Tracing PAH Size in Prominent Nearby Mid-Infrared Environments
  We present observations from the First Light Infrared TEst CAMera (FLITECAM)
on board the Stratospheric Observatory for Infrared Astronomy (SOFIA), the
Spitzer Infrared Array Camera (IRAC) and the Spitzer Infrared Spectrograph
(IRS) SH mode in three well-known Photodissocation Regions (PDRs), the
reflection nebulae (RNe) NGC 7023 and NGC 2023 and to the southeast of the
Orion Bar, which are well suited to probe emission from Polycyclic Aromatic
Hydrocarbon molecules (PAHs). We investigate the spatial behaviour of the
FLITECAM 3.3 um filter as a proxy for the 3.3 um PAH band, the integrated 11.2
um PAH band, and the IRAC 8.0 um filter as a proxy for the sum of the 7.7 and
8.6 um PAH bands. The resulting ratios of 11.2/3.3 and IRAC 8.0/11.2 provide an
approximate measure of the average PAH size and PAH ionization respectively. In
both RNe, we find that the relative PAH ionization and the average PAH size
increases with decreasing distance to the illuminating source. The average PAH
sizes derived for NGC 2023 are greater than those found for NGC 7023 at all
points. Both results indicate that PAH size is dependent on the radiation field
intensity. These results provide additional evidence of a rich carbon-based
chemistry driven by the photo-chemical evolution of the omnipresent PAH
molecules within the interstellar medium. In contrast, we did not detect a
significant variation in the average PAH size found in the region southeast of
the Orion Bar and report a peculiar PAH ionization radial profile.
