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

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.Comment: 17 pages, 12 figures, 1 table; Accepted for publication in A&

Effect of dust size and structure on scattered light images of protoplanetary discs

We study scattered light properties of protoplanetary discs at near-infrared wavelengths for various dust size and structure by performing radiative transfer simulations. We show that different dust structures might be probed by measuring disk polarisation fraction as long as the dust radius is larger than the wavelength. When the radius is larger than the wavelength, disc scattered light will be highly polarised for highly porous dust aggregates, whereas more compact dust structure tends to show low polarisation fraction. Next, roles of monomer radius and fractal dimension for scattered light colours are studied. We find that, outside the Rayleigh regime, as fractal dimension or monomer radius increases, colours of the effective albedo at near-infrared wavelengths vary from blue to red. Our results imply that discs showing grey or slightly blue colours and high polarisation fraction in near-infrared wavelengths might be explained by the presence of large porous aggregates containing sub-microns sized monomers.Comment: Accepted for publication in MNRAS, 18 pages, 19 figure

Dust Destruction by Charging: A Possible Origin of Grey Extinction Curves of Active Galactic Nuclei

Observed extinction curves of active galactic nuclei (AGNs) are significantly different from those observed in the Milky Way. The observations require preferential removal of small grains at the AGN environment; however, the physics for this remains unclear. In this paper, we propose that dust destruction by charging, or Coulomb explosion, may be responsible for AGN extinction curves. Harsh AGN radiation makes a dust grain highly charged through photoelectric emission, and grain fission via the Coulomb explosion occurs when the electrostatic tensile stress of a charge grain exceeds its tensile strength. We show that the Coulomb explosion can preferentially remove both small silicate and graphite grains and successfully reproduce both flat extinction curves and the absence of 2175\AA~bump.Comment: 8 pages, 6 Figures; Accepted for publication in Ap