# Unveiling Dust Aggregate Structure in Protoplanetary Disks by Millimeter-wave Scattering Polarization

## TL;DR

This study uses 3D radiative transfer simulations to show how dust aggregate structure and porosity affect millimeter-wave polarization in protoplanetary disks, aiding understanding of planetesimal formation.

## Contribution

It demonstrates the influence of dust porosity and fractal dimension on polarization signals, providing observational constraints on dust evolution in disks.

## Key findings

- Lower porosity and higher fractal dimension aggregates match observed polarization.
- Highly porous aggregates cannot explain the observed polarization.
- Multi-wavelength polarimetry helps constrain dust porosity.

## Abstract

Dust coagulation in a protoplanetary disk is the first step of planetesimal formation. However, a pathway from dust aggregates to planetesimals remains unclear. Both numerical simulations and laboratory experiments have suggested the importance of dust structure in planetesimal formation, but it is not well constrained by observations. We study how dust structure and porosity alters polarimetric images at millimeter wavelength by performing 3D radiative transfer simulations. Aggregates with different porosity and fractal dimension are considered. As a result, we find that dust aggregates with lower porosity and/or higher fractal dimension are favorable to explain observed millimeter-wave scattering polarization of disks. Aggregates with extremely high porosity fail to explain the observations. In addition, we also show that particles with moderate porosity show weak wavelength dependence of scattering polarization, indicating that multi-wavelength polarimetry is useful to constrain dust porosity. Finally, we discuss implications for dust evolution and planetesimal formation in disks.