Radial variations in grain sizes and dust scale heights in the protoplanetary disk around HD 163296 revealed by ALMA polarization observation

TL;DR

This study uses ALMA polarization observations and radiative transfer modeling to reveal radial variations in grain size and dust scale height in the HD 163296 protoplanetary disk, providing insights into disk structure and turbulence.

Contribution

It presents the first detailed modeling of polarization to constrain grain size and dust scale height variations in a protoplanetary disk, highlighting the role of these parameters in polarization signatures.

Findings

Maximum grain size varies from 140 microns in gaps to different sizes in rings.

Dust scale height is less than one-third of the gas scale height inside 70 au.

Turbulence parameter alpha varies, indicating a dead zone at the ring boundary.

Abstract

The HD 163296 disk shows ring and gap substructures with ALMA observations. In addition, this is the only disk where the rings and gaps are spatially resolved in millimeter-wave polarization measurements. In this paper, we conduct radiative transfer modeling that includes self-scattering polarization to constrain the grain size and its distribution. We found that the grain size and dust scale height are the key parameters for reproducing the radial and azimuthal distributions of the observed polarization signature. Radial variation is mainly determined by grain size. The polarization fraction is high if the particle size is $\sim \lambda/2\pi$; it is low if the particle size is larger or smaller than this. In contrast, azimuthal variation in polarization is enhanced if the dust scale height is increased. Based on detailed modeling of the HD 163296 polarization, we found the following radial variations in the grain size and dust scale height. The maximum grain size was 140 microns in the gaps and significantly larger or smaller in the rings. The dust scale height is less than one-third the gas scale height inside the 70 au ring, and two-thirds the gas scale height outside the 70 au ring. Furthermore, we constrained the gas turbulence to be $\alpha\lesssim1.5\times10^{-3}$ in the 50 au gap and $\alpha\sim 0.015-0.3$ in the 90 au gap. The transition of the turbulence strength at the boundary of the 70 au ring indicates the existence of a dead zone.