Mid-IR imaging of the transitional disk of HD169142: Measuring the size
  of the gap
  The disk around the Herbig Ae star HD\,169142 was imaged and resolved at 18.8
and 24.5\,$\mu$m using Subaru/COMICS. We interpret the observations using a 2D
radiative transfer model and find evidence for the presence of a large gap. The
MIR images trace dust that emits at the onset of the strong rise in the
spectral energy distribution (SED) at 20\,$\mu$m, therefore are very sensitive
to the location and characteristics of the inner wall of the outer disk and its
dust. We determine the location of the wall to be 23$^{+3}_{-5}$\,AU from the
star. An extra component of hot dust must exist close to the star. We find that
a hydrostatic optically thick inner disk does not produce enough flux in the
NIR and an optically thin geometrically thick component is our solution to fit
the SED. Considering the recent findings of gaps and holes in a number of
Herbig Ae/Be group I disks, we suggest that such disk structures may be common
in group I sources. Classification as group I should be considered a support
for classification as a transitional disk, though improved imaging surveys are
needed to support this speculation.
