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 \micron 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 \micron 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″ 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 .