PDRs4All XIX. The 6 to 9 $μ$m region as a probe of PAH charge and size in the Orion Bar

TL;DR

This study uses JWST MIRI-MRS data from the Orion Bar PDR to dissect the charge state and size distribution of PAHs via the 6–9 μm AIBs. By analyzing spatial morphologies, cross-band correlations, and synthetic JWST filter images, the authors identify two PAH-size groups and demonstrate that the 6.2/11.2 μm ratio is the most robust tracer of PAH ionization, while 6.2/8.6 and 7.7/8.6 serve as size proxies for the emitting PAHs. They also test JWST photometric prescriptions (filters like F770W, F1130W, etc.) to trace PAH ionization fractions in edge-on versus face-on geometries, showing that photometry can effectively substitute for IFU spectroscopy in extended objects. The findings provide practical diagnostics for PAH chemistry in PDRs and guide future extragalactic JWST studies of PAH emission.

Abstract

Infrared emission from polycyclic aromatic hydrocarbons (PAHs) play a major role in determining the charge balance of their host environments that include photo-dissociation regions (PDRs) in galaxies, planetary nebulae, and rims of molecular clouds. We aim to investigate the distribution and sizes of charged PAHs across the key zones of the Orion Bar PDR. We employ JWST MIRI-MRS observations of the Orion Bar from the Early Release Science program ''PDRs4All'' and synthetic images in the JWST MIRI filters. We investigate the spatial morphology of the AIBs at 6.2, 7.7, 8.6, and 11.0 $μ$m that commonly trace PAH cations, and the neutral PAH-tracing 11.2 $μ$m AIB, their (relative) correlations, and the relationship with existing empirical prescriptions for AIBs. The 6.2. 7.7, 8.6, 11.0, and 11.2 $μ$m AIBs are similar in spatial morphology, on larger scales. Analyzing three-feature intensity correlations, two distinct groups emerge: the 8.6 and 11.0 $μ$m vs. the 6.2 and 7.7 $μ$m AIBs. We attribute these correlations to PAH size. The 6.2 and 7.7 $μ$m AIBs trace cationic, medium-sized PAHs. Quantum chemical calculations reveal that the 8.6 $μ$m AIB is carried by large, compact, cationic PAHs, and the 11.0 $μ$m AIB's correlation to it implies, so is this band. The 6.2/8.6 and 7.7/8.6 PAH band ratios thus probe PAH size. We conclude that the 6.2/11.2 AIB ratio is the most reliable proxy for charged PAHs, within the cohort. We outline JWST MIRI imaging prescriptions that serve as effective tracers of the PAH ionization fraction as traced. This study showcases the efficacy of the 6-9 $μ$m AIBs to probe the charge state and size distribution of the emitting PAHs, offering insights into the physical conditions of their host environments. JWST MIRI photometry offers a viable alternative to IFU spectroscopy for characterizing this emission in extended objects.

Figures (10)

• Figure 1: The AIB template spectrum for the Orion Bar Atomic PDR, showcasing the 6.2, 7.7, 8.6, 11.0, and 11.2 µm AIBs shaded in red and indicated by the vertical solid blue lines. The blue curve shows the underlying "global" continuum, the orange curve is the "local" continuum (lc), and the black round symbols mark the anchor points used for the continua determination.
• Figure 2: Spatial distribution of the surface brightnesses of the 6.2, 7.7, 8.6, 11.0, and 11.2 µm AIBs in the Orion Bar PDR, in units of ${\rm W\, m^{-2}\,sr^{-1}}$, and brightness ratios relative to the 11.2 µm AIB. The analysis utilizes the 7.7 and 8.6 µm bands measured using a local continuum (lc; Fig. \ref{['fig:ADPR_spec']}). $\theta^1$ Ori C is located toward the top right of each map. For each map, the range of the corresponding color bar is set between 0.5$\%$ and 99.5$\%$ percentile level for the data, while zero pixels, edge pixels, and pixels covering the two proplyds as seen in the MIRI mosaic, indicated by the black circles, are masked out. The contours trace peak emission for the 11.0 µm AIB (white) and the 11.2 µm AIB (teal). The rectangular apertures of the template spectra for the H ii region, the atomic PDR, DF 1, DF 2, and DF 3, from top to bottom, are shown in gray, the gray lines delineate the IF and the three dissociation fronts, DF 1, DF 2 and DF 3, and the dashed, diagonal white line in the top left map indicates the cut across the MIRI mosaic (position angle of 155.79°).
• Figure 3: Normalized surface brightnesses and their ratios for the 6.2, 7.7, 8.6, 11.0, and 11.2 µm AIBs as a function of distance to the IF (0.228 pc or 113.4 from $\theta^1$ Ori C) along a cut crossing the mosaic (see Fig.~\ref{['fig:maps']} for the location of the cut). Normalization factors are listed in ${\rm W\, m^{-2}\,sr^{-1}}$ in parentheses for each surface brightness. As the cut is not perpendicular to the IF and distances are given along the cut, a correction factor of cos(19.58$\degr$) $=$ 0.942 needs to be applied to obtain a perpendicular distance from the IF.
• Figure 4: Correlations of the 7.7 [lc]/11.2, 8.6 [lc]/11.2, and 11.0/11.2 AIB surface brightness ratios. The Spearman correlation coefficients 'R’ for the variables excluding (including) the points from the H ii region (black), and regions beneath the IF (teal and dark blue), are printed in red (black) in the panels. Only surface brightnesses from every other spaxel are considered in the correlation analyses. The data points are colored according to regions in the mosaic where those pixels are located (top right panel). This visual region-color scheme is as follows: blue (atomic PDR), dark blue (east IF), teal (west IF), purple (DF 1), yellow (region between DF 1 and DF 2), orange (DF 2), light brown (region west of DF 2), brown (region between DF 2 and DF 3), pink (DF 3), and light purple (region beyond DF 3). The contours highlighting DF 2 and DF 3 are identified through enhanced emission of H$_2$ 0-0 S(3) in these DFs. The red circular regions on the color-region map (top right panel) correspond to the protoplanetary disks within the line-of-sight of observations, which have been masked in this analysis.
• Figure 5: Two strongest correlations between ratios of the synthetic images involving F770W, F1130W, and F1500W and the measured 7.7/11.2 spectroscopic PAH emission in the Orion Bar. Here the 7.7 µm feature is measured using the global continuum (Fig. \ref{['fig:ADPR_spec']}). Lines of best-fit through the data excluding (including) the points from the H ii region (black), and the regions near the IF (teal and dark blue), their equations and the Spearman correlation coefficients R are shown in red (black). The star symbols highlight the average values for the atomic PDR (blue), the H ii region (black) and the region near the IF (teal and dark blue).