PDRs4All: XVIII. The evolution of the PAH ionisation and PAH size distribution across the Orion Bar

Abstract

We investigate the evolution of the PAH population's charge state and size across key physical zones in the Orion Bar, which include the HII region, the atomic PDR (APDR), and three HI/H2 dissociation fronts (DF1, DF2, and DF3). Utilising the NASA Ames PAH Infrared Spectroscopic Database (PAHdb) and the pyPAHdb spectral modelling tool, we analysed the MIRI-MRS observations of the Orion Bar from the "PDRs4All" ERS Program. pyPAHdb modelling reveals the fractional contribution of the different PAH charge states and sizes to the total PAH emission across the Orion Bar. Cationic PAH emission peaks in the APDR region, where neutral PAHs have minimal contribution. Emission from neutral PAHs peaks in the HII region that consists of emission from a face-on PDR associated to the background OMC-1 molecular cloud, and in the molecular cloud regions past DF2. PAH anions are observed deep within the DF2 and DF3 zones. The average PAH size ranges between ~$60-74$ Nc. The modelling reveals regions of top-down PAH formation at the ionisation front, and bottom-up PAH formation within the molecular cloud region. The PAH ionisation parameter $γ$ ranges between ~$2-9 x 10^4$. Intensity ratios tracing PAH ionisation scale well with $γ$ in regions encompassing edge-on or face-on PDR emission, but their correlation weakens within the molecular cloud zone. Modelling of the $5-15$ $μ$m PAH spectrum with pyPAHdb achieves comprehensive characterization of the net contribution of neutral and cationic PAHs across different environments, whereas empirical PAH proxy intensity ratio tracers can be highly variable and unreliable outside regions dominated by PDR emission. The derived average PAH size in the different physical zones is consistent with a view of PAHs being more extensively subjected to ultraviolet processing closer to the ionisation front, and less affected within the molecular cloud.

Figures (14)

• Figure 1: Spectral decomposition of the five Orion Bar template spectra in the 5--15 $\mu$m wavelength region. The template spectra are shown in blue, the spectrum after the emission lines removal is shown in orange, the fitted baseline in green, and the resulting isolated PAH emission spectrum in red.
• Figure 2: PAH size (top sub-panels) and charge breakdown (bottom sub-panels) for the spectral templates of the APDR (left) and DF3 (right). Observations are shown in black, total fit in red, and the separate breakdown components are given in the legend of each panel.
• Figure 3: PAH charge breakdown maps. The map of neutral PAHs is shown in the top panel, the PAH cations map in the middle panel, and the PAH anions map in the bottom panel. The colorbar shows the respective charge fractions, and, for each map, the range is scaled between 0.5% and 99.5% percentile. The colourmap has been chosen to emphasize structure.
• Figure 4: PAH size breakdown maps. The map of large PAHs (N$_{\textrm{C}}$$> 70$) is shown in the top panel, the medium-sized PAHs ($50 <$ N$_{\textrm{C}}$$\leq 70$) map in the middle panel, and the small PAHs (N$_{\textrm{C}}$$\leq 50$) in the bottom panel. The colour bar shows the respective size fractions, and for each map, the range is scaled between 0.5% and 99.5% percentile.
• Figure 5: Map of the PAH ionisation parameter ($\gamma$) obtained from the pyPAHdb modelling. White lines indicate the ionisation and dissociation fronts.