Detailed radial scale height profile of dust grains as probed by dust self-scattering in HL Tau

TL;DR

This study uses high-resolution polarization observations of HL Tau to map the radial dust scale height profile, revealing a thick inner disk and a settled outer disk, offering insights into dust settling and turbulence in planet formation.

Contribution

It provides the first detailed radial profile of dust scale height in HL Tau, linking polarization features to disk geometry and turbulence variations.

Findings

Inner disk has a large aspect ratio ($H/R \,\ge\, 0.15$), indicating a thick dust layer.

Outer disk is thin with $H/R < 0.05$, suggesting settled dust.

Turbulence varies radially, increasing from $10^{-5}$ at 100 au to $10^{-2.5}$ at 20 au.

Abstract

The vertical settling of dust grains in a circumstellar disk, characterized by their scale height, is a pivotal process in the formation of planets. This study offers in-depth analysis and modeling of the radial scale height profile of dust grains in the HL Tau system, leveraging high-resolution polarization observations. We resolve the inner disk's polarization, revealing a significant near-far side asymmetry, with the near side being markedly brighter than the far side in polarized intensity. This asymmetry is attributed to a geometrically thick inner dust disk, suggesting a large aspect ratio of $H/R \ge 0.15$. The first ring at 20 au exhibits an azimuthal contrast, with polarization enhanced along the minor axis, indicating a moderately thick dust ring with $H/R \approx 0.1$. The absence of the near-far side asymmetry at larger scales implies a thin dust layer, with $H/R < 0.05$. Taken together, these findings depict a disk with a turbulent inner region and a settled outer disk, requiring a variable turbulence model with $\alpha$ increasing from $10^{-5}$ at 100 au to $10^{-2.5}$ at 20 au. This research sheds light on dust settling and turbulence levels within protoplanetary disks, providing valuable insights into the mechanisms of planet formation.