First science results from SOFIA/FORCAST: The mid-infrared view of the
  compact HII region W3A
  The massive star forming region W3 was observed with the faint object
infrared camera for the SOFIA telescope (FORCAST) as part of the Short Science
program. The 6.4, 6.6, 7.7, 19.7, 24.2, 31.5 and 37.1 \um bandpasses were used
to observe the emission of Polycyclic Aromatic Hydrocarbon (PAH) molecules,
Very Small Grains and Big Grains. Optical depth and color temperature maps of
W3A show that IRS2 has blown a bubble devoid of gas and dust of $\sim$0.05 pc
radius. It is embedded in a dusty shell of ionized gas that contributes 40% of
the total 24 \um emission of W3A. This dust component is mostly heated by far
ultraviolet, rather than trapped Ly$\alpha$ photons. This shell is itself
surrounded by a thin ($\sim$0.01 pc) photodissociation region where PAHs show
intense emission. The infrared spectral energy distribution (SED) of three
different zones located at 8, 20 and 25\arcsec from IRS2, show that the peak of
the SED shifts towards longer wavelengths, when moving away from the star.
Adopting the stellar radiation field for these three positions, DUSTEM model
fits to these SEDs yield a dust-to-gas mass ratio in the ionized gas similar to
that in the diffuse ISM. However, the ratio of the IR-to-UV opacity of the dust
in the ionized shell is increased by a factor $\simeq$3 compared to the diffuse
ISM.
