Characterizing the PAH Emission in the Orion Bar

TL;DR

This study uses infrared spectroscopy to analyze PAH emissions in the Orion Bar PDR, revealing how PAH characteristics vary with environmental factors and providing insights into their chemical evolution and environmental tracing capabilities.

Contribution

It provides detailed spatial characterization of PAH emissions in the Orion Bar, highlighting the influence of viewing angle and radiation field on PAH properties, which is novel in PDR studies.

Findings

PAH emission peaks between ionization and PDR fronts.

Variations in PAH band intensities depend on viewing angle and radiation strength.

PAHs effectively trace environmental properties like FUV radiation and ionization.

Abstract

We present 5--14~$\mu$m spectra at two different positions across the Orion Bar photodissociation region (PDR) obtained with the Infrared Spectrograph onboard the Spitzer Space Telescope and 3.3~$\mu$m PAH observations obtained with the Stratospheric Observatory for Infrared Astronomy (SOFIA). We aim to characterize emission from Polycyclic Aromatic Hydrocarbon (PAH), dust, atomic and molecular hydrogen, argon, sulfur, and neon as a function of distance from the primary illuminating source. We find that all the major PAH bands peak between the ionization front and the PDR front, as traced by H$_{2}$, while variations between these bands become more pronounced moving away from this peak into the face-on PDRs behind the PDR front and at the backside of the \HII\, region. While the relative PAH intensities are consistent with established PAH characteristics, we report unusual behaviours and attribute these to the PDR viewing angle and the strength of the FUV radiation field impinging on the PDRs. We determine the average PAH size which varies across the Orion Bar. We discuss subtle differences seen between the cationic PAH bands and highlight the photo-chemical evolution of carbonaceous species in this PDR environment. We find that PAHs are a good tracer of environmental properties such as the strength of the FUV radiation field and the PAH ionization parameter.