How large are the monomers of dust aggregates in planet-forming disks?: Insights from quantitative optical and near-infrared polarimetry

TL;DR

This study uses optical and near-infrared polarimetric observations combined with numerical modeling to constrain the size of dust monomers in planet-forming disks, finding they are no larger than 0.4 micrometers, similar to solar system dust subunits.

Contribution

It provides the first meaningful observational constraint on dust monomer sizes in planet-forming disks using polarimetric data and numerical simulations.

Findings

Monomer size is constrained to be no greater than 0.4 micrometers.

Degree of polarization depends sensitively on monomer size.

Results link dust properties in disks to solar system and interstellar dust sizes.

Abstract

Context: The size of the constituent particles (monomers) of dust aggregates is one of the most uncertain parameters directly affecting collisional growth of aggregates in planet-forming disks. Despite its importance, the monomer size has not yet been meaningfully constrained by disk observations. Aims: We attempt to derive the monomer size from optical and near-infrared (IR) polarimetric observations of planet-forming disks. Methods: We perform a comprehensive parameter survey on the degree of linear polarization of light scattered by dust aggregates, using an exact numerical method called the $T$-matrix method. We investigate the effect of the monomer size, aggregate size, porosity, and composition on the degree of polarization. The obtained results are then compared with observed polarization fractions of several planet-forming disks at optical and near-IR wavelengths. Results: It is shown that the degree of polarization of aggregates depends sensitively on the monomer size unless the monomer size parameter is smaller than one or two. Comparing the simulation results with the disk observations, we find that the monomer radius is no greater than $0.4~\mu$m. The inferred monomer size is therefore similar to subunit sizes of the solar system dust aggregates and the maximum size of interstellar grains. Conclusions: Optical and near-IR quantitative polarimetry will provide observational grounds on the initial conditions for dust coagulation and thereby planetesimal formation in planet-forming disks.