Scientists are elucidating the complex behaviour of polycyclic aromatic hydrocarbons (PAHs) within the Orion Bar, a prominent photodissociation region. Alexandros Maragkoudakis, Christiaan Boersma, and Els Peeters, from their respective institutions, alongside Louis J. Allamandola, Pasquale Temi, and Vincent J. Esposito et al., present a detailed analysis of PAH charge states and size distributions across this crucial interstellar environment. Their research, utilising data from the “PDRs4All” ERS Program and the pyPAHdb modelling tool, demonstrates how PAH evolution varies significantly between the HII region, the APDR, and dissociation fronts, revealing insights into both top-down and bottom-up formation processes. This work is significant because it establishes a robust method for characterising PAH contributions using comprehensive spectral modelling, offering a more reliable approach than traditional empirical tracers and furthering our understanding of PAH processing in harsh ultraviolet environments.

Utilising the NASA Ames PAH Infrared Spectroscopic Database and the pyPAHdb spectral modelling tool, the work reveals detailed insights into the lifecycle of PAHs and their response to intense ultraviolet light.

The modelling reveals that cationic PAH emission is strongest in the atomic PDR, where neutral PAHs are minimal. Conversely, emission from neutral PAHs peaks in the HII region and molecular cloud regions beyond the second dissociation front. Notably, PAH anions are observed deep within the second and third dissociation fronts, indicating a complex interplay of ionisation and molecular formation.
This detailed mapping of PAH charge states provides a new understanding of the chemical processes occurring within these regions. A key finding of this study is the determination that the average size of the PAHs in the Orion Bar ranges from approximately ~60-74 NC, offering a precise measurement of their physical dimensions.

The research indicates that PAHs closer to the ionisation front are more extensively processed by ultraviolet radiation, leading to changes in their size and structure. Within the molecular cloud, however, PAHs are less affected by this radiation, preserving their original characteristics. This variation supports a model of ‘top-down’ PAH formation at the ionisation front and ‘bottom-up’ formation within the molecular cloud.

Furthermore, the study demonstrates that empirical intensity ratios used to trace PAH ionisation are reliable in regions dominated by edge-on or face-on PDR emission, but become less accurate within the molecular cloud zone. Comprehensive characterisation of neutral and cationic PAH contributions is achieved through pyPAHdb modelling of the 5, 15μm spectrum, surpassing the limitations of simpler empirical tracers. The pyPAHdb tool was central to this work, allowing for the separation of overlapping PAH emission features and the determination of the fractional contribution of different PAH charge states and sizes to the total emission.

Modelling was conducted by comparing the observed spectra with a library of theoretical PAH spectra within the PAHdb, refining the model until a robust fit was achieved. This process yielded quantitative measurements of PAH properties, including charge state and size distribution, in each of the defined physical zones.

Specifically, the average PAH size in the Orion Bar was determined to range between ~60, 74 NC, providing a concrete measurement of their physical dimensions. The research revealed regions of top-down PAH formation at the ionisation front and bottom-down PAH formation within the molecular cloud region, indicating differing formation pathways dependent on the local environment. Furthermore, the PAH ionisation parameter γ was found to range between ~2, 9 × 10⁴, correlating with ultraviolet radiation levels and providing insights into the processing of PAHs in these harsh environments.

PAH size, charge and distribution correlate with physical conditions in the Orion Bar, suggesting their origin and evolution within this region

Researchers determined the average size of polycyclic aromatic hydrocarbons (PAHs) within the Orion Bar to be approximately ~60-74 NC, establishing a concrete measurement of their physical dimensions in this interstellar region. Detailed analysis of the Orion Bar’s PAH population reveals a predictable relationship between PAH charge state, size, and the varying physical conditions across key zones, including the HII region, the atomic PDR, and dissociation fronts DF1, DF2, and DF3.

Modelling with the pyPAHdb tool demonstrates that cationic PAH emission peaks in the atomic PDR, where neutral PAHs contribute minimally to the overall emission. Neutral PAH emission is strongest in the HII region, associated with a face-on PDR linked to the OMC-1 molecular cloud, and also in the molecular cloud regions beyond DF2.

Observations reveal the presence of PAH anions concentrated within the DF2 and DF3 zones, indicating specific formation environments. Small and medium-sized PAHs constitute approximately 70% of the total PAH emission across the mosaic, with peak emission from smaller PAHs located between the DF2 and DF3 zones.

The PAH ionisation parameter, γ, ranges between ~2 −9 × 104, providing a quantitative measure of the ionisation levels within the studied regions. Intensity ratios used as empirical tracers of PAH ionisation correlate well with γ in areas exhibiting edge-on or face-on PDR emission, but this correlation diminishes within the molecular cloud zone.

This work utilizes the NASA Ames PAH Infrared Spectroscopic Database and pyPAHdb to comprehensively characterise the contributions of neutral and cationic PAHs across diverse environments, demonstrating the limitations of empirical PAH proxies outside of PDR-dominated regions. The derived average PAH size is consistent with the expectation that ultraviolet processing is more significant closer to the ionisation front, while PAHs within the molecular cloud are less affected by this radiation.

PAH charge and size correlate with ultraviolet radiation in the Orion Bar, suggesting a radiation-driven process

Scientists have established a clear relationship between the charge and size of polycyclic aromatic hydrocarbons (PAHs) and the ultraviolet radiation levels within the Orion Bar. Analysis of mid-infrared observations reveals variations in PAH properties across different regions of this interstellar environment, including the HII region, the photodissociation region, and dissociation fronts.

Detailed modelling utilising the NASA Ames PAH database and the pyPAHdb tool demonstrates that cationic PAH emission is strongest where ultraviolet radiation is minimal, while neutral and anionic PAHs are prevalent in areas exposed to higher radiation levels. This research provides a more nuanced understanding of PAH lifecycle processes, crucial as these molecules are fundamental building blocks of interstellar matter and contribute to star and planet formation.

The average size of the PAHs studied, ranging from approximately 60 to 74 carbon atoms, offers a concrete measurement of their physical dimensions within the Orion Bar. Furthermore, the findings suggest distinct formation pathways for PAHs, with top-down formation occurring at ionisation fronts and bottom-up formation within molecular clouds.

While empirical intensity ratios effectively trace PAH ionisation in regions dominated by photodissociation regions, their reliability diminishes within molecular clouds. The derived PAH sizes support the idea that ultraviolet processing significantly affects these molecules closer to ionisation fronts, with less alteration occurring deeper within molecular clouds.

The authors acknowledge that empirical PAH tracers can be unreliable outside of regions with strong photodissociation region emission. Future research could focus on expanding these analyses to other star-forming regions to further refine our understanding of PAH evolution and their role in the interstellar medium.

👉 More information
🗞 PDRs4All: XVIII. The evolution of the PAH ionisation and PAH size distribution across the Orion Bar
🧠 ArXiv: https://arxiv.org/abs/2601.23282