Observed polarized scattered light phase functions of planet-forming disks

TL;DR

This study extracts and analyzes the polarized scattering phase functions of dust particles in planet-forming disks, revealing two categories linked to different aggregate structures, which enhances understanding of dust properties and disk dynamics.

Contribution

First to extract full polarized phase functions from a broad sample of disks, linking observed scattering behaviors to dust aggregate structures using VLT/SPHERE data.

Findings

Polarized phase functions fall into two categories.

One category matches fractal, porous aggregates.

The other aligns with more compact, less porous aggregates.

Abstract

Dust particles are the building blocks from which planetary bodies are made. A major goal of the studies of planet-forming disks is to constrain the properties of dust particles and aggregates in order to trace their origin, structure, and the associated growth and mixing processes in the disk. Observations of scattering and/or emission of dust in a location of the disk often lead to degenerate information about the kind of particles, such as size, porosity, or fractal dimension of aggregates. Progress can be made by deriving the full (polarizing) scattering phase function of such particles at multiple wavelengths. This has now become possible by careful extraction from scattered light images. Such an extraction requires knowledge about the shape of the scattering surface in the disk and we discuss how to obtain such knowledge as well as the associated uncertainties. We use a sample of disk images from observations with VLT/SPHERE to, for the first time, extract the phase functions of a whole sample of disks with broad phase angle coverage. We find that polarized phase functions come in two categories. Comparing the extracted functions with theoretical predictions from rigorous T-Matrix computations of aggregates, we show that one category can be linked back to fractal, porous aggregates, while the other is consistent with more compact, less porous aggregates. We speculate that the more compact particles become visible in disks where embedded planets trigger enhanced vertical mixing.