Tracing PAH Size in Prominent Nearby Mid-Infrared Environments

Abstract

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.

arXiv comment20 pages, 11 figures, accepted for publication in ApJ

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• PDRs4All: A JWST Early Release Science Program on radiative feedback from massive stars
• High angular resolution near-IR view of the Orion Bar revealed by Keck/NIRC2
• Polycyclic Aromatic Hydrocarbon emission model in photodissociation regions I: Application to the 3.3, 6.2, and 11.2 $\mu$m bands
• Orion Bar as a window to the evolution of PAHs
• Characterizing Spatial Variations of PAH Emission in the Reflection Nebula NGC 1333
• Characterizing the PAH Emission in the Orion Bar