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

Dust properties in protoplanetary disks remain poorly understood despite their central role in planetary formation. A detailed observational characterization of their properties is crucial to improving our understanding of grain growth. Recent studies, based on analyses of scattered-light images and scattering phase functions, provide promising insights into dust characterization. However, the phase function alone is insufficient to constrain dust properties efficiently. In particular, degeneracies between dust size, composition, and morphology remain a major obstacle to linking observations to realistic dust models. We investigate the behavior of the disk color as a function of dust size, shape, and composition. The goal is to combine this with phase function measurements to most effectively constrain the dust properties. We used the 3D radiative transfer code MCFOST to generate synthetic scattered-light images of protoplanetary disks from the visible to the near-infrared, in both total and polarized intensity. We considered compact spherical grains using Mie theory. We used the optical properties computed with the T-matrix method or the discrete dipole approximation for irregular particles and for compact and fractal aggregates from the AggScatVIR database. We measured the disk colors directly from the synthetic images. We obtain five main results. (i) For aggregates larger than the wavelength, the monomer size plays a crucial role in determining the colors, whereas the aggregate size has a negligible effect, especially for low fractal dimensions. (ii) The polarized color is more sensitive to the dust properties than the total intensity color, likely because it combines the wavelength dependences of both the scattered intensity and the degree of polarization. (iii) The effective medium theory is a reliable approximation only for relatively compact aggregates composed of small monomers and becomes inaccurate for highly porous aggregates or those composed of larger monomers. (iv) The effective albedo is useful for predicting disk colors, but radiative transfer calculations are still required to account for the multiple scattering effect and quantitatively interpret disk colors. (v) For HD 100546, the red optical to near-infrared colors exclude compact grains and organic-rich compositions but are compatible with fractal or compact aggregates of amorphous carbon composition containing large monomers (200--400 nm). Our study of color based on synthetic data and its application to the example of the disk around HD 100546 shows that combining color measurements, or a color measurement with a polarized scattering phase function, offers a promising approach to breaking the degeneracy between dust models. Extending this approach to larger samples of disks with multiwavelength observations should provide better constraints on the properties and evolution of dust and motivate the use of color in future studies of protoplanetary disks.