Context. The James Webb Space Telescope (JWST), with its high spatial resolution and sensitivity, enabled the first detection of several v = 1-0 rovibrational emission lines of hydrogen deuteride HD in the Orion Bar, a prototypical photodissociation region (PDR). This provides an incentive to examine the physics of HD in dense and strongly irradiated PDRs. Aims. Using the latest data available on HD excitation by collisional, radiative, and chemical processes, our goal is to unveil HD formation and excitation processes in PDRs by comparing our state-of-the-art PDR model with observations made in the Orion Bar and discuss if and how HD can be used as a complementary tracer of physical parameters (thermal pressure and intensity of the UV field) in the emitting region. Methods. We computed detailed PDR models using an upgraded version of the Meudon PDR code (including radiative, collisional, and formation pumping excitation of HD rovibrational levels). Model results were then compared to spectro-imaging data acquired with the NIRSpec instrument on board JWST using population-excitation diagrams and synthetic emission spectra. Results. The models predict that HD is mainly produced in the gas phase via the reaction D + H-2 -> H + HD at the front edge of the PDR, contrary to H-2 (which forms on grain surfaces), and that the D/HD transition is located slightly closer to the edge than the H/H-2 transition. Rovibrational levels are excited by UV pumping. In the observations, HD rovibrational emission is detected close to the H/H-2 dissociation fronts of the Orion Bar, and it peaks where vibrationally excited H-2 peaks, rather than at the maximum emission of pure rotational H-2 levels. We detected lines emitted from five different levels of HD (v = 1) from which we can derive an excitation temperature around T-ex similar to 480-710 K. Our comparison to PDR models showed that a range of thermal pressure P = (3-9) x 10(7) K cm(-3) with no strong constraints on the intensity of the UV field G(0) are compatible with HD observations. This range of pressure is consistent with previous estimates from H-2 observations with JWST. Conclusions. This study provides a new detailed analysis of HD formation and excitation in PDRs. State-of-the-art PDR models with parameters best reproducing other tracers' emission are compatible with HD observations, highlighting the coherence of the different studies. This is also the first time that observations of HD emission lines in the near-infrared have been used to put constraints on the thermal pressure in the PDR, even though the lines are very faint.

Context . The James Webb Space Telescope (JWST), with its high spatial resolution and sensitivity, enabled the first detection of several v = 1–0 rovibrational emission lines of hydrogen deuteride HD in the Orion Bar, a prototypical photodissociation region (PDR). This provides an incentive to examine the physics of HD in dense and strongly irradiated PDRs. Aims . Using the latest data available on HD excitation by collisional, radiative, and chemical processes, our goal is to unveil HD formation and excitation processes in PDRs by comparing our state-of-the-art PDR model with observations made in the Orion Bar and discuss if and how HD can be used as a complementary tracer of physical parameters (thermal pressure and intensity of the UV field) in the emitting region. Methods . We computed detailed PDR models using an upgraded version of the Meudon PDR code (including radiative, collisional, and formation pumping excitation of HD rovibrational levels). Model results were then compared to spectro-imaging data acquired with the NIRSpec instrument on board JWST using population–excitation diagrams and synthetic emission spectra. Results . The models predict that HD is mainly produced in the gas phase via the reaction D + H 2 → H + HD at the front edge of the PDR, contrary to H 2 (which forms on grain surfaces), and that the D/HD transition is located slightly closer to the edge than the H/H 2 transition. Rovibrational levels are excited by UV pumping. In the observations, HD rovibrational emission is detected close to the H/H 2 dissociation fronts of the Orion Bar, and it peaks where vibrationally excited H 2 peaks, rather than at the maximum emission of pure rotational H 2 levels. We detected lines emitted from five different levels of HD ( v = 1) from which we can derive an excitation temperature around T ex ~ 480–710 K. Our comparison to PDR models showed that a range of thermal pressure P = (3–9) × 10 7 K cm −3 with no strong constraints on the intensity of the UV field G 0 are compatible with HD observations. This range of pressure is consistent with previous estimates from H 2 observations with JWST. Conclusions . This study provides a new detailed analysis of HD formation and excitation in PDRs. State-of-the-art PDR models with parameters best reproducing other tracers’ emission are compatible with HD observations, highlighting the coherence of the different studies. This is also the first time that observations of HD emission lines in the near-infrared have been used to put constraints on the thermal pressure in the PDR, even though the lines are very faint.

Using JWST/MIRI observations, we report the detection of CO2 ice in the dusty torus of the planetary nebula NGC 6302, an environment generally considered hostile to fragile molecular species and ices due to intense UV irradiation. This detection accompanies cold (20-50 K) gas-phase CO(2 )along the same sightlines. The ice absorption profile exhibits a double-peak profile, which is characteristic of pure crystalline CO(2 )ice. The CO(2 )gas-to-ice ratio is higher by more than an order of magnitude than in young stellar objects, which indicates distinct ice formation or processing mechanisms in evolved stellar environments. This discovery demonstrates that the dusty torus provides sufficient shielding to harbor ice chemistry, and that ice-mediated surface reactions must be incorporated into chemical models of planetary nebulae.

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 μm 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.

Context. Infrared emission from polycyclic aromatic hydrocarbons (PAHs) plays a major role in determining the charge balance of their host environments that include photodissociation regions (PDRs) in galaxies, planetary nebulae, and rims of molecular clouds.Aims. We aim to investigate the distribution and sizes of charged PAHs across the key zones of the Orion Bar PDR, i.e., the ionization front, the atomic PDR, and the dissociation fronts. Methods. We employed JWST MIRI-MRS observations of the Orion Bar from the Early Release Science program "PDRs4All" and synthetic images in the JWST MIRI filters. We investigated the spatial morphology of the aromatic infrared bands (AIBs) at 6.2, 7.7, 8.6, and 11.0 mu m (commonly tracing PAH cations) and the neutral PAH-tracing 11.2 mu m AIB, their (relative) correlations, and their relationship with existing empirical prescriptions for AIBs. Results. The 6.2, 7.7, 8.6, 11.0, and 11.2 mu m AIBs are similar in spatial morphology on larger scales. Aside from the 11.0 mu m AIB, these AIBs exhibit enhanced intensities at the dissociation fronts. Analyzing three-feature intensity correlations, two distinct groups emerge: the 8.6 and 11.0 mu m AIBs versus the 6.2 and 7.7 mu m AIBs. We attribute these correlations to PAH size. The 6.2 and 7.7 mu m AIBs trace cationic, medium-sized PAHs. Quantum chemical calculations reveal that the 8.6 mu m AIB is carried by large, compact, cationic PAHs, and the 11.0 mu m AIB's correlation with it implies that this band is as well. 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 by the 7.7/11.2 PAH emission. Conclusions. This study showcases the efficacy of the 6-9 mu m AIBs in probing the charge state and size distribution of 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.

Context. Infrared emission from polycyclic aromatic hydrocarbons (PAHs) plays a major role in determining the charge balance of their host environments that include photodissociation regions (PDRs) in galaxies, planetary nebulae, and rims of molecular clouds. Aims. We aim to investigate the distribution and sizes of charged PAHs across the key zones of the Orion Bar PDR, i.e., the ionization front, the atomic PDR, and the dissociation fronts. Methods. We employed JWST MIRI-MRS observations of the Orion Bar from the Early Release Science program “PDRs4All” and synthetic images in the JWST MIRI filters. We investigated the spatial morphology of the aromatic infrared bands (AIBs) at 6.2, 7.7, 8.6, and 11.0 μm (commonly tracing PAH cations) and the neutral PAH-tracing 11.2 μm AIB, their (relative) correlations, and their relationship with existing empirical prescriptions for AIBs. Results. The 6.2, 7.7, 8.6, 11.0, and 11.2 μm AIBs are similar in spatial morphology on larger scales. Aside from the 11.0 μm AIB, these AIBs exhibit enhanced intensities at the dissociation fronts. Analyzing three-feature intensity correlations, two distinct groups emerge: the 8.6 and 11.0 μm AIBs versus the 6.2 and 7.7 μm AIBs. We attribute these correlations to PAH size. The 6.2 and 7.7 μm AIBs trace cationic, medium-sized PAHs. Quantum chemical calculations reveal that the 8.6 μm AIB is carried by large, compact, cationic PAHs, and the 11.0 μm AIB's correlation with it implies that this band is as well. 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 by the 7.7/11.2 PAH emission. Conclusions. This study showcases the efficacy of the 6−9 μm AIBs in probing the charge state and size distribution of 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.

We report observations of comet 81P/Wild 2, target of the Stardust sample return mission, on UT 2023 March 20 and 24 at a heliocentric distance (r_H) of 1.85 au using the NIRSpec and MIRI integral field unit spectrographs on board the James Webb Space Telescope (JWST). This study is the first compositional comparison between JWST remote-sensing spectroscopy of a solar system object against terrestrial analysis of its returned samples. We securely detected molecular emission from H_2O, CH_4, C_2H_6, CH_3OH, CO, CO_2, ^13CO_2, OCS, HCN, and CN and find molecular abundances consistent within 2σ with those reported during previous perihelion passages. The water ortho-to-para ratio was 2.76±0.05, and the ^12CO_2/^13CO_2 ratio was 85±4. Thermal emission from the nucleus and dust was detected and modeled, providing an effective nucleus radius of 1.77±0.04 km and a dust composition (relative mass fraction of the submicron grains) of ∼36% amorphous carbon, ∼25% amorphous Mg:Fe olivine, ∼23% Mg-rich crystalline olivine, and ∼15% amorphous Mg:Fe pyroxene. The crystalline mass fraction of the sub-micron grains in the coma was 0.362±0.003. Comparison of the JWST-derived thermal model against the fine-grained materials in Stardust returned samples demonstrates complementarity between the missions, with each most sensitive to a different population of the coma dust grains.

Conspicuous excess emission is present in the near-infrared (NIR) region in various objects, including reflection nebulae, planetary nebulae, and nearby galaxies. However, the spatial distribution and spectral shape of the excess emission remain poorly understood. We studied the NIR continuum emission spectroscopically and obtained its spatial distribution relative to the aromatic infrared band (AIB) at 3.3um in the Orion Bar prototypical photodissociation region (PDR). We aim to characterize its spectral shape and discuss its origin. We employed 3D spectroscopic data of the Orion Bar taken with the integrated field unit of NIRSpec on JWST from the Early Release Science program "PDRs4All." Contribution from the foreground ionized gas was estimated using the Cloudy code and subtracted. The observed regions were divided into nine physically distinct regions and an average spectrum was derived for each region. The nine regions, including the ionized gas, atomic PDR, and molecular PDR, clearly show remaining continuum in the region 1–4.5um. The continuum at wavelengths longer than 2.7um shows good correlations with the 3.3um AIB, while the correlation of the continuum at 1.2um is not significant. We further find that the NIR continuum in the Orion Bar can be approximated by a summation of two blackbodies. The low-temperature component correlates with the AIB well, while the high-temperature component does not. The average spectra also show absorption features at 3.0 and 4.27um, which are attributed to the presence in the spectra of water ice and CO2 ice. We discuss possible origins of the NIR continuum, among which recurrent fluorescence from carbon clusters better explains the observed low-temperature component. The presence of ice species suggests a contribution from a deeper layer of the PDR along the line of sight producing characteristic ice absorption features.

Context. Polycyclic aromatic hydrocarbons (PAHs) constitute a significant fraction of the Universe's carbon budget, playing a key role in the cosmic carbon cycle and dominating the mid-infrared spectra of astrophysical environments in which they reside. Although PAHs are known to form in the circumstellar envelopes of post-asymptotic giant branch stars, their formation and evolution are still not well understood. Aims. We aim to understand how pristine complex hydrocarbons and PAHs in circumstellar environments transition to the PAHs observed in the interstellar medium. Methods. The mid-infrared PAH spectra (5-18 mu m) of the planetary nebula, NGC 7027, were investigated using spectral cubes from JWST MIRI-MRS. Results. We report the first detection of spatially resolved variations of the PAH spectral profiles across class & Ascr;, & Ascr;& Bernoullis;, and & Bernoullis; in all major PAH bands (6.2, 7.7, 8.6, and 11.2 mu m) within a single source, NGC 7027. These variations are linked to morphological structures within NGC 7027. Clear correlations are revealed between the 6.2, 7.7, and 8.6 mu m features, where the red components (6.26, 7.8, and 8.65 mu m) exhibit a strong correlation and the same is found for the blue components of the 6.2 and 7.7 mu m features (6.205 and 7.6 mu m). The blue component of the 8.6 mu m feature (8.56 mu m) appears to be independent of the other components. We link this behavior to differences in the molecular structure of their PAH subpopulations. Decomposition of the 11.2 mu m band confirms two previously identified components, with the broader 11.25 mu m component attributed to emission from very small grains or PAH clusters rather than PAH emission. Conclusions. We show that PAH profile classes generally vary with proximity to the central star's UV radiation field, suggesting class & Bernoullis; PAHs represent more processed species while class & Ascr; PAHs remain relatively pristine, challenging current notions on the spectral evolution of PAHs.

Context. JWST observations of the Orion Bar have revealed rich and diverse polycyclic aromatic hydrocarbon (PAH) emission. These observations allow for the first time a comprehensive characterisation of the charge state and size of the PAH population on morphologically resolved photodissociation regions (PDR) scales, properties closely linked to physical conditions of their inhabiting environments. Aims. We investigate the evolution of the PAH population's charge state and size across key physical zones in the Orion Bar, which include the H ii region, the atomic PDR (APDR), and three bright H I/H-2 dissociation fronts (DF1, DF2, and DF3). We connect changes in the PAH charge and size as probed by empirical emission proxies with the varying physical properties of their surrounding environments. Methods. 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' JWST Early Release Science Program. Decomposition and modelling were performed on the 5-15 mu m spectrum across the entire JWST mosaic, as well as on the weighted average spectra of the five key physical zones. Results. 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 make a minimal contribution. Emission from neutral PAHs peaks in the H ii region that consists of emission from a face-on PDR associated with the background OMC-1 molecular cloud, and in the molecular cloud regions past DF2. The PAH anions are observed deep within the DF2 and DF3 zones. Small and medium-sized PAHs make up similar to 70% of the PAH emission across the mosaic, with the peak of the small PAH emission found between the DF2 and DF3 zones. The average PAH size in the Orion Bar ranges between similar to 60-74 N-C. 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, gamma, ranges between similar to 2-9 & times; 10(4). Intensity ratios that are empirical tracers of PAH ionisation (I-6.2/I-11.2, I-7.7/I-11.2, I-8.6/I-11.2) scale well with gamma in regions encompassing edge-on or face-on PDR emission, but their correlation weakens within the molecular cloud zone. Conclusions. Modelling of the 5-15 mu m PAH spectrum with pyPAHdb achieves a comprehensive characterisation 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.

We report the detection of a set of new near-infrared emission features between 3.5 and 5.2 mu m in JWST/NIRSpec observations of Tc 1, the planetary nebula known for displaying the cleanest and most prominent mid-infrared cosmic fullerene spectrum. These broad features share the same spatial distribution as the well-known C60 and C70 mid-infrared emission bands, peaking in an asymmetric ring approximately 5 ''-6 '' from the central star. Through comparison with new anharmonic quantum chemical calculations, we demonstrate that these features arise from C60 combination bands, marking their first detection in an astrophysical environment. The total energy radiated in the combination bands amounts to similar to 17% of the total energy emitted from all C60 modes, with direct implications for fullerene cooling models. These near-infrared combination bands offer a promising new window for identifying and studying the molecular astrophysics of C60 in sources where mid-infrared spectra are more complex.

Aims. Our goal is to use the first detection of CH+ and CH3+ infrared rovibrational emission in the Orion Bar and in the protoplanetary disk d203-506 to probe their formation and excitation mechanisms and constrain the physico-chemical conditions of the environment. Methods. We used spectro-imaging acquired using both the NIRSpec and MIRI-MRS instruments on board JWST to study the infrared CH+ and CH3+ spatial distribution at very small scales (down to 0.1 '') and compared it to excited H-2 emission. We studied their excitation in detail, and in the case of CH+, we compared the observed line intensities with chemical formation pumping models based on recent quantum dynamical calculations. Throughout this study, we compare the emission of these molecules in two environments: the Bar a photodissociation region - and a protoplanetary disk (d203-506), both of which are irradiated by the Trapezium cluster. Results. We detected CH+ and CH3+ vibrationally excited emission both in the Bar and d203-506. These emissions originate from the same region as highly excited H-2 (high rotational and rovibrational levels) and correlate less with the lower rotational levels of H-2 (J ' < 5) or the emission of aromatic and aliphatic infrared bands. Our comparison between the Bar and d203-506 revealed that both CH+ and CH3+ excitation and/or formation are highly dependent on gas density. The excitation temperature of the observed CH+ and CH3+ rovibrational lines is around T similar to 1500 K in the Bar and T similar to 800 K in d203-506. Moreover, the column densities derived from the rovibrational emission are less than 0.1% of the total known (CH+) and expected (CH3+) column densities. These different results show that CH+ and CH3+ level populations strongly deviate from local thermodynamical equilibrium. The CH+ rovibrational supra-thermal emission (v = 1 and v = 2) can be explained by chemical formation pumping with excited H-2 via C+ + H-2* = CH+ + H. The difference in the population distribution of the H-2* energy levels between the Orion Bar and d203-506 then result in different excitation temperatures. These results support a gas phase formation pathway of CH+ and CH3+ via successive hydrogen abstraction reactions. However, we do not find any evidence of CH3+ emission in the JWST spectrum, which may be explained by the fact its spectroscopic signatures could be spread in the JWST spectra. Finally, the observed CH+ intensities coupled with a chemical formation pumping model provide a diagnostic tool to trace the local density. Conclusions. Line emission from vibrationally excited CH+ and CH3+ provides new insight into the first steps of hydrocarbon gas-phase chemistry in action. This study highlights the need for extended molecular data of detectable molecules in the interstellar medium in order to analyze the JWST observations.

Young (≲ 10 Myr) planetary-mass companions (PMCs) provide valuable insights into the formation and early evolution of planetary systems. To date, only a dozen such objects have been identified through direct imaging. Using JWST/NIRCam observations towards the Orion Nebula, obtained as part of the PDRs4All Early Release Science program, we have identified a faint point source near the M-type star V2376 Ori. Follow-up spectroscopic observations with the MUSE instrument on the VLT confirm that the source, V2376 Ori b, is indeed a young planetary-mass companion. It is a member of Orion D, around 80 pc in the foreground of the Trapezium cluster of Orion and with an age of approximately 7 ± 3 Myr. We fit the SED of V2376 Ori b to infer a mass of ∼ 20 M_ Jup. The MUSE spectrum reveals several accretion tracers. Based on the Hα line intensity, we estimate an accretion rate of ∼10^-6.5 ± 0.7 M_Jup yr^-1, which is comparable to that of young PMCs such as PDS 70b. In addition, the MUSE data cube reveals extended emission in the [O ii] doublet at 7320 and 7330 Å, which is interpreted as evidence of a dynamical interaction between the two sources that, potentially, involves mass transfer between their individual accretion disks. These results demonstrate that JWST/NIRCam imaging surveys of young stellar associations can uncover new PMCs, which can then be confirmed and characterized through ground-based spectroscopic follow-up.

The chemical composition of exoplanets is thought to be influenced by the composition of the disks in which they form. JWST observations have unveiled a variety of chemical species in numerous nearby disks, which show substantial variations in the C/O abundance ratio. However, little is known about the composition and C/O ratio of disks around young stars in clusters exposed to strong ultraviolet radiation from nearby massive stars, which are representative of the environments where most planetary systems form, including ours. Here we present JWST spectroscopy of d203-504, a young 0.7 M circle dot star in the Orion nebula with a 30 au disk irradiated by nearby massive stars. These observations reveal spectroscopic signatures of CO, H2O, CH3+ and polycyclic aromatic hydrocarbons. Water and CO are detected in absorption in the inner disk (r less than or similar to 1 au), where the estimated gas-phase C/O ratio is 0.48, consistent with the solar value and that of the Orion nebula. By contrast, CH3+ and polycyclic aromatic hydrocarbons are found in the extended surface layers of the disk. These results suggest that gas in the inner disk is chemically shielded from ultraviolet radiation, whereas the surface layers of the disk experience ultraviolet-induced chemistry, potentially depleting their carbon content.

Context. JWST continues to deliver incredibly detailed infrared (IR) images of star-forming regions in the Milky Way and beyond. IR emission from star-forming regions is very spectrally rich due to emission from gas-phase atoms, ions, and polycyclic aromatic hydrocarbons (PAHs). Physically interpreting IR images of these regions relies on assumptions about the underlying spectral energy distribution in the imaging bandpasses. Aims. We aim to provide empirical prescriptions to derive line, PAH, and continuum intensities from JWST images. These prescriptions will facilitate the interpretation of images in a wide variety of astrophysical contexts. We also measure the level of agreement between JWST imaging and integral field spectroscopy. Methods. We use JWST PDRs4All Near-Infrared Camera (NIRCam) and Mid-Infrared Instrument (MIRI) imaging and Near-Infrared Spectrograph (NIRSpec) integral field unit (IFU) and MIRI Medium Resolution Spectrograph (MRS) spectroscopic observations of the Orion Bar, the prototypical photodissociation region (PDR), to directly compare and cross-calibrate imaging and IFU data at ~100 AU resolution over a region where the radiation field and ISM environment evolves from hot ionized gas to warm neutral gas followed by cold molecular gas. We study the relative contributions of line, PAH, and continuum emission to the NIRCam and MIRI filters as functions of local physical conditions, and investigate filter combinations that represent selected line and PAH emission. Results. We provide empirical prescriptions based on NIRCam and MIRI images that may be used to derive intensities of strong emission lines and PAH features. Within the range of the environments probed in this study, these prescriptions accurately predict Pa α, Brα, and PAH 3.3 μm and 11.2 μm intensities, while those for [Fe II] 1.644 μm, H2 1–0 S(1) 2.12 μm and 0–0 S(9) 4.69 μm, and PAH 7.7 μm show more complicated environmental dependencies. Conclusions. Linear combinations of JWST NIRCam and MIRI images provide effective tracers of ionized gas, H2, and PAH emission in PDRs. We expect these recipes to be useful for both the Galactic and extragalactic communities. The flux calibration between imaging and spectroscopy is found to agree within 1–20% for NIRCam and NIRSpec, and 2–7% for MIRI Imager and MRS.

Planetary nebulae are sites where ejected stellar material evolves into complex molecules, but the precise physical conditions and chemical routes that govern these processes are unclear. The presence of abundant carbon-rich molecules in O-rich environments poses particular challenges. Here we report the first detection of methyl cation (CH 3 + ) in any planetary nebula, observed in the O-rich nebula NGC 6302 using JWST MIRI/Medium Resolution Spectrometer observations. CH 3 + is a key driver of organic chemistry in UV-irradiated environments. Spatially resolved observations reveal that CH 3 + is colocated with 12 CO, H 2 , H ii , HCO + , and polycyclic aromatic hydrocarbons. LTE modeling of the CH 3 + emission yields excitation temperatures of 500–800 K in the inner bubble and torus, rising to 1000–2000 K in the outer bubble of NGC 6302, with column densities ranging from ∼10 11 to 10 13 cm −2 . This detection suggests that hydrocarbon radical chemistry must be incorporated into planetary nebulae chemical models. Further near-IR observations are crucial to map different chemical networks operating in these environments.

Planetary nebulae are sites where ejected stellar material evolves into complex molecules, but the precise physical conditions and chemical routes that govern these processes are unclear. The presence of abundant carbon-rich molecules in O-rich environments poses particular challenges. Here we report the first detection of methyl cation (CH ${}_{3}^{+}$ ) in any planetary nebula, observed in the O-rich nebula NGC 6302 using JWST MIRI/Medium Resolution Spectrometer observations. CH ${}_{3}^{+}$ is a key driver of organic chemistry in UV-irradiated environments. Spatially resolved observations reveal that CH ${}_{3}^{+}$ is colocated with ^12 CO, H _2 , H ii , HCO ^+ , and polycyclic aromatic hydrocarbons. LTE modeling of the CH ${}_{3}^{+}$ emission yields excitation temperatures of 500–800 K in the inner bubble and torus, rising to 1000–2000 K in the outer bubble of NGC 6302, with column densities ranging from ∼10 ^11 to 10 ^13 cm ^−2 . This detection suggests that hydrocarbon radical chemistry must be incorporated into planetary nebulae chemical models. Further near-IR observations are crucial to map different chemical networks operating in these environments.

Polycyclic aromatic hydrocarbons (PAHs) are responsible for strong mid-IR emission features near star-forming regions. It is well known that low-metallicity environments exhibit weaker PAH emission, but it is not clear how metallicity affects the properties of the emitting PAH population. We present a detailed study of the PAH emission in a region of 30 Doradus (30 Dor), a well-known low-metallicity star-forming environment in the Large Magellanic Cloud and we compare it to PAH emission in the Orion Bar to investigate the characteristics of the PAH population and how the environments affect the resulting IR emission. We analyze JWST observations of 30 Dor that include imaging (NIRCam and MIRI) and spectroscopy (NIRSpec integral-field unit (IFU) and MIRI Medium Resolution Spectroscopy (MRS)). We extracted NIRSpec/IFU and MIRI/MRS spectra from 18 apertures that cover the morphological structures present within the observed region of 30 Dor. We characterize the profiles and relative intensities of PAH emission in these apertures. The detailed profiles of the PAH emission bands in 30 Dor are all similar and match with one of the dissociation fronts (DF2) in the Orion Bar, but their relative band ratios show a much larger range than in the Orion Bar. The PAH emission in 30 Dor originates from a population with a lower or similar ionization fraction than in the Orion Bar, and a size distribution that has more small-sized PAHs. Since smaller PAHs typically photofragment before larger PAHs, our findings support the hypothesis that the lower PAH emission due to lower metallicities is the result of the inhibition of growth toward larger PAHs rather than photofragmentation.

We have conducted a sensitivity analysis on the mid-infrared spectral decomposition of galaxies and the modeling of the polycyclic aromatic hydrocarbon (PAH) emission spectrum with the NASA Ames PAH Infrared Spectroscopic Database (PAHdb) to assess the variance on the average galaxy PAH population properties under a grid of different modeling parameters. We find that the short-low and short-low+long-low Spitzer-IRS decomposition with PAHFIT provides consistent modeling and recovery of the 5–15 μ m PAH emission spectrum. For PAHdb modeling, application of a redshift to the calculated spectra to account for anharmonic effects introduces a 15%–20% variance on the derived parameters, while its absence improves the fits by ∼13%. The 4.00- α release of PAHdb achieves the complete modeling of the 6–15 μ m PAH spectrum, including the full 6.2 μ m band, improving the average fitting uncertainty by a factor of 2. The optimal PAHdb modeling configuration requires selection of pure PAHs without applying a redshift to the bands. Although quantitatively the PAHdb-derived parameters change under different modeling configurations or database versions, their variation follows a linear scaling, with previously reported trends remaining qualitatively valid. PAHdb modeling of JWST observations, and JWST observations smoothed and resampled to the Spitzer-IRS resolution and dispersion have consistent PAHdb derived parameters. Decomposition with different codes, such as PAHFIT and CAFE, produce PAH emission spectra with noticeable variation in the 11–15 μ m region, driving a ∼7% difference in the neutral PAH fraction under PAHdb modeling. A new library of galaxy PAH emission templates is delivered to be utilized in galaxy spectral energy distribution modeling.

Context. Photodissociation regions (PDRs) exhibit strong emission bands between 3–20 μm known as the aromatic infrared bands (AIBs), and they originate from small carbonaceous species such as polycyclic aromatic hydrocarbons (PAHs) excited by UV radiation. The AIB spectra observed in Galactic PDRs are considered a local analog for those seen in extragalactic star-forming regions. Recently, the PDRs4All JWST program observed the Orion Bar PDR, revealing the subcomponents and profile variations of the AIBs in very high detail. Aims. We present the Python version of PAHFIT, a spectral decomposition tool that separates the contributions by AIB subcomponents, thermal dust emission, gas lines, stellar light, and dust extinction. We aim to provide a configuration that enables highly detailed decompositions of JWST spectra of PDRs (3.1–26 μm) and to test if the same configuration is suitable to characterize AIB emission in extragalactic star forming regions. Methods. We determined the central wavelength and FWHM of the AIB subcomponents by fitting selected segments of the Orion Bar spectra and compiled them into a “PDR pack” for PAHFIT. We tested the PDR pack by applying PAHFIT to the full 3.1–26 μm PDRs4All templates. We applied PAHFIT with this PDR pack and the default continuum model to seven spectra extracted from the central star forming ring of the galaxy NGC7469. Results. We introduce an alternate dust continuum model to fit the Orion Bar spectra, as the default PAHFIT continuum model mismatches the intensity at 15–26 μm. Using the PDR pack and the alternate continuum model, PAHFIT reproduces the Orion Bar template spectra with residuals of a few percent. A similar performance is achieved when applying the PDR pack to the NGC7469 spectra. We provide PAHFIT-based diagnostics that trace the profile variations of the 3.3, 3.4, 5.7, 6.2, and 7.7 μm AIBs and thus the photochemical evolution of the AIB carriers. The 5.7 μm AIB emission originates from at least two subpopulations, one more prominent in highly irradiated environments and one preferring more shielded environments. Smaller PAHs as well as very small grains or PAH clusters both thrive in the more shielded environments of the molecular zone in the Orion Bar. Based on these new diagnostics, we show and quantify the strong similarity of the AIB profiles observed in NGC7469 to the Orion Bar template spectra.