The size of monomers of dust aggregates in planet-forming disks

Insights from quantitative optical and near-infrared polarimetry

¹ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098XH Amsterdam, The Netherlands
e-mail: r.tazaki@uva.nl
² Astronomical Institute, Graduate School of Science, Tohoku University, 6-3 Aramaki, Aoba-ku, Sendai 980-8578, Japan

Received: 5 March 2022
Accepted: 14 April 2022

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 performed 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 investigated the effect of the monomer size, aggregate size, porosity, and composition on the degree of polarization. The obtained results were then compared with observed polarization fractions of several planet-forming disks at optical and near-IR wavelengths.

Results. We show that the degree of polarization of aggregates acutely depends 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 μm. 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.