Why are protoplanetary disks so faint in scattered light?

(sub)micron-sized dust grains are thought to populate the upper layers of protoplanetary disks. Such small grains should be efficient scatterers in the optical and near-infrared, yet these disks are observed to be very faint at those wavelengths.

In this paper, we propose this observed faintness of disks can be explained by the presence of larger, but fluffy dust aggregates rather than small compact particles. The large size causes extreme forward scattering, resulting in a relatively faint disk with a grey to red color, while the phase function appears relatively isotropic at intermediate angles, mimicking the appearance of smaller grains. 

We demonstrate that extreme forward scattering can explain the brightness and color of HD 100546, by comparing detailed radiative transfer models with anisotropic scattering to new and archival scattered light images from the Hubble space telescope.

link: Mulders et al. 2013 (arXiv)

Does the structure of a protoplanetary disk depend on the mass of its host star?

The final outcome of the planet formation process depends on the properties of the protoplanetary disks planets form in.  Some of these properties vary with stellar spectral type due to different stellar masses and luminosity, while others don't. Previous work had found indications that these disks become flatter towards lower stellar masses.

In this work, we investigate whether the disk structure varies with stellar mass from low-mass brown dwarfs to intermediate-mass Herbig Ae stars. Using radiative transfer models including vertical structure calculations with self-consistent dust settling, we constrain the strength of turbulent mixing from fitting a sample of SEDs for each stellar mass group. We find no significant variations in turbulent mixing strength, and develop a set of scaling relations that show the disk structure is independent of stellar mass when considering regions of similar temperature.

link: Mulders & Dominik 2012 (arXiv)