation wavelength. The paper concludes that self-scattering plays a dominant role in polarization production, indicating constraints on maximum grain size ($\sim150 \, \mu\text{m}$) assuming compact spherical dust particles.

Implications and Model Predictions

The constraints on grain size embedded within these observations challenge previous models of dust dynamics which required larger, millimeter-sized grains for effective trapping within disk vortices. The possibility of unresolved substructures or fluffy aggregates may provide clarity on such discrepancies, offering insights into grain growth processes and dynamics in protoplanetary disks.

Future Directions

The observations push forward the necessity to reassess existing models of dust distribution and dynamics, particularly concerning grain size and structure. Future research may leverage these insights to refine grain growth scenarios and magnetic field interaction models, potentially exploring the local influence of vortex phenomena in greater detail.

The intricate interaction between polarized emissions and grain properties observed in the HD 142527 circumstellar disk propels the discourse on protoplanetary disk physics, offering pivotal data investigating grain size distributions and their implications for disk evolution and planet-forming processes.