The colors of protoplanetary disks - Impact of nonspherical dust particles on the polarized and total intensity colors

Abstract

Dust properties in protoplanetary disks remain poorly understood despite their central role in planet formation. Recent studies based on analyses of scattered-light images and phase functions provide promising insights into dust characterization, but the phase function alone is insufficient to constrain dust properties. Degeneracies between dust remain a major obstacle to linking observations to realistic dust models. We investigate the behavior of the disk color as a function of dust. We used MCFOST to generate synthetic scattered-light images from the visible to the near-infrared, in total and polarized intensity. We considered compact spherical grains using, irregular particles and compact and fractal aggregates using optical properties from the AggScatVIR database. Disk colors were measured from these images. For aggregates larger than the wavelength, monomer size influences color, whereas aggregate size has little effect, especially at low fractal dimensions. Polarized color is more sensitive to dust properties than total intensity color, because it combines the wavelength dependences of scattered intensity and polarization degree. Effective medium theory is reliable only for compact aggregates with small monomers, becoming inaccurate for highly porous aggregates or larger monomers. Effective albedo helps predict disk colors, but radiative transfer is needed to account for multiple scattering. For HD 100546, red optical-to-near-infrared colors exclude compact grains and organic-rich compositions but are consistent with amorphous-carbon aggregates, compact or fractal, containing large monomers. These results show that combining color measurements with polarized scattering phase functions offers a promising way to break degeneracies between dust models. Applying this approach to larger disk samples could better constrain dust properties and evolution.