Why circumstellar disks are so faint in scattered light: The case of HD 100546

Scattered light images of circumstellar disks play an important role in characterizing the planet forming environments around young stars. The characteristic size of the scattering dust grains can be estimated from the observed brightness asymmetry between the front and back side of the disk, for example using standard Mie theory. However such models often overpredict their brightness by one or two orders of magnitude, and have difficulty explaining very red disk colors. We aim to develop a dust model that explains simultaneously the observed disk surface brightness, colors and asymmetry in scattered light, focusing on constraining grain sizes. We use the 2D radiative transfer code MCMax with anisotropic scattering to explore the effects of grain size on synthetic scattered light images of circumstellar disks. We compare the results with surface brightness profiles of the protoplanetary disk HD 100546 in scattered light at wavelengths from 0.4 to 2.2 micron. We find that extreme forward scattering by micron sized particles lowers the /effective/ dust albedo and creates a faint and red disk that /appears/ only slightly forward scattering. For the outer (>~100 AU) disk of HD 100546 we derive a minimum grain size of 2.5 micron, likely present in the form of aggregates. Intermediate sized grains are too bright, whereas smaller grains are faint and scatter more isotropically, but also produce disk colors that are too blue. Observed surface brightness asymmetries alone are not sufficient to constrain the grain size in circumstellar disks. Additional information, such as the brightness and colors of the disk are needed to provide additional constraints.