VLT/NACO Polarimetric Differential Imaging of HD100546 - Disk Structure
  and Dust Grain Properties between 10-140 AU
  We present polarimetric differential imaging (PDI) data of the circumstellar
disk around the Herbig Ae/Be star HD100546 obtained with VLT/NACO. We resolve
the disk in polarized light in the H and Ks filter between ~0.1-1.4" (i.e.,
~10-140 AU). The innermost disk regions are directly imaged for the first time
and the mean apparent disk inclination and position angle are derived. The
surface brightness along the disk major axis drops off roughly with S(r) ~ r^-3
but has a maximum around 0.15" suggesting a marginal detection of the main disk
inner rim at ~15 AU. We find a significant brightness asymmetry along the disk
minor axis in both filters with the far side of the disk appearing brighter
than the near side. This enhanced backward scattering and a low total
polarization degree of the scattered disk flux of 14%(+19%/-8%) suggests that
the dust grains on the disk surface are larger than typical ISM grains.
Empirical scattering functions reveal the backward scattering peak at the
largest scattering angles and a second maximum for the smallest scattering
angles. This indicates a second dust grain population preferably forward
scattering and smaller in size. It shows that, relatively, in the inner disk
regions (40-50 AU) a higher fraction of larger grains is found compared to the
outer disk regions (100-110 AU). Finally, our images reveal distinct
substructures between 25-35 AU physical separation from the star and we discuss
the possible origin for the two features in the context of ongoing planet
formation.
