PDRs4All IV. An embarrassment of riches: Aromatic infrared bands in the Orion Bar

Context. Mid-infrared observations of photodissociation regions (PDRs) are dominated by strong emission features called aromatic infrared bands (AIBs). The most prominent AIBs are found at 3.3, 6.2, 7.7, 8.6, and 11.2 µ m. The most sensitive, highest-resolution infrared spectral imaging data ever taken of the prototypical PDR, the Orion Bar, have been captured by JWST . These high-quality data allow for an unprecedentedly detailed view of AIBs. Aims. We provide an inventory of the AIBs found in the Orion Bar, along with mid-IR template spectra from five distinct regions in the Bar: the molecular PDR (i.e. the three H 2 dissociation fronts), the atomic PDR, and the H ii region. Methods. We used JWST NIRSpec IFU and MIRI MRS observations of the Orion Bar from the JWST Early Release Science Program, PDRs4All (ID: 1288). We extracted five template spectra to represent the morphology and environment of the Orion Bar PDR. We investigated and characterised the AIBs in these template spectra. We describe the variations among them here. Results. The superb sensitivity and the spectral and spatial resolution of these JWST observations reveal many details of the AIB emission and enable an improved characterization of their detailed profile shapes and sub-components. The Orion Bar spectra are dominated by the well-known AIBs at 3.3, 6.2, 7.7, 8.6, 11.2, and 12.7 µ m with well-defined profiles. In addition, the spectra display a wealth of weaker features and sub-components. The widths of many AIBs show clear and systematic variations, being narrowest in the atomic PDR template, but showing a clear broadening in the H ii region template while the broadest bands are found in the three dissociation front templates. In addition, the relative strengths of AIB (sub-)components vary among the template spectra as well. All AIB profiles are characteristic of class A sources as designated by Peeters et al. (2002a), except for the 11.2 µ m AIB profile deep in the molecular zone, which belongs to class B 11 . 2 . Furthermore, the observations show that the sub-components that contribute to the 5.75, 7.7, and 11.2 µ m AIBs become much weaker in the PDR surface layers. We attribute this to the presence of small, more labile carriers in the deeper PDR layers that are photolysed away in the harsh radiation field near the surface. The 3.3 / 11.2 AIB intensity ratio decreases by about 40% between the dissociation fronts and the H ii region, indicating a shift in the polycyclic aromatic hydrocarbon (PAH) size distribution to larger PAHs in the PDR surface layers, also likely due to the e ff ects of photochemistry. The


Introduction
A major component of the infrared (IR) emission near starforming regions in the Universe consists of a set of broad emission features at 3.3, 6.2, 7.7, 8.6, 11.2, and 12.7 µm (e.g.Tielens 2008, and references therein).These mid-IR emission features, referred to as aromatic infrared bands (AIBs), are generally attributed to vibrational emission from polycyclic aromatic hydrocarbons and related species upon absorption of interstellar far-ultraviolet (FUV; 6-13.6 eV) photons (Léger & Puget 1984;Allamandola et al. 1985).The AIB spectrum is very rich and consists of the main bands listed above and a plethora of weaker emission features.Moreover, many AIBs are in fact blends of strong and weak bands (e.g.Peeters et al. 2004a).The AIB emission is known to vary from source to source and spatially within extended sources in terms of the profile and relative intensities of the features (e.g.Joblin et al. 1996;Hony et al. 2001;Berné et al. 2007;Sandstrom et al. 2010;Boersma et al. 2012;Candian et al. 2012;Stock & Peeters 2017;Peeters et al. 2017).These remarkably widespread emission features have been described in many diverse astronomical sources, including protoplanetary disks (e.g.Meeus et al. 2001;Vicente et al. 2013), H ii regions (e.g.Bregman 1989;Peeters et al. 2002b), reflection nebulae (e.g.Peeters et al. 2002a;Werner et al. 2004), planetary nebulae (e.g.Gillett et al. 1973;Bregman 1989;Beintema et al. 1996), the interstellar medium (ISM) of galaxies ranging from the Milky Way (Boulanger et al. 1996), the Magellanic Clouds (e.g.Vermeij et al. 2002;Sandstrom et al. 2010), starburst galaxies, luminous and ultra-luminous IR galaxies, and high-redshift galaxies (e.g.Genzel et al. 1998;Lutz et al. 1998;Peeters et al. 2004b;Yan et al. 2005;Galliano et al. 2008), as well as in the harsh environments of galactic nuclei (e.g.Smith et al. 2007;Esquej et al. 2014;Jensen et al. 2017).
A useful observational proxy for studying AIBs is the spectroscopic classification scheme devised by Peeters et al. (2002a) which classifies each individual AIBs based on their profile shapes and precise peak positions (classes A, B, and C).While the AIBs observed in a given source generally belong to the same class, this is not always the case.In particular, the classes in the 6 to 9 µm region do not always correspond to those of the 3.3 and 11.2 AIBs (van Diedenhoven et al. 2004).Class A sources are the most common -they exhibit the "classical" AIBs, with a 6.2 µm AIB that peaks between 6.19 and 6.23 µm, a 7.7 µm complex in which the 7.6 µm sub-peak is stronger than the 7.8 µm sub-peak, and the 8.6 µm feature peaks at 8.6 µm.Class B sources can be slightly redshifted compared to class A, while at the same time the 7.7 µm complex peaks between 7.8 and 8 µm.Class C sources show a very broad emission band peaking near 8.2 µm, and typically do not exhibit the 6.2 or 7.7 µm AIBs.
These three classes were found to show a strong correlation with the type of object considered.The most common AIB spec-⋆ Tim Lee sadly passed away on Nov 3, 2022.trum, class A, is identified in the spectra of photodissociation regions (PDRs), Hii regions, reflection nebulae, the ISM, and galaxies.The most widely used template for class A sources has been the spectrum of the Orion Bar (Peeters et al. 2002a;van Diedenhoven et al. 2004).Class B sources are isolated Herbig Ae/Be stars and a few evolved stars; in fact, evolved star spectra can belong to either of the classes.Class C sources include post-AGB and Herbig Ae/Be stars, as well as a few T-Tauri disks (Peeters et al. 2002a;Bouwman et al. 2008;Shannon & Boersma 2019).More recent work has developed analogous classification schemes for other AIBs and has included a new class D (e.g.van Diedenhoven et al. 2004;Sloan et al. 2014;Matsuura et al. 2014).
Observations using space-based IR observatories -in particular the Short-Wavelength Spectrometer (SWS; de Graauw et al. 1996) on board the Infrared Space Observatory (ISO; Kessler et al. 1996) and the Infrared Spectrograph (IRS; Houck et al. 2004) on board the Spitzer Space Telescope (Werner et al. 2004) -have revealed the richness of AIBs (for a review see e.g.Peeters et al. 2004a;Tielens 2008).However, obtaining a full understanding of the photochemical evolution underlying AIBs has been limited by insufficient spatial and spectral resolution (e.g.Spitzer-IRS) or by limited sensitivity and spatial resolution (e.g.ISO/SWS) of these IR facilities.
JWST is set to unravel the observed complexity of AIBs, as it offers access to the full wavelength range of importance for AIB studies at medium spectral resolution and at unprecedented spatial resolution and sensitivity.JWST is able to resolve, for the first time, where and how the photochemical evolution of polycyclic aromatic related species, the carriers of AIBs, occurs while providing a detailed view of the resulting AIB spectral signatures.The PDRs4All JWST Early Release Science Program observed the prototypical highly irradiated PDR, the Orion Bar (Berné et al. 2022;Habart et al. 2023;Peeters et al. 2023).The Orion Bar PDR has a G 0 which varies with position from about 1 × 10 4 to 4 × 10 4 Habings (e.g.Marconi et al. 1998;Peeters et al. 2023) and it has a gas density which varies from of a few 10 4 cm −3 in the atomic PDR to ∼ 10 6 cm −3 in the molecular region (e.g.Parmar et al. 1991;Tauber et al. 1994;Young Owl et al. 2000;Bernard-Salas et al. 2012a;Goicoechea et al. 2016;Joblin et al. 2018;Habart et al. 2023).Given the proximity of Orion (414 pc; Menten et al. 2007), the PDRs4All dataset takes full advantage of JWST's spatial resolution to showcase the AIB emission in unprecedented detail.
In this paper, we present five MIRI-MRS template spectra representing key regions of the Orion Bar PDR.Combined with corresponding JWST NIRSpec-IFU template spectra (Peeters et al. 2023), we present an updated inventory and characterization of the AIB emission in this important reference source.We describe the observations, data reduction, and the determination of the underlying continuum in our template spectra in Sect. 2. In Sect.3, we give a detailed account of the observed AIB bands and sub-components along with their vibrational assignments.We compare our findings with previous works and discuss the AIB profiles and the AIB variability in the Orion Bar in Sect. 4. Finally, we summarize our results and narrate a picture of the origins and evolution of the AIB emission in Sect. 5.

MIRI-MRS observations and data reduction
On 30 January 2023, JWST observed the Orion Bar PDR with the Mid-Infrared Instrument (MIRI) in medium resolution spectroscopy (MRS) mode (Wells et al. 2015;Argyriou et al. 2023) as part of the PDRs4All Early Release Science program (Berné et al. 2022).We obtained a 1×9 pointing mosaic in all four MRS channels (channels 1, 2, 3, and 4), and all three sub-bands within each channel (short, medium, and long).We applied a 4-point dither optimised for extended sources and use the FASTR1 readout pattern adapted for bright sources.We integrated for 521.7 s using 47 groups per integration and 4 integrations.The resulting datacube thus spans the full MRS wavelength range (4.90 to 27.90 µm) with a spectral resolution ranging from R ∼ 3700 in channel 1 to ∼ 1700 in channel 4 and a spatial resolution of 0.207 ′′ at short wavelengths to 0.803 ′′ at long wavelengths, corresponding to 86 and 332 AU, respectively at the distance of the Orion Nebula.
The mosaic was positioned to overlap the PDRs4All JWST Near Infrared Spectrograph (NIRSpec) IFU (Böker et al. 2022) observations of the Orion Bar (Peeters et al. 2023, Fig. 1).Given the different fields of view of the MRS channels (∼ 3 ′′ in channel 1 to ∼ 7 ′′ in channel 4), the spatial footprint with full wavelength coverage is limited by the field of view (FOV) of channel 1.The footprint shown in Fig. 1 represents the area with full MRS wavelength coverage, noting that the MRS data in channels 2-4 exist beyond the area shown, but we choose to use only the sub-set of data with full wavelength coverage.The NIRSpec IFU and MIRI MRS datasets combined provide a perpendicular cross-section of the Orion Bar from the H ii region to the molecular zone at very high spatial and spectral resolution from 0.97 to 27.9 µm.
We reduced the MIRI-MRS data using version 1.9.5.dev10+g04688a77 of the JWST pipeline1 , and JWST Calibration Reference Data System2 (CRDS) context 1041.We ran the JWST pipeline with default parameters except the following.The master background subtraction, outlier detection, fringe-and residual-fringe correction steps were all turned on.Cubes were built using the drizzle algorithm.The pipeline combined all pointings for each sub-band, resulting in 12 cubes (4 channels of 3 sub-bands each) covering the entire field of view.
We stitched the 12 sub-band cubes into a single cube by reprojecting all of the cubes onto a common spatial grid using channel 1 short as a reference.We then scaled the spectra to match in flux where they overlap using channel 2 long as the reference.This stitching algorithm is part of the "Haute Couture" algorithm described in Canin et al., (in preparation).
While the pipeline and reference files produce high-quality data products, a few artefacts still remain in the data.The most important artifacts for our analysis are fringes and flux calibration that are not yet finalised (see Appendix B).Neither of these artefacts have a strong impact on our results, as discussed in Appendix B, although we do limit our analysis to wavelengths ≤ 15 µm due to the presence of artefacts beyond this range.

Extracting template spectra from key regions
We extracted MIRI template spectra using the same extraction apertures as Peeters et al. (2023) 3 .These apertures are selected to represent the key physical zones of the Orion Bar PDR: the H ii region, the atomic PDR, and the three bright H i / H 2 dissociation fronts (DF 1, DF 2, and DF 3) corresponding to three molecular hydrogen (H 2 ) filaments that were identified in the NIRSpec FOV (Fig. 1).We emphasize that the remaining areas in the MRS spectral map will be analysed at a later time.We note that the AIB emission detected in the H ii region template originates from the background PDR.Combined with the NIR-Spec templates of Peeters et al. (2023), these spectra capture all of the AIB emission in each of the five regions.In this paper, we focus on the inventory and characterization of the AIBs found in these template spectra.We refer to Peeters et al. (2023) and Van De Putte et al. (2023) for the inventories of the gas lines from atoms and small molecules extracted from NIRSpec and MIRI MRS data, respectively.For a detailed description of the Orion Bar PDR morphology as seen by JWST we refer to Habart et al. (2023) and to Peeters et al. (2023).

Measuring the underlying continuum
The AIB emission is perched on top of the continuum emission from stochastically heated very small grains (e.g.Smith et al. 2007).Different spectral decomposition methods have deduced additional emission components referred to as: 1) emission from evaporating very small grains (based on the blind signal separation method; Berné et al. 2007;Pilleri et al. 2012;Foschino et al. 2019) and 2) plateau emission due to large PAHs, PAH clusters and nanoparticles (Bregman et al. 1989;Roche et al. 1989;Peeters et al. 2012;Boersma et al. 2014;Sloan et al. 2014;Peeters et al. 2017).
In order to identify and characterize AIBs, we subtracted estimates of the continuum emission in each template spectrum.We computed a linear continuum for NIRSpec and a spline continuum anchored at selected wavelengths for MIRI data.Furthermore, we adopted the same anchor points for all five templates.While our measurements of the full-width at half-maximum (FWHM) of AIBs (see Table 1) do depend on the selected continuum, we note that our main goal -to catalog AIBs and their sub-components qualitatively -does not require highly precise estimates of the continuum.The FWHM4 of each AIB complex was measured by normalizing the continuum-subtracted template spectrum by the peak intensity of the AIB and then calculating the FWHM of the entire AIB complex, that is, without taking into consideration blends, components, and/or sub-components that make up the AIB complex.The measured FWHM strongly depends on the estimated continuum emission; however, this does not impact qualitative trends in FWHM from template to template.The integrated flux of each AIB was computed from the continuumsubtracted spectra.We refer to Peeters et al. (2023) for details on how the 3.3 and 3.4 µm AIB fluxes were measured.

AIB characteristics and assignments
The superb quality of the Orion Bar observations combined with the increased spectral resolution compared to prior IR space observations reveals an ever-better characterization of the AIBs in terms of sub-components, multiple components making up a "single" band, and the precise shapes of the band profiles (see Fig. 2 for an overview and Figs. 3 and 4 for zoom-ins of selected AIBs in the five template spectra).
We offer a detailed description of the spectral characteristics of the AIB emission as seen by JWST as well as current vibrational assignments in Sects.3.1 to 3.6.The detailed AIB inventory is listed in Table A.1.We note that we consider all spectrally resolved emission features to be candidate AIBs.To assess whether a candidate AIB is real or an artefact, we compare the template spectra in the location of the candidate AIB to the spectrum of the calibration standard star 10 Lac (see Appendix B for details).Due to the very high signal-to-noise ratio (S/N), the spectra reveal an abundance of weak features, either as standalone features, or as shoulders of other bands.Occasionally these shoulders are only visible as a change in the slope along the wing of a stronger AIB and, in such cases, we estimated the central wavelength of the weak AIB visually based on the AIB profile of the main component.
Hereafter, all mentions of nominal AIBs, given in boldface in col. 1 of Table A .1, namely 3.3, 3.4, 5.25, 5.75, 6.2, 7.7, 8.6, 11.2, 12.0, 12.7, and 13.5 µm, do not indicate the precise peak positions of these AIBs.The precise peak positions of these nominal AIBs are reported in terms of wavelength in col. 3 of Table A.1.We note that we converted the positions in wavelength to wavenumber by rounding to the nearest integer in units of cm −1 and so the precision of the reported wavenumbers does not reflect the instrumental precision of the peak position of the AIBs.
3.1.The 3.2-3.5 µm (3125-2860 cm −1 ) range The 3 µm spectral region is dominated by the 3.3 and 3.4 µm AIBs that peak at 3.29 and 3.4 µm, respectively.While some studies have found that the peak of the 3.3 µm AIB shifts toward longer wavelengths (van Diedenhoven et al. 2004), our measurements of the peak position of this band in each template are consistent with the nominal value of 3.29 µm.The band profiles, however, show some slight differences among the templates.The width of the 3.29 µm band varies slightly (see Table 1 and Fig. 5).The templates in order of increasing 3.29 µm band width are: atomic PDR, DF 1, H ii region, DF 2, and DF 3.While the small increase in width on the red side may be attributed to underlying broad plateau emission (see Peeters et al. 2023), the blue wing broadens by ∼5 cm −1 on a total band width of ∼ 37.5 cm −1 (Table 1 and Peeters et al. 2023).
Article number, page 4 of 25 The 3.29 µm band is characteristic for the CH stretching mode in PAHs.The peak position is somewhat dependent on molecular structure, for example the number of adjacent hydrogens on a ring and steric hindrance between opposing hydrogens in so-called 'bay' regions.Molecular symmetry has a more important effect as it controls the number of allowed transitions and the range over which IR activity is present (Maltseva et al. 2015(Maltseva et al. , 2016)).For a given PAH, the initial excitation energy has a very minor influence on the peak position (Mackie et al. 2022).Earlier works have also demonstrated this minor influence (Joblin et al. 1995;Pech et al. 2002).Additionally, Tokunaga & Bernstein (2021) found that the peak position and width of the 3.3 µm feature must be fitted simultaneously as both depend on the carrier.There is also a weak dependence of peak position on the charge state, but since the CH stretch is very weak in cations (Allamandola et al. 1999;Peeters et al. 2002a), this is of no consequence.These modes are very much influenced by resonance effects with combination bands involving CC modes and CH in-plane bending modes (Mackie et al. 2015(Mackie et al. , 2016)).Overall, in the emission spectra of highly excited species, the differences in peak position mentioned here will be too subtle compared to the impact of molecular symmetry when attempting to identify the carrier(s).The observed narrow width of the 3.3 µm AIB implies then emission by very symmetric PAHs (Pech et al. 2002;Ricca et al. 2012;Mackie et al. 2022).
The very weak shoulder on the blue side, namely, at ≃ 3.246 µm, has the same strength relative to the main feature in all template spectra, suggesting it is part of the same emission complex.Its peak wavelength may point toward the stretching mode of aromatic CH groups in bay regions or, alternatively, the effect of resonant interaction in a specific species (van Diedenhoven et al. 2004;Candian et al. 2012;Mackie et al. 2015) or aromatic CH in polyaromatic carbon clusters (Dubosq et al. 2023).In a recent analysis of the 3.3 µm AIB in the Red Rectangle, Tokunaga et al. (2022) found differences in the spectra of this source compared to earlier analyses (e.g.Tokunaga et al. 1991;Candian et al. 2012) due to the treatment of Pfund emission lines from the standard star.Candian et al. (2012) fit the 3.3 µm AIB in each spaxel of their IFU cube with two components and analysed spatial variations in the integrated intensities of these components.While issues with the standard star spectrum would affect all spectra in the cube (Tokunaga et al. 2022), spatial variations in integrated intensities should not be affected.
The AIB spectra reveal a plethora of bands longward of the 3.29 µm feature between ≃ 3.4 and ≃ 3.6 µm (Table A .1;Peeters et al. 2023;Sloan et al. 1997).As the relative strengths of these sub-components show variations from source to source and within sources (e.g.Joblin et al. 1996;Pilleri et al. 2015;Peeters et al. 2023), they are generally ascribed to different emitting groups on PAHs.Here, we note that the emission profile of the 3.4 µm band varies between the five templates, broadening to longer wavelength (Peeters et al. 2023), indicating the presence of multiple components in the main 3.4 µm band at 3.395, 3.403, and 3.424 µm.The other bands do not show such profile variations.Bands in this wavelength range are due to the CH stretching mode in aliphatic groups and assignments to methyl (CH 3 ) groups attached to PAHs and to superhydrogenated PAHs have been proposed (Joblin et al. 1996;Bernstein et al. 1996;Maltseva et al. 2018;Buragohain et al. 2020;Pla et al. 2020).As for the aromatic CH stretching mode, the peak position is sensitive to resonances with combination bands of CC modes and CH in-plane bending modes (Mackie et al. 2018).Typically, methylated PAHs show a prominent band around 3.4 µm, but its peak position falls within a wide range, ≃ 0.17 µm (Maltseva   5.75 and 5.878, 6.2, 8.6, 11.2 and 12.0, 12.7, 13.5, and 14.2 µm AIBs (indicated in the y-axis label of each panel).The panels show wavelength ranges that are also shown in Fig. 3, except for the panels that are centered on the 13.5 and 14.2 µm AIBs (small panels in the lower right).These figures illustrate the overall similarity and subtle differences in AIB profiles from region to region.et al. 2018;Buragohain et al. 2020).For hydrogenated PAHs, the main activity is at slightly longer wavelengths, ≃ 3.5 µm within a somewhat narrower range (≃ 0.05 µm).As the extra hydrogens in superhydrogenated PAHs are relatively weakly bound (1.4-1.8 eV; Bauschlicher & Ricca 2014), astronomical models imply that superhydrogenated PAHs quickly lose all these sp 3 hydrogens in strongly irradiated PDRs (e.g. when G 0 /n(H) > 0.03; Andrews et al. 2016).
For further analysis of the AIB emission in this region, including the many weaker features listed in Table A.1, we refer to Peeters et al. (2023).1).All values are normalised to those from the atomic PDR.The ordering of the regions on the x-axis is arbitrary, but was chosen so that the data roughly show an increasing trend in FWHM from left to right (Sect.4.3).
blue shoulder centered at ∼ 5.18 µm and extending to about 5.205 µm, followed by a sharp blue rise to a peak at 5.236 µm and a strong red wing extending to about 5.38 µm.A detailed inspection of the profiles reveals a very weak feature at ∼ 5.30 µm superposed on the red wing.Comparing the five template spectra, we conclude that the 5.25 µm feature broadens, in particular on the red side, with the narrowest feature seen in the atomic PDR, and then increasing in width in the H ii region, DF 1, DF 2, and DF 3 (Table 1 and Fig. 5).Besides the broadening, the observed profiles are very similar for the five templates, implying that the main feature consists of a single band.Inspection of the template spectra (Fig. 4) reveals that the 5.75 µm band is a blend of three bands at 5.642, 5.699, and 5.755 µm (e.g.comparing the atomic PDR and DF 3 spectra in Fig. 4).The MIRI MRS spectra clearly exhibit a new, symmetric feature at 5.878 µm.We also report a tentative detection of two very weak features at 5.435 and 5.535 µm.
The spectra of PAHs show weak combination bands in this wavelength range generated by modes of the same type, for example, out-of-plane (OOP) modes (Boersma et al. 2009b,a).3)-( 7) FWHM (cm −1 ) of AIB complex from indicated template spectrum; (8) peak-to-peak variation in FWHM (cm −1 ) observed in the templates (see Fig. 5).
Combination bands involving in-plane modes occur at shorter wavelengths (3.8 − 4.4 µm) and are typically an order of magnitude weaker (Mackie et al. 2015(Mackie et al. , 2016)).Combination bands involving the OOP bending modes typically result in a spectrum with two relatively simple AIBs near 5.25 and 5.75 µm.
For small PAHs, the ratio of the intrinsic strength of these bands to the OOP modes increases linearly with PAH size (Lemmens et al. 2019).This ratio increases further for the larger PAHs studied in Lemmens et al. (2021).However, whether the correlation continues linearly is yet to be confirmed.
3.3.The 6.2 µm (1610 cm −1 ) AIB The interstellar 6.2 µm band is one of the main AIBs.The profile peaks at 6.212 µm (1610 cm −1 ) and has a steep blue rise, a pronounced red wing, and a blue broad shoulder centered at ∼ 6.07 µm.Comparing the five template spectra, we conclude that the feature broadens toward the blue side at the same time that the red wing becomes more pronounced (by about 8.2 cm −1 on a total width of ≃ 33.6 cm −1 ; Table 1).Pending confirmation, the peak position possibly varies (its value ranges from 6.2115 to 6.2161 µm).There is a distinct weaker feature at 6.024 µm (1660 cm −1 ) superposed on the blue shoulder.This symmetric feature has a constant width and varies in intensity independently of the main feature (see Fig. 4).This suggests that the 6.024 µm band is an independent component.We note that the observed strength variations of the 6.024 µm band do not affect the conclusion on the broadening of the blue side of the 6.2 µm band.
There is a very weak feature perched on the red wing at 6.395 µm (1564 cm −1 ) in the template of the atomic region.It may be obscured by the stronger red wing in the other template spectra.A very subtle change in slope of the red wing may also be present near 6.5 µm in some templates (e.g. the atomic PDR).Pure aromatic CC stretching modes fall between 6.1 and 6.5 µm.In the 6-9 µm wavelength range, the number of bands and their precise positions will depend on charge, molecular structure, size, and heterogeneity.In particular, their intrinsic strengths are very sensitive to the charge state of the species, increasing by about a factor of 10 for cations (Allamandola et al. 1999;Peeters et al. 2002a;Bauschlicher et al. 2008).Given the Article number, page 8 of 25 observed strength of the 6.2 µm band relative to the CH stretching and OOP bending modes that dominate the neutral spectra, this AIB is attributed to PAH cations (Allamandola et al. 1999).In the past, the peak position was somewhat of an enigma.In early comparisons with harmonic calculations, this band arose at too red a wavelength in PAH cations.This problem was compounded by the adoption of a redshift of 15 cm −1 to account for anharmonic effects during the emission cascade (for a summary, see Bauschlicher et al. 2009).However, model studies have revealed that anharmonicity introduces a red wing on the profile but does not lead to an appreciable redshift of the peak (Mackie et al. 2022).Recent experimental and quantum chemical studies of neutral, symmetric PAHs have shown that the mismatch between the experimental and interstellar 6.2 µm band positions is less severe than thought (Lemmens et al. 2021).Furthermore, quantum chemical studies on PAH cations have employed the cc-pVTZ basis set that better accounts for treatment of polarization in PAHs.With this basis set used in density functional theory calculations, the calculated peak position of the aromatic CC stretching mode in cations is in much better agreement with the observations (Ricca et al. 2021), but this still needs to be confirmed by experimental studies on PAH cations.The discrepancy noted in earlier studies between the peak position of the 6.2 µm AIB and the aromatic CC stretch in PAHs has prompted a number of suggestions.Specifically, incorporation of heteroatoms such as N into the ring backbone or coordination of atoms such as Si, the presence of aliphatic structures, protonated PAHs, and/or (pentagonal) defects will induce blue shifts in the peak position of this mode (Hudgins et al. 2005;Pino et al. 2008;Joalland et al. 2009;Carpentier et al. 2012;Galué 2014;Tsuge et al. 2018;Wenzel et al. 2022;Rap et al. 2022).Further studies are warranted to assess whether these suggestions are still relevant.
The observed 6.024 µm band is at too short a wavelength to be an aromatic CC stretching vibration.Rather, this position is characteristic of the C=O stretch in conjugated carbonyl groups; that is, as quinones or attached to aromatic rings (Allamandola et al. 1989b;Sarre 2019).This band has not been the focus in many quantum chemical studies.We also note that the very weak feature at 6.395 µm is likely another aromatic CC stretching mode.
3.4.The 7.7 µm (1300 cm −1 ) AIB complex It has been well established that the 7.7 µm AIB is a blend of several features (Bregman 1989;Cohen et al. 1989;Peeters et al. 2002a).The JWST spectra reveal that the main component at 7.626 µm is accompanied by moderately strong bands at 7.8 and 7.85 µm.The 7.8 µm component appears narrower in DF 2 and DF 3, peaking near 7.743 µm, although this may arise due to differences in the red wing of the 7.626 µm component or this may reflect the lack of a different component at 7.775 µm present in the atomic PDR, the H ii region and DF 1.In any case, given the observed variations between the templates, these bands are independent components.The 7.7 µm AIB complex as a whole broadens significantly from the atomic to the molecular region (by about 18.8 cm −1 , Table 1).In addition to these moderately strong components, there are also weak features at 7.24 and 7.43 µm and between the 7.7 and 8.6 complexes at 8.223 and 8.330 µm.Very weak features are also present at shorter wavelengths (6.638, 6.711, 6.850, 6.943, 7.05, and 7.10 µm).
Bands in this wavelength range are due to modes with a mixed character of CC stretching and CH in-plane bending vibrations.As mentioned in Sect.3.3, the strength of these modes is very dependent on the charge state of the species and the interstellar 7.7 µm AIB is generally attributed to PAH cations.The spectra of very symmetric PAHs become more complex with increasing size and the main band(s) in the 7.5 − 8.0 µm range shift systematically with size toward longer wavelength from about 7.6 to about 7.8 µm or even larger (Bauschlicher et al. 2008(Bauschlicher et al. , 2009;;Ricca et al. 2012).These quantum chemical calculations point toward compact PAHs in the size range 24 to 100 C atoms as the carriers and probably more toward the smaller size for the main 7.626 µm band and slightly larger for the two moderate components.Detailed spectral decompositions of ISO-SWS and Spitzer-IRS observations agree with these conclusions (Joblin et al. 2008;Shannon & Boersma 2019).The very weak features at 7.24 and 7.43 µm are likely also CC stretching modes.The CH deformation modes of aliphatic groups also occur around 6.8 and 7.2 µm, but these modes are weaker compared to the CH stretching modes of aliphatic groups around 3.4 µm (Wexler 1967;Yang et al. 2016;Dartois et al. 2005) and, given the weakness of the 3.4 µm AIB, we deem that identification unlikely for the weak features at 7.24 and 7.43 µm detected in all templates.The very weak features at 6.850 and 6.943 µm are only present in DF 2 and DF 3. As both these templates also show the strongest 3.4 µm emission, these bands may arise from CH deformation modes of aliphatic groups (Wexler 1967;Arnoult et al. 2000).
3.5.The 8.6 µm (1160 cm −1 ) AIB This AIB peaks at 8.60 µm (1163 cm −1 ).The apparent shift toward shorter wavelengths in the DF 3 spectrum as well as the apparent broadening of the band are likely caused by the change in the underlying "continuum" due to the 7.7 µm AIB and/or plateau emission and/or very small grain emission.The change in slope in the blue wing at 8.46 and 8.54 µm suggests the presence of more than one component in this AIB.However, these components seem to be very weak compared to the main band.There is a similar change in slope at 8.74 µm and potentially at 8.89 µm in all template spectra and this likely has a similar origin.Since these features are very weak, we label them as tentative.
The 8.6 µm AIB is due to CH in-plane bending modes in PAHs, but this mode has a large CC stretching admixture.The intensity of this band increases significantly and it shifts to longer wavelength, producing the very prominent band that appears near 8.5 µm in the spectra of large (N C ∼ 100, N C being the number of C atoms in a PAH molecule) compact PAHs (Bauschlicher et al. 2008).For even larger, compact PAHs, this band starts to dominate the spectra in the 7 − 9 µm range and these species are excluded as carriers of the typical AIB emission (Ricca et al. 2012).In large polyaromatic and aliphatic systems, the geometrical distortions of the C-C backbone and defects, partly related to the hydrogen content, shift the position of this band (Carpentier et al. 2012;Dartois et al. 2020).The weaker features on the blue side of the main band may be due to somewhat smaller and/or less symmetric PAHs while the longer wavelength feature may be due to a minor amount of somewhat larger symmetric, compact PAHs.
3.6.The 10-20 µm (500-1000 cm −1 ) range This wavelength range is dominated by the strong AIB at 11.2 µm, a moderately strong AIB at 12.7 µm and a plethora of weaker AIBs at 10.95, 11.005, 12.0, 13.5, 13.95, 14.21, and 16.43 µm.The 11.2 µm AIB clearly displays two components at 11.207 and 11.25 µm, along with a tentative component at 11.275 µm.The AIB peaks at the first component (11.207 µm) in the atomic PDR, the H ii region, and DF 1, while it peaks at the second component (11.25 µm) in DFs 2 and 3.These two components may shift to longer wavelengths in DFs 2 and 3, however, such shifts are still yet to be confirmed.The relative strengths of these two components vary across the five templates, indicating they are independent components.The combined 11.2 µm profile is asymmetric with a steep blue rise and a red wing.The AIB broadens significantly (by ∼ 5.9 cm −1 on a total width of ∼ 12.2 cm −1 ; Table 1) through the atomic PDR, DF 1, the H ii region, DF 2, and DF 3 in increasing order.This broadening is driven by changes in the red wing though similar but very small changes in the steepness of the blue side are present.An additional weaker component may be present on the red wing at 11.275 µm.In addition, similar to the 5.25 µm AIB, the 11.2 µm AIB displays a broad blue, slow-rising, shoulder from ∼ 10.4 µm up to the start of the steep blue wing.A wellknown weaker AIB is present at 11.005 µm and is superposed on this blue shoulder.
The 12.0 µm band peaks at 11.955 µm and may have a second component at 12.125 µm.The template spectra furthermore display elevated emission between the red wing of the 11.2 µm band and the 12.2 µm band (see e.g.DF 3) suggestive of more complex AIB emission than expected based on the presence of these two bands.However, due to an artefact at 12.2 µm (see Appendix B), confirmation of the second component, the 12.0 µm profile, and this elevated emission between the 11.2 and 12.0 µm bands requires further improvements to the calibration.
The 12.7 µm band is very complex displaying a terraced blue wing and a steep red decline.It peaks at 12.779 µm except in the atomic PDR where it peaks at a second component at 12.729 µm.The strengths of these components vary independently from each other.Three additional terraces are located near 12.38, 12.52, and 12.625 µm and a red shoulder near 12.98 µm suggests the presence of an additional component.Given the complexity of the 12.7 µm band, spatial-spectral JWST maps are required to understand its spectral decomposition into its numerous components.The entire 12.7 µm complex significantly broadens largely on the blue side but also on the red side.We report a broadening (by ∼ 14.2 cm −1 on a total width of ∼ 21.9 cm −1 ; Table 1 and Fig. 5) through the atomic PDR, DF 1, the H ii region, DF 2, and DF 3 in increasing order.
The 13.5 µm band peaks at 13.55 µm and may be accompanied by two additional components at 13.50 and 13.62 µm.The 13.5 µm band seems to broaden as well.We note that several artefacts exist just longwards of this band (Appendix B), hampering its analysis.Hence, future improvements to the calibration and additional observations on a wider range of sources will have to confirm this broadening.These artefacts also limit the detection and analysis of bands in the 14-15 µm range.We detected a band at 14.21 µm and potentially at 13.95 µm, although the latter is just to the red of the artefact at 13.92 µm.
Bands in the 11-14 µm range are attributed to CH OOP bending modes.The peak position and pattern of these bands is very characteristic for the molecular edge structure of the PAH; that is, the number of adjacent H's 5 .The bands making up the 11.2 µm AIB can be ascribed to neutral species with solo H's (Hony et al. 2001;Bauschlicher et al. 2008).The cationic solo 5 Some earlier comparisons of the OOP modes pattern with laboratory and quantum chemical studies included a 15 cm −1 shift to account for anharmonicity.Recent model studies have shown that such a shift is not warranted (Mackie et al. 2022).
H OOP band falls at slightly shorter wavelength than the corresponding solo H OOP band of neutral PAHs and the 11.0 µm AIB has been attributed to cations (Hudgins & Allamandola 1999;Hony et al. 2001;Rosenberg et al. 2011).The 12.7 AIB complex is due to either duo H's in neutral PAHs or trio H's in cations.For species with both solo and duo H's, coupling of the duo with the solo CH OOP modes splits the former into two bands.The sub-components in the 12.7 µm AIB may reflect this coupling and/or it may be caused by contributions of more than one species with duo's.
The weak 12.0 µm AIB can be attributed to OOP modes of duo H's, while the 13.5 µm AIBs likely have an origin in OOP modes of quartet H's in pendant aromatic rings (Hony et al. 2001;Bauschlicher et al. 2008).The weak bands near 14.2 µm could be due to OOP modes of quintet H's.Alternatively, for larger PAHs, CCC skeletal modes are present in this wavelength range (Ricca et al. 2012).
We also detected a band at 16.43 µm.Other weaker bands are present in this region but due to calibration issues (Appendix B), we refrained from characterizing them.

Comparison to previous observations
Overall, in terms of spectral inventory, the observed AIB emission in the 3 µm range is consistent with prior high-quality ground-based observations of the Orion Bar (e.g.Sloan et al. 1997).Likewise, the main characteristics of the AIB emission are also detected in prior observations of the Orion Bar carried out with ISO-SWS (Verstraete et al. 2001;Peeters et al. 2002a;van Diedenhoven et al. 2004) and Spitzer-IRS in short-low mode (Knight et al. 2022a).Furthermore, in retrospect, many (weaker) bands and sub-components of the AIB emission seen by JWST may also be recognised in the ISO-SWS observation of the Orion Bar, but they were too weak and too close to the S/N limit to be reported in previous works.However, as these JWST data have an unparalleled combination of extremely high S/N, spectral resolution and, most importantly, superb spatial resolution, these spectral imaging data reveal already known bands and subcomponents in unprecedented detail allowing for a much improved characterization of the AIB emission.In addition, these spectral imaging data reveal previously unreported components (blends) and sub-components of the AIB emission (indicated in Table A.1 and discussed in Sect.3).
The AIBs at 5.75, 7.7, 8.6, 11.2, and 12.7 µm have complex sub-components.Boersma et al. (2009b) noted that the 5.75 AIB has an unusual profile, resembling a blended double-peaked feature.The JWST template spectra indicate that the band is composed of three components with variable strengths.New components are also seen in the 12.7 µm band.Shannon et al. (2016) reported that the 12.7 µm band shifts to longer wavelengths at larger distances from the illuminating star in reflection nebulae.This is consistent with the behavior of this band in the Orion Bar reported here where it reflects the relative intensities of the two components at 12.729 and 12.779 µm.These authors also reported a change in the blue wing.The JWST data now characterizes the components (i.e.terraces) in the blue wing and their relative intensities.
While the sub-components of the 8.6 µm AIB have not, to our knowledge, been reported in the literature, several studies detail sub-components in the 7.7, 11.2, and 12.7 µm AIBs.The 7.7 µm AIB complex is composed of two main sub-components at ∼7.626 and ∼7.8 µm (Cohen et al. 1989;Bregman 1989; Article number, page 10 of 25 Peeters et al. 2002a).The 7.7 µm AIB complex is distinguished into four classes (A, B, C, and D) primarily based on its peak position (Peeters et al. 2002a;Sloan et al. 2014;Matsuura et al. 2014).Spectral-spatial imaging has revealed that the (class A) 7.7 µm profile varies within extended ISM-type sources and depends on the local physical conditions: the 7.8 µm component gains in prominence relative to the 7.626 µm component and is accompanied by increased emission "between" the 7.7 µm and 8.6 µm AIBs in regions with less harsh radiation fields (Bregman & Temi 2005;Berné et al. 2007;Pilleri et al. 2012;Boersma et al. 2014;Peeters et al. 2017;Stock & Peeters 2017;Foschino et al. 2019;Knight et al. 2022b).Our findings using the JWST Orion Bar templates (Fig. 3) are consistent with these past results.Pilleri et al. (2012) attributed this to an increased contribution of evaporating very small grains (eVSGs).In addition, the JWST data reveal that the 7.8 µm component is composed of three components whose relative contribution varies.
Likewise, the 11.2 µm AIB has been classified into class A 11.2 , B 11.2 , and A(B) 11.2 .Class A 11.2 peaks in the 11.20-11.24µm range and displays a less pronounced red wing relative to the peak intensity (corresponding to a FWHM of ∼0.17 µm), class B 11.2 peaks at ∼11.25 µm and shows a more pronounced red wing (FWHM of ∼0.20 µm), and class A(B) 11.2 is a mix with a peak position as that of class A 11.2 and prominence of its red wing as that of class B 11.2 (resulting in a FWHM of ∼0.21 µm; van Diedenhoven et al. 2004).As for the 7.7 AIB complex, ISM-type sources display a class A 11.2 profile.Recent spectralimaging data however indicated that the 11.2 µm profile shifts to slightly longer wavelengths accompanied with a stronger red wing relative to the peak intensity in two (out of 17) positions of the Orion Veil (Boersma et al. 2012) and in two reflection nebulae (Boersma et al. 2013;Shannon et al. 2016).These authors classified these profile variations as a shift from class A 11.2 to class A(B) 11.2 , which, in the case of the two reflection nebulae, occurred when moving away from the illuminating star.Boersma et al. (2014) linked the change in the 11.2 µm profile to a change in the 7.7 µm AIB complex (probed by the 11.2/11.3and 7.6/7.8intensity ratios, respectively).A change in peak position along with a broadening of the profile is consistent with the JWST templates of the Orion Bar.As discussed in Sect.3, the change in the peak position of the 11.2 µm AIB reflects the relative importance of two components at 11.207 and 11.25 µm that are now clearly discerned in the JWST data.Furthermore, thanks to the increased spectral and spatial resolution, we conclude that DF 3 belongs to class B 11.2 (Fig. 6).Hence, the Orion Bar exhibits class A 11.2 profiles near the surface of the PDR which evolved from class B 11.2 profiles deeper in the molecular zone.
The ISO-SWS observations of the Orion Bar (taken in a 14 ′′ × 20 ′′ aperture) resemble the atomic PDR template, even when centered on DF 3 6 .This resemblance is due to the fact that the AIB emission is significantly stronger in the atomic PDR compared to the molecular PDR (Habart et al. 2023;Peeters et al. 2023) and it dominates the emission within the large ISO/SWS aperture.Hence, the JWST spectrum of the atomic PDR in the Orion Bar (Fig. 2) serves as the updated, high-resolution, more detailed template spectrum for class A AIB emission.The DF 2 and DF 3 templates, which probe regions deep in the molecular PDR, no longer exhibit class A 11.2 profiles while the 3.3, 6.2, 7.7, and 8.6 µm AIBs still clearly belong to class A. A similar situation, where individual targets are found to belong to two classes, has been reported for two targets: the planetary nebula Hb 5 and 6 ISO-SWS observation with TDT of 69501806 (uniquely identifies the ISO observation).the Circinus galaxy (van Diedenhoven et al. 2004).These authors furthermore found that the other two galaxies in their sample display class A profiles for the 3.3, 6.2, 7.7, and 8.6 µm AIBs, while displaying a class A(B) 11.2 AIB profile.This suggests that, out of the main AIBs, the 11.2 µm AIB is the cleanest indicator of the shift from class B to class A.

AIB profiles
Broadly speaking, the prominent AIBs can be separated into three groups: 1) bands with a steep blue rise and a pronounced red wing.The 5.25,6.2,and 11.2 µm AIBs are clear examples.The profiles also often show a shoulder on the blue side, which is considerably weaker than the red wing; 2) bands that are clear blends of multiple components.This group includes the 3.4, 5.75, 7.7, and 12.7 µm AIBs.They typically comprise three or more sub-components.;and 3) bands that seem to be symmetric, often resembling Gaussian profiles.This group includes the 3.3, 5.878, and 8.6 µm AIBs, as well as the 6.024 and 11.005 µm AIBs.These divisions are not entirely strict.The Article number, page 11 of 25 group 1 AIBs typically have very weak features perched on the red and/or blue side of the profile.Likewise, it is conceivable that each of the blended components in the group 2 AIBs might have an intrinsic profile with relatively sharp blue rise and a more gradual red wing that is obfuscated by blending.For example, while the 3.403 µm AIB is often blended with a feature at 3.424 µm, this component is very weak and the 3.403 µm profile resembles that of group 1.We note that while the 11.2 µm AIB is also a blend of three components, the character of the profile is dominated by the presence of a steep blue rise and a pronounced red wing rather than the presence of the sub-components.Therefore, we list the 11.2 µm AIB in group 1.
Profiles with a steep blue rise and a pronounced red wing are characteristic for the effects of anharmonicity (Pech et al. 2002;Mackie et al. 2022).Detailed models have been developed that follow the emission cascade for a highly excited, single PAH and that include the effects of anharmonic interactions based on quantum chemical calculations (Mackie et al. 2022).These models do not contain free parameters besides the size of the emitting species (i.e. the average excitation level after absorption of a FUV photon) and the resulting profiles agree qualitatively well with the observations of group 1 AIBs (Mackie et al. 2022).The results show that the wavelength extent of the red wing depends on the details of the anharmonic coupling coefficients with other modes.The strength of the wing relative to the peak emission is sensitive to the excitation level of the emitting species after absorption of the UV photon (i.e. the initial average energy per mode) and the cascade process (e.g.how fast the energy is "leaking" away through radiative cooling).The steepness of the blue rise is controlled by rotational broadening.Analysis of the profile of the 11.2 µm AIB observed by ISO/SWS suggests emission by a modestly sized PAH (N C ∼ 30; Mackie et al. 2022) but that conclusion has to be reassessed given the presence of more than one component in this AIB in the JWST spectra of the Orion Bar.
Not all bands will show equally prominent anharmonic profiles.In particular, the far-infrared modes in small PAHs are very harmonic in nature (Lemmens et al. 2020) and their profiles would not develop red wings.Likewise, the aromatic CH stretching modes are very susceptible to resonances with combination bands (Maltseva et al. 2015(Maltseva et al. , 2016;;Mackie et al. 2015Mackie et al. , 2016) ) and this interaction dominates their profiles (Mackie et al. 2022).

AIB variability in Orion
Spatial-spectral maps carry much promise to untangle the complexity of the AIBs and possibly link observed variations to the presence of specific carriers.The first forays into this field were based on Spitzer spectral maps.Analysis of the spatial behaviour of individual AIBs and AIB components revealed their interdependence as well as new components (e.g.Boersma et al. 2009b;Peeters et al. 2012;Boersma et al. 2013Boersma et al. , 2014;;Shannon et al. 2016;Peeters et al. 2017).Analyses based on blind signal separation methods have uncovered several distinct components and spectral details (Berné et al. 2007;Joblin et al. 2008;Pilleri et al. 2012;Foschino et al. 2019), but the increased spectral and spatial resolution, as well as the higher sensitivity of JWST, can be expected to take this to a new level.Indeed, some of the spectral details uncovered by previous spatial-spectral studies are now directly detected in the presented JWST data of the Orion Bar.Applications of blind signal separation techniques, as well as spectral fitting using PAHFIT (Smith et al. 2007) and the Python PAH Database (Matthew J. Shannon & Christiaan Boersma 2018) on the full spectral map of the Orion Bar may be a promising ground for additional detections and potential identifications.Here, we address spatial-spectral variations on inspection of the five template spectra.More detailed analyses are deferred to future studies.
While the spectra are rich in components (Table A.1), there is little diversity between the template spectra.All templates show evidence for each sub-components, except possibly for a few very weak bands whose presence may be easily lost in the profiles of nearby strong bands.The most obvious variations are the increased prominence of the sub-components in the 7.7 µm AIB at 7.743 and 7.85 µm and in the 11.2 µm AIB at 11.25 µm in the DF 2 and DF 3 spectra, and the variation in the relative strength of the sub-components of the weak 5.75 µm band.Similarly, the width of many AIBs (3.3, 5.25, 6.2, 7.7, 11.2, and 12.7) broadens significantly (Table 1).This broadening is also systematic, with the FWHM being smallest in the atomic PDR, increasing subsequently in DF 1, followed by the H ii region, then DF 2, and finally DF 3 (Table 1 and Fig. 5).The only exception to this systematic trend is the H ii region having a FWHM smaller than DF 1 (but larger than the atomic PDR) for the 5.25 and 7.7 µm AIBs.As pointed out in Sect.2.2, we note that the AIB emission in the H ii region originates from the background PDR.
There is also some region-to-region variation in the relative strengths of the main AIBs (Fig. 5).The largest variations are seen in the 3.4 µm AIB to 3.3 µm AIB ratio (∼ 100% greater in DF 3 than in the atomic PDR) and in the 3.3 µm AIB to 11.2 µm AIB ratio (∼ 40% greater in DF 3 than in the atomic PDR).Both of these ratios are largest in DF 2 and DF 3, and decrease in the atomic region.Variations in the strength of the CC modes and in-plane CH bending modes (6.2, 7.7, and 8.6 µm) relative to the 11.2 µm OOP modes are more modest (at the 10-20% level).Larger variations occur in the relative strength of the moderate bands, namely, the 5.25, 5.878, 6.024, and 11.955 µm bandswhich are much more pronounced in DF 2 and DF 3 than in the atomic zone.As noted in Sect.4.1, the ISO-SWS data of the Orion Bar resembles the atomic PDR template.Hence, the range in spectral variability within class A AIBs is well represented by the five templates for all AIBs except the 11.2 µm AIB.For the 11.2 µm AIB, the presented data not only showcase the class A AIB variability but also the shifts from class A to class A(B) and then to class B. It is expected that future JWST observations probing a large range of physical conditions and environments further extend the spectral variability in the AIB emission.
The anharmonic profile of bands due to smaller PAHs will tend to have a less steep blue rise due to the increase in the rotational broadening as well as a slight increase in the width and a more pronounced red wing due the higher internal excitation for the same photon energy (Mackie et al. 2022;Tielens 2021).Hence, the presence of somewhat smaller PAHs may be at the origin (of some) of the overall profile variations in the 3.4, 5.25, 6.2, and 11.2 µm AIBs in the JWST template spectra.We note that the spectrum of the DF 1 template resembles much more the atomic region template than the DF 2 and DF 3 dissociation front templates (Figs. 3 and 4, as well as Table 1).This is likely due to the terraced-field-like structure of the molecular PDR resulting in a strongly enhanced line-of-sight visual extinction through the foreground atomic PDR toward DF 1 compared to DF 2 and DF 3 (Habart et al. 2023;Peeters et al. 2023).Hence, a large contribution from the atomic region in the foreground is contributing to the emission toward DF 1.The overall similarity of the spectra suggests that the PAH family is very robust but has a small amount of additional species in the DF 2 and DF 3 zones that is not present in the surface layers of the PDR.In the Orion Bar, the PDR material is advected from the molecular zone to Article number, page 12 of 25 the ionization front at about 1 km s −1 (Pabst et al. 2019) over ≃ 20, 000 yr.In that period, a PAH will have absorbed some 10 8 UV photons and, yet, apparently the effect on the composition of the interstellar PAH family is only minor as it only results in a change in the prominence of the sub-components in the 7.7 and 11.2 µm AIBs.This likely reflects that for moderate-to-large PAHs (N C ≳ 30), photofragmentation is a minor channel compared to IR emission and, moreover, when fragmentation occurs, the H-loss channel dominates over C loss (Allain et al. 1996a,b;Zhen et al. 2014b,a;Wenzel et al. 2020) and then rapidly followed by rehydrogenation with abundant atomic H (Montillaud et al. 2013;Andrews et al. 2016).We note that the UV field increases by about two orders of magnitude between the H 2 dissociation front and the PDR surface but the atomic H abundance increases by a similar factor as H 2 is increasingly photolysed near the surface.Hence, the ratio of the local FUV field to the atomic hydrogen density, G 0 /n(H), which controls the photoprocessing (Andrews et al. 2016), does not vary much among the five template regions.We thus suggest that the additional species in the deeper layers of the Orion Bar causing the increased prominence of sub-components in the 5.75 (at 5.755 µm), 7.7, and 11.2 µm AIBs, are aromatic species and/or functional groups and/or pendant rings that are more susceptible to photolysis.
Photolytic processing of the PAH family with position in the Orion Bar may also leave its imprint on the PAH size distribution and this will affect the relative strength of the AIBs.The 3.3/11.2AIB ratio has long been used as an indicator of the size of the emitting species (Allamandola et al. 1989b;Pech et al. 2002;Ricca et al. 2012;Mori et al. 2012;Croiset et al. 2016;Maragkoudakis et al. 2020;Knight et al. 2021Knight et al. , 2022a) ) as this ratio is controlled by the 'excitation temperature' of the emitting species and, hence, for a fixed FUV photon energy, by the size.For the five JWST template regions, the 3.3/11.2AIB ratio is observed to vary by about 40%, being largest in DF 3 and decreasing toward DF 2, followed by DF 1, the atomic PDR and the H ii region.The variation in this ratio is slightly less than what is observed in the reflection nebulae NGC 7023 and in the larger Orion region (e.g. the Orion Bar and the Veil region beyond the Orion Bar) and corresponds to an increase in the typical size of the emitting species by about 40% toward the surface (Croiset et al. 2016;Knight et al. 2021Knight et al. , 2022a;;Murga et al. 2022).Hence, we link the decreased prominence of the sub-component in the 5.75 µm (at 5.755 µm), 7.7 µm (at 7.743 and 7.775 µm), and 11.2 µm AIBs (at 11.275 µm) as well as the variation in the 3.3/11.2AIB ratio to the effects of photolysis as material is advected from the deeper layers of the PDR to the surface.
Variations in the 6.2/11.2ratio (Fig. 5) are generally attributed to variations in the ionised fraction of PAH (Peeters et al. 2002a;Galliano et al. 2008;Stock et al. 2016;Boersma et al. 2018).This ratio is only 12% stronger in DF 3 than the surface of the PDR.The limited variations in the 6.2/11.2AIB ratio is at odds with those measured by Spitzer and ISO.Specifically, this ratio is observed to increase by about 50% across the Orion Bar when approaching the Trapezium cluster (Knight et al. 2022a).Moreover, Galliano et al. (2008) measured an increase in this ratio by almost a factor of 2 over (a much wider swath of) the Orion Bar.The PAH ionization balance is controlled by the ionization parameter, G 0 T 1/2 /n e with G 0 , T , and n e the intensity of the FUV field, the gas temperature, and the electron density.The high spatial resolution of JWST allows for a clear separation of the emission at the H 2 dissociation fronts and the PDR surface.The limited variation in the 6.2/11.2AIB ratio is somewhat surprising because the PAH ionizing photon flux differs by about a factor of 40 between the dissociation fronts (located at A V = 2 mag) and the PDR surface; the gas temperature will also increase (slightly) toward the surface, while the electron abundance remains constant over this region (Tielens & Hollenbach 1985).It is also possible that PAH cations contribute an appreciable amount to the 11.2 µm band (Shannon et al. 2016;Boersma et al. 2018).Further modelling will be important to fully understand the complexity of the Orion Bar (A.Sidhu et al., in prep.).
The 3.4/3.3AIB ratio is observed to decrease by about 100% from DF 3 to the atomic PDR (Fig. 5).Such variations have also been seen in other nebulae and attributed to photofragmentation processes (Joblin et al. 1996).The 3.4 µm AIB is attributed to aliphatic CH modes in the form of a minor amount of H bonded to sp 3 C atoms either in the form of methyl groups or as superhydrogenated PAHs (Schutte et al. 1993;Bernstein et al. 1996;Joblin et al. 1996).The abundance of superhydrogenated PAHs is expected to be very small throughout the Orion Bar as such extra H's are readily lost in the strong FUV radiation field (Andrews et al. 2016).Methyl groups are also more easily photolysed than aromatic H's (energy barriers are 3.69, 4.00, and 4.47 eV for CH 2 -H, -CH 3 and aromatic H loss, respectively; Tielens 2021).Recent experiments report that, for cations, this methyl group photolysis can lead to quite stable tropylium formation (loss of H followed by isomerization to a seven-membered ring; Jochims et al. 1999;Zhen et al. 2016;Wenzel et al. 2022).However, further investigation is required to firmly establish the importance of this fragmentation route for conditions present in the interstellar medium.If borne out, the reaction of the tropylium cation with atomic H has a calculated barrier of 3.2 kcal mol −1 (1600 K; Bullins et al. 2009) and, hence, under warm, dense Hrich conditions, the methyl group could be reformed.Hence, in a "suitable" PDR, the species may cycle back and forth between a methyl functional group and the tropylium structure until eventually -CH 3 loss occurs.In any case, it can be expected that the stronger UV field nearer to the surface will reduce the number of CH methyl groups compared to the number of CH aromatic bonds.Further experimental and quantum chemical studies will have to address the competition between the various channels involved in the chemistry of methylated PAHs in PDRs.
The increased importance of PAH photolysis near the surface of the Orion Bar PDR is in line with the GrandPAH hypothesis that only the most resilient species can sustain the harsh conditions of strong FUV radiation fields.Thus, a limited number of compact, large PAHs will dominate the interstellar PAH family in these conditions (Andrews et al. 2015;Tielens 2013).If the conditions are right, large PAHs may even be stripped of all their H's and isomerize to the fullerene, C 60 (Boersma et al. 2012;Berné et al. 2015;Zhen et al. 2014a).We also realize that the presence of somewhat smaller PAHs and the increased importance of aliphatic functional groups deep in the PDR may reflect the importance of ion-molecule and/or radical chemistry during the preceding dark cloud core phase modifying and/or forming PAHs in a bottom-up scenario akin to that proposed for the formation of benzonitrile, indene, and cyanonaphthalene (McGuire et al. 2018;Cernicharo et al. 2021;McGuire et al. 2021).

Conclusions
The superb sensitivity and spectral resolution of JWST have revealed an ever-better characterization of the AIBs in the Orion Bar in terms of sub-components, multiple components making up a "single" band, and band profiles.In addition, the unprecedented spatial resolution of the spectral imaging data showcases the interdependence of the numerous AIB components.
Article number, page 13 of 25 We extracted five template spectra in apertures positioned on the H ii region, the atomic PDR, and the three dissociation fronts DFs 1, 2, and 3.The spectra display a wealth of detail and many weak features have now been firmly identified, their peak positions quantified, and their profiles established.At the same time, the spectra are really very simple.There are a limited number of strong bands with well defined peak positions and red-shaded profiles characteristic for anharmonic interactions.And there is little diversity between the templates.A modest variation is observed in the relative intensities of the main AIBs and of the sub-components of an AIB as well as a systematic broadening of the FWHM of many AIBs (smallest in the atomic PDR and largest in DF 3).Consequently, these templates demonstrate the spectral variations in the class A AIB emission as well as the shift from class B 11.2 (DF 3) to class A 11.2 (atomic PDR).The comparison of the template spectra with the ISO-SWS spectrum of the Orion Bar underscores that the spectrum of the atomic region is the "poster child" for the class A spectrum (Fig. 2; Peeters et al. 2002a;van Diedenhoven et al. 2004).This comparison also demonstrates that in a large aperture, PDRs such as Orion are expected to show class A spectra.Conversely, PDRs with more gentle physical conditions (e.g. in the DF 3) are expected to display a slightly modified class A AIB spectrum (except for the 11.2 µm AIB), showcasing broader AIBs and an increased prominence of minor sub-components with respect to the exemplar class A spectrum.In the case of the 11.2 µm AIB, more gentle physical conditions broaden the AIB and increase the prominence of minor sub-components seen in class A, resulting in a class B 11.2 µm AIB profile.Hence, the templates suggest a shift from class B 11.2 (DF 3) to class A 11.2 (atomic PDR).Further modelling of the PDR physics and chemistry may help to pinpoint the physical and chemical processes that drive these spatial-spectral variations in the Orion Bar.Furthermore, we expect that similar studies of a variety of sources with JWST will provide deeper insight in the origin of the A, B, C, and D classes identified by using measurements obtained with ISO and Spitzer (Peeters et al. 2002a;van Diedenhoven et al. 2004;Sloan et al. 2014;Matsuura et al. 2014).In any case, the spatial-spectral variations in the Orion Bar provide a framework in which AIB spectra of extragalactic regions of massive star formation can be analysed in terms of the physical conditions in their PDRs.
An analysis of the Spitzer spectra of a variety of objects revealed that the mid-IR spectra at the brightest spots in PDRs show remarkably similar AIBs and this has been taken to imply that in the harsh conditions of these positions, the PAH family is dominated by a few species that can withstand the harsh conditions in PDRs (Andrews et al. 2015;Tielens 2013).For now, the limited diversity in the AIB characteristics of the Orion Bar templates points in the same direction.Indeed, a broad distribution of PAHs would result in much more sub-structure and variation behavior.In addition, the disappearance of the (weak) 11.25 µm component of the 11.2 µm AIB in the PDR surface layers implies that photochemistry is important: only the most robust species survive in the harsh conditions at the surface of the PDR.We note that while the cosmic AIB emission can be classified in four classes (A, B, C, and D), interstellar AIB emission invariably belongs to class A. The Orion Bar spectrum is the "poster child" of the class A spectrum -out of all the AIBs found in the Orion Bar template spectra, only the 11.2 µm AIB shows some indication of a class B contribution.Only very distinctly different classes of objects with unique histories display classes B, C, and D AIB emission.This too implies that the interstellar PAH family consists of a small set of very robust species.
Moreover, we conclude that the profiles of the 5.25, 6.2, and 11.2 µm AIBs are controlled by anharmonicity, rather than by blending of a number of bands, while variations in the widths of these bands in the different template spectra are related to variations in the excitation of the emitting species in those positions.As a corollary, this implies that these bands are likely dominated by emission of a single carrier, further supporting the GrandPAH hypothesis (Mackie et al. 2022).However, it remains to be seen whether this spectral similarity still holds when a much larger range of objects is investigated at the higher spectral resolution of JWST.
The much higher spatial resolution of JWST provides further insight in the processes that might be relevant for the composition of the PAH family.Specifically, as argued in Sect.4, the decreased prominence of the minor features in the 5.75, 7.7, and 11.2 µm AIBs in the atomic region indicates the loss of a subpopulation of the PAH family (Berné & Tielens 2012;Montillaud et al. 2013;Berné et al. 2015;Andrews et al. 2016) or the loss of very small grains (Pilleri et al. 2012(Pilleri et al. , 2015) ) in this region.Likewise, the decrease in the methyl group coverage, as evidenced in the variation of the 3.4/3.3AIB ratio, indicates the loss of more loosely bound functional groups in the surface layers or their conversion to aromatic moieties.Models suggest that photolysis of PAHs is controlled by the strength of the UV field over the atomic hydrogen density (Montillaud et al. 2013;Andrews et al. 2016;Berné et al. 2015) and over much of the Orion Bar PDR, G 0 /n(H) ≃ 1 (Tielens & Hollenbach 1985;Bernard-Salas et al. 2012b), implying that the PAHs that are lost from the PAH family are small (N C ≲ 50).
Hence, the picture that emerges from the analysis of the template spectra is the increased importance of FUV processing in PDR surface layers, resulting in a "weeding out" of the weakest links of the PAH family.These less resistant species are possibly formed from small hydrocarbons during the preceding dark cloud phase of the region in a bottom-up chemical scenario (Cuadrado et al. 2015;McGuire et al. 2021), feeding the gas with small hydrogen rich photolytically produced species (Alata et al. 2015).The UV processing of the PAH family will start with the loss of the smallest PAHs but as the PDR material is advected to the surface, larger and larger PAHs will become susceptible to photoprocessing.Similar scenarios have been proposed recently with data not obtained with JWST, albeit with slightly different numbers (e.g.N C ≲ 50 in Murga et al. 2022).In favourable conditions, the processing of very large PAHs (N C ≳ 60) may lead to the formation of C 60 (Berné & Tielens 2012;Berné et al. 2015).While the present data around 18.9 µm are still marred by instrumental artefacts, future searches for the signature of fullerenes in the MIRI spectra of these surface layers may reveal whether such a scenario plays a role under the conditions of the Orion Bar.
JWST is poised to obtain high-quality spectral imaging observations of a large sample of environments probing the full range of physical conditions that are relevant for AIB emission.These observations promise to capture the complexity of AIB emission with the unprecedented detail that is needed in order to advance our understanding of the photochemical evolution of large carbonaceous molecules.
Table A.1.Catalogue of AIB features detected in the five template spectra of the Orion Bar.The method used to select these features is described in Sect.3. The precise peak positions of nominal AIBs -whose wavelengths appear in boldface in this table -are indicated in Col. 3. We note that we converted the positions in wavelength to wavenumber by rounding to the nearest integer in units of cm −1 , and so the precision of the reported wavenumbers do not reflect the precision of the peak position of the AIBs.

Fig. 2 .
Fig. 2. AIB spectrum as seen by JWST using the Orion Bar atomic PDR template spectrum (Sect.2.2) as an example.Red shaded regions indicate emission from AIBs while blue curves indicate the underlying continuum.Figure is adapted from Peeters et al. (2004a).

Fig. 4 .
Fig.4.Continued from Fig.3.From left to right, top to bottom: Zoom-ins on the template spectra normalised by the peak flux of the5.25, 5.75  and 5.878, 6.2, 8.6, 11.2 and 12.0, 12.7, 13.5, and 14.2 µm AIBs (indicated in the y-axis label of each panel).The panels show wavelength ranges that are also shown in Fig.3, except for the panels that are centered on the 13.5 and 14.2 µm AIBs (small panels in the lower right).These figures illustrate the overall similarity and subtle differences in AIB profiles from region to region.

3. 2 .Fig. 5 .
Fig.5.Variation from region to region in the ratios of AIB integrated fluxes (top) and the widths of AIBs (bottom; data from Table1).All values are normalised to those from the atomic PDR.The ordering of the regions on the x-axis is arbitrary, but was chosen so that the data roughly show an increasing trend in FWHM from left to right (Sect.4.3).

Fig. 6 .
Fig. 6.Comparison of the 11.2 µm profile in the five template spectra with a class A 11.2 µm profile represented by the ISO-SWS spectrum of the Orion Bar H2S1 (van Diedenhoven et al. 2004, top panel) and a class B 11.2 µm profile represented by the ISO-SWS spectrum of HD 44179 (van Diedenhoven et al. 2004, bottom panel).
Fig. C.1.The MIRI-MRS template spectra with their respective continua.The anchor points for each continuum are represented by filled circles.See Sect.2.3 and Appendix C for details.
Fig. C.2.The NIRSpec template spectra (grating g395h, filter f290lp) from Peeters et al. (2023) along with their respective continua, zoomed in on the wavelength range that is relevant to the 3.3 µm AIB.See Sect.2.3 and Appendix C for details.

Table 1 .
FWHM of AIBs and AIB complexes measured from five template spectra extracted from NIRSpec-IFU (first row) and MIRI-MRS (rows 2-6) observations of the Orion Bar.Notes.The FWHM of the 8.6 µm AIB cannot be determined, for the peak intensity of the plateau between the 7.7 µm and 8.6 µm AIBs is greater than half the peak intensity of the 8.6 µm AIB (see the third panel of Fig.3).