The spatial extent of Polycyclic Aromatic Hydrocarbons emission in the
  Herbig star HD 179218
  We investigate in the mid-IR the spatial properties of the PAHs emission in
the disk of HD179218. We obtained mid-IR images in the PAH1, PAH2 and Si6
filters at 8.6, 11.3 and 12.5 mu, and N band low-resolution spectra using
CanariCam on the GTC. We compared the PSFs measured in the PAH filters to the
PSF derived in the Si6 filter, where the thermal continuum dominates. We
performed radiative transfer modelling of the spectral energy distribution and
produced synthetic images in the three filters to investigate different spatial
scenarios. Our data show that the disk emission is spatially resolved in the
PAHs filters, while unresolved in the Si6 filter. An average FHWM of 0.232",
0.280" and 0.293" is measured in the three filters. Gaussian disk fitting and
quadratic subtraction of the science and calibrator suggest a lower-limit
characteristic angular diameter of the emission of circa 100 mas (circa 40 au).
The photometric and spectroscopic results are compatible with previous
findings. Our radiative transfer (RT) modelling of the continuum suggests that
the resolved emission results from PAH molecules on the disk atmosphere being
UV-excited by the central star. Geometrical models of the PAH component
compared to the underlying continuum point at a PAH emission uniformly extended
out to the physical limits of the disk's model. Also, our RT best model of the
continuum requires a negative exponent of the surface density power-law, in
contrast to earlier modelling pointing at a positive exponent. Based on spatial
and spectroscopic considerations as well as on qualitative comparison with
IRS48 and HD97048, we favor a scenario in which PAHs extend out to large radii
across the flared disk surface and are at the same time predominantly in an
ionized charge state due to the strong UV radiation field of the 180 L_sun
central star.
