Fractal aggregates of sub-micron-sized grains in the young planet-forming disk around IM Lup

TL;DR

This study uses multi-wavelength scattered light observations of the IM Lup disk to determine that dust aggregates are fractal with a dimension of 1.5, composed of ~200 nm monomers, supporting fractal coagulation in early planet formation.

Contribution

First detailed comparison of observed scattered light with complex-shaped dust models, revealing fractal aggregate properties in a young planet-forming disk.

Findings

Dust aggregates have a fractal dimension of 1.5.

Monomer size is approximately 200 nm.

Dust composition likely includes amorphous carbon.

Abstract

Despite rapidly growing disk observations, it remains a mystery what primordial dust aggregates look like and what the physical and chemical properties of their constituent grains (monomers) are in young planet-forming disks. Confrontation of models with observations to answer this mystery has been a notorious task because we have to abandon a commonly used assumption, perfectly spherical grains, and take into account particles with complex morphology. In this Letter, we present the first thorough comparison between near-infrared scattered light of the young planet-forming disk around IM Lup and the light-scattering properties of complex-shaped dust particles. The availability of scattering observations at multiple wavelengths and over a significant range of scattering angles allows for the first determination of the monomer size, fractal dimension, and size of dust aggregates in a planet-forming disk. We show that the observations are best explained by fractal aggregates with a fractal dimension of 1.5 and a characteristic radius larger than $\sim2~\mu$m. We also determined the radius of the monomer to be $\sim200$ nm, and monomers much smaller than this size can be ruled out on the premise that the fractal dimension is less than 2. Furthermore, dust composition comprising amorphous carbon is found to be favorable to simultaneously account for the faint scattered light and the flared disk morphology. Our results support that planet formation begins with fractal coagulation of sub-micron-sized grains. All the optical properties of complex dust particles computed in this study are publicly available.