Orion Bar as a window to the evolution of PAHs

TL;DR

This study uses multi-telescope IR observations of the Orion Bar to model PAH evolution, revealing how PAH properties vary with environment and supporting the attribution of certain IR features to specific PAH types.

Contribution

It introduces a model linking PAH size, hydrogenation, and ionization states to observed IR band ratios, providing insights into PAH evolution in PDRs.

Findings

PAH populations vary with distance from ionizing source

Model matches observed IR band ratios well

Large cations dominate at PDR surface, small neutral PAHs deeper in cloud

Abstract

We investigate the mid-infrared (IR) emission in the Orion Bar photodissociation region, using archival photometric and spectroscopic observations from UKIRT, Spitzer, ISO, and SOFIA telescopes. Specifically, we consider flux densities of the emission bands at 3.3, 3.4, 3.6, 6.6, 7.7, 11.2~$\mu$m in several locations and a spectrum from 3 to 45~$\mu$m in one location. We study the behaviour of band flux ratios, which are sensitive to external conditions, as revealed by their variations with the distance from an ionizing source. Assuming that the mid-IR emission arises mostly from polycyclic aromatic hydrocarbons (PAHs), and that a weak emission feature at 3.4~$\mu$m is related to PAHs with extra hydrogen atoms (H-PAHs), we trace variations of the ratios using a model for PAH evolution. Namely, we estimate how populations of PAHs of different sizes, hydrogenation and ionization states change across the Orion Bar over a time interval approximately equal to its lifetime. The obtained ensembles of PAHs are further used to calculate the corresponding synthetic spectra and band flux densities. The model satisfactorily describes the main features of the ratios $I_{3.6}/I_{11.2}$, $I_{7.7}/I_{11.2}$, $I_{7.7}/I_{3.6}$ and $I_{3.3}/I_{3.4}$. We conclude that the best coincidence between modelling and observations is achieved if C loss of PAHs is limited by the number of carbon atoms $N_{\rm C}=60$, and the band at 3.4~$\mu$m may indeed be attributed to H-PAHs. We confirm that large cations dominate at the surface of the PDR but small neutral PAHs and anions are abundant deeper in the molecular cloud.