HD 100546 colors favor large carbon dust aggregates

The red optical-to-near-infrared colors of the disk around HD 100546 point to dust made of amorphous-carbon aggregates with large component grains. Models show that polarized-light colors can reveal dust properties more clearly than total brightness alone, while the shape and makeup of the particles affect how confidently those properties can be inferred.

Star system: HD 100546 | Distance from the Sun: 359 light years

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HD 100546 colors favor large carbon dust aggregates

Star system: HD 100546 | Distance from the Sun: 359 light years

The red colors of scattered light from the disk around HD 100546 are consistent with aggregates made of amorphous carbon and containing large component grains. They rule out compact grains and organic-rich compositions as explanations for those colors. The aggregates may be compact or fractal, meaning their components may be packed more densely or arranged in a more open structure.

To examine how dust affects disk colors, models generated synthetic images across visible and near-infrared wavelengths. The images represented both total scattered light and polarized light, whose waves oscillate in a preferred direction. Researchers tested spherical and irregular grains, as well as compact and fractal aggregates. Comparing these modeled colors showed how particle size, shape, and composition change the light from a disk.

For aggregates larger than the wavelength of light, the size of their smaller component grains, called monomers, affects their colors. The overall aggregate size has little effect, especially for aggregates with a less complex, lower-fractal structure. Polarized-light color responds more strongly to dust properties than total-intensity color because it reflects changes in both scattered brightness and the degree of polarization. A simplified method for estimating the optical properties of mixtures works for compact aggregates with small monomers, but becomes unreliable for more porous aggregates or larger monomers. Dust reflectivity can help predict colors, but models must also account for light scattering multiple times within the disk.

Key findings
HD 100546's red optical-to-near-infrared disk colors are consistent with amorphous-carbon aggregates containing large monomers.
The colors rule out compact grains and organic-rich compositions as explanations.
Polarized-light color is more sensitive to dust properties than total-intensity color.
For aggregates larger than the light wavelength, monomer size influences color, while aggregate size has little effect in many cases.
Effective-medium estimates become inaccurate for highly porous aggregates or aggregates with larger monomers.

Scientific study

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