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.

Recent James Webb Space Telescope observations reveal unprecedented details in the 2000-1650 cm-1 (∼5-6 μm) region, including the well-known 5.25 and 5.75 μm bands. This range, dominated by overtone and combination bands mainly involving C-H out-of-plane (CHoop) bending fundamentals, provides a powerful yet underexplored probe of astronomical polycyclic aromatic hydrocarbons (PAHs) and their nitrogen-containing analogues (PANHs). We present a combined experimental and theoretical investigation of 27 neutral PAHs and PANHs using matrix-isolation spectroscopy and second-order vibrational perturbation theory to disentangle effects of PAH structures and nitrogen substitution in the 2000-1650 cm-1 region. Coupling among the underlying fundamentals subdivides this region, accommodating the distinct characteristics of the 5.25 and 5.75 μm bands. The higher-frequency subregion (2000-1870 cm-1; 5.00-5.35 μm) reflects PAH edge topology, where the highest-frequency band shifts from quartet + bay structures (∼5.13 μm) to quartet+solo (∼5.18 μm) and pericondensed PAHs (∼5.20 μm), consistent with the relatively stable astronomical 5.25 μm band. The lower-frequency subregion (1870-1650 cm-1; 5.35-6.06 μm) shows greater structural variability, consistent with the composite 5.75 μm band. Nitrogen substitution broadens the spectral distribution by shifting bands by ∼10 cm-1. From our neutral PAHs/PANHs set, we infer that the 5-6 μm region reflects contributions from neutral astronomical PAHs, dominated by pericondensed PAHs, with secondary contributions from catacondensed PAHs with limited quartets and increasing solos. Contributions from irregular edges containing quartets C-H groups and nitrogen-substitution are modest. A corresponding study of cations is needed to fully characterize the 5-6 μm emission, disentangling cation and neutral contributions from the CHoop region.

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.

Studying the physicochemical properties of ice in astronomical environments is crucial to understanding the chemical processes involved in cosmic events such as comet and planet formation. The physical characteristics and chemical evolution on the surfaces of cosmic objects such as comets or interstellar grains offer key insights into these processes. This study focuses on α-pinene, a carbon- and hydrogen-rich molecule, which serves as a model for investigating radical-driven synthesis of more complex molecules under space-like conditions. It also provides a useful analogy for complex terrestrial organic molecules and sheds light on how organic matter interacts with water and radiation in extraterrestrial environments. In this work, we simulate the effects of heavy-ion cosmic ray bombardment on chiral molecules in the interstellar medium by analyzing the radiolysis of a C10H16/H2O (1:1) mixture irradiated with 61.3 MeV 84Kr15+ ions. Fourier Transform Infrared (FTIR) spectroscopy is employed to monitor the chemical evolution of ice samples at 10 K, both before and after irradiation. We identify 12 C n H m and ten C n H m O k molecules, including complex products such as naphthalene (C10H8), glycolaldehyde (HCOCH2OH), and methyl formate (HCOOCH3). The most abundant hydrogenated product is acetylene (C2H2), followed by naphthalene (C10H8), while the most abundant oxygenated molecules are vinyl alcohol (CH2CHOH) and ethanol (CH3CH2OH). Notably, the formation of CO2 is minimal in this experiment. The destruction cross-sections of α-pinene and water in the (1:1) mixture are determined to be 3.5 and 6.4 × 10-13 cm2, respectively. The formation cross-sections for the products resulting from radiolysis are on average 2 × 10-14 cm2 for hydrocarbons and 0.6 × 10-14 cm2 for the oxygenated products.

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.

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-AGB stars, their formation and evolution are still not well-understood. We aim to understand how pristine complex hydrocarbons and PAHs in circumstellar environments transition to the PAHs observed in the ISM. The mid-infrared PAH spectra (5-18 micron) of the planetary nebula, NGC 7027, are investigated using spectral cubes from JWST MIRI-MRS. We report the first detection of spatially-resolved variations of the PAH spectral profiles across class A, AB, and B in all major PAH bands (6.2, 7.7, 8.6, and 11.2 micron) 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 micron features, where the red components (6.26, 7.8, 8.65 micron) exhibit a strong correlation and the same is found for the blue components of the 6.2 and 7.7 (6.205 and 7.6 micron). The blue component of the 8.6 (8.56 micron) appears to be independent of the other components. We link this behaviour to differences in molecular structure of their PAH subpopulations. Decomposition of the 11.2 micron band confirms two previously identified components, with the broader 11.25 micron component attributed to emission from very small grains of PAH clusters rather than PAH emission. We show that PAH profile classes generally vary with proximity to the central star's UV radiation field, suggesting class B PAHs represent more processed species while class A PAHs remain relatively pristine, challenging current notions on the spectral evolution of PAHs.

Molecular dynamics (MD) calculations were carried out to simulate the solar wind irradiation, namely, H+, of methane-ammonia ices. To mimic a continuous ion bombardment of the ice, multiple impact cycles were performed on the ice target. Each impact cycle involved seven 0.829 keV H+ (total energy of 5.8 keV and a velocity of 400 km/s) impacting the surface for a duration of 0.5 ps, which was shown in previous work to be a sufficient time for any product resulting from H+ impacts of the ice to form and stabilize. At the end of each cycle, the ice was quenched to 15 K to prevent excessive heating and sublimation. The dominant radiolysis species formed in our simulations were those obtained from the reaction of methyl and amino radicals, namely, ethane, hydrazine, and methylamine. The formation of methylamine, the building block of the amino acid glycine, is in agreement with observations and previous irradiation experiments. Additional species resulting from progressive impact-mediated hydrogen loss of simple two-radical products, namely, ethyl, methanimine, aminomethyl, and diimine, were produced in significant quantities in our simulations and in previous irradiation experiments. Unsaturated molecules, such as vinyl, ethylene, and acetylene, were formed to a lesser extent by impact-mediated hydrogen loss. Larger product species, such as methanediamine, requiring the reaction of up to four radicalized ice molecules did form throughout the course of our simulations and were also obtained in previous irradiation experiments. Methanediamine is a precursor to nucleobases.

Context. The infrared universe is dominated by emission from polycyclic aromatic hydrocarbons (PAHs) observed as aromatic infrared bands (AIBs). JWST has produced a rich trove of information on these PAH signatures. Aims. We aim to investigate the photochemical evolution of PAHs in photodissociation regions (PDRs), focusing on their molecular edge structures across key zones, including the H II region, the ionization front, the atomic PDR, the dissociation front, and the molecular PDR. Methods. We utilized JWST's MIRI-MRS observations of the Orion Bar for the PDRs4All JWST Early Release Science program. We investigated the spectral and spatial characteristics of 10-15 mu m AIBs. Results. The AIBs at 10.6, 10.8, 11.0, 11.2, 12.0, 12.7, 13.5, 14.0, and 14.2 mu m share large-scale spatial morphologies, peaking in the atomic PDR and gradually declining with distance from the PDR surface. Correlations between the AIBs reveal that they are largely carried by PAHs. Profile variations and subcomponents of the 11.2 and 12.0 mu m AIBs reveal a carrier that behaves independently of PAHs, which we attribute to very small grains (VSGs) and/or PAH clusters. We ascribe the 11.0 and 11.207 mu m AIBs, part of the 12.0 mu m AIB, and the 12.7, 13.5, and 14.2 mu m AIBs to CHoop modes and discuss their hydrogen-adjacency assignments. We propose that the 10.6, 10.8, and 14.0 mu m AIBs do not arise from CHoop modes. We derived the relative amounts of solo, duo, trio, and quartet CH groups to infer the molecular structures. These suggest that PAHs are dominated by solo and trio CH groups throughout the PDR. We attribute the decrease in duo and quartet CH groups relative to the solo CH groups toward the PDR surface to the effects of photolysis of the labile hydrogens. Conclusions. The 10-15 mu m AIBs are powerful probes of the PAH molecular structures. This study showcases the spatial and spectral variability in CHoop features due to photochemical processing of PAHs, and the differentiated spectral characteristics of PAHs and VSGs, in a prototypical PDR.

Context.Mid-infrared emission features are important probes of the properties of ionized gas and hot or warm molecular gas, which are difficult to probe at other wavelengths. The Orion Bar photodissociation region (PDR) is a bright, nearby, and frequently studied target containing large amounts of gas under these conditions. Under the “PDRs4All” Early Release Science Program for JWST, a part of the Orion Bar was observed with MIRI integral field unit (IFU) spectroscopy, and these high-sensitivity IR spectroscopic images of very high angular resolution (0.2″) provide a rich observational inventory of the mid-infrared (MIR) emission lines, while resolving the HIIregion, the ionization front, and multiple dissociation fronts.Aims.We list, identify, and measure the most prominent gas emission lines in the Orion Bar using the new MIRI IFU data. An initial analysis summarizes the physical conditions of the gas and demonstrates the potential of these new data and future IFU observations with JWST.Methods.The MIRI IFU mosaic spatially resolves the substructure of the PDR, its footprint cutting perpendicularly across the ionization front and three dissociation fronts. We performed an up-to-date data reduction, and extracted five spectra that represent the ionized, atomic, and molecular gas layers. We identified the observed lines through a comparison with theoretical line lists derived from atomic data and simulated PDR models. The identified species and transitions are summarized in the main table of this work, with measurements of the line intensities and central wavelengths.Results.We identified around 100 lines and report an additional 18 lines that remain unidentified. The majority consists of HIrecombination lines arising from the ionized gas layer bordering the PDR. The HIline ratios are well matched by emissivity coefficients from H recombination theory, but deviate by up to 10% because of contamination by HeIlines. We report the observed emission lines of various ionization stages of Ne, P, S, Cl, Ar, Fe, and Ni. We show how the NeIII/NeII, SIV/SIII, and ArIII/ArIIratios trace the conditions in the ionized layer bordering the PDR, while FeIII/FeIIand NiIII/NiIIexhibit a different behavior, as there are significant contributions to FeIIand NiIIfrom the neutral PDR gas. We observe the pure-rotational H2lines in the vibrational ground state from 0–0S(1) to 0–0S(8), and in the first vibrationally excited state from 1–1S(5) to 1–1 S(9). We derive H2excitation diagrams, and for the three observed dissociation fronts, the rotational excitation can be approximated with one thermal (~700 K) component representative of an average gas temperature, and one nonthermal component (~2700 K) probing the effect of UV pumping. We compare these results to an existing model of the Orion Bar PDR, and find that the predicted excitation matches the data qualitatively, while adjustments to the parameters of the PDR model are required to reproduce the intensity of the 0–0 S (6) to S (8) lines.

Molecular dynamics simulations were performed to characterize reaction products, resulting from solar wind irradiation, namely, H+, of methane and methane-water ices. In our approach, we used seven 0.829 keV H+ (total energy of 5.8 keV), with a velocity of 400 km/s, to hit the icy surface simultaneously, and we repeated this process multiple times to simulate continuous irradiation while quenching the ice to 15 K after each irradiation to prevent excessive heating and sublimation. Our simulations produced complex organic molecules previously obtained in laboratory experiments. For methane ice, molecules containing two carbons were predominant, with ethane and ethyl radicals being the most abundant, followed by ethylene, vinyl radical, and acetylene. Hydrocarbons containing three carbons (e.g., propane, propene, and propyl) were minor products, and only a few molecules containing four carbon atoms (e.g., iso-butene, 1-methylpropylidene, and 2-buten-2-yl) formed. Products that can be formed from the reaction of 1-3 impact fragmentation events, ethane, ethyl radical, and ethylene, monotonically increased over time, while products of 3 or more impact fragmentation events, vinyl, propane, and acetylene, formed over longer time scales. The number of methane complexes decreased over time. For a methane/water (1:1) ice mixture, most of the products consisted of methyl-water complexes, and their number increased with time. All the other oxygenated and nonoxygenated products formed in small amounts due to the water solvation of radicals. For a methane/water (4:1) ice mixture, the methyl-water complexes constituted 45% of the total products, with oxygenated and nonoxygenated products being formed in almost equal amounts. For methane-water ices, the proportions of alkanes, alkenes, and alkynes were very similar to those of pure methane. Dimethyl ether and ethanol formed for both 1:1 and 4:1 methane-water ices.

A systematic study was performed on the spectral properties of polycyclic aromatic hydrocarbons (PAHs) with edge defects using harmonic density functional theory calculations. Their potential astronomical relevance was assessed through direct comparison with NIRSpec and MIRI-MRS spectra of the atomic photodissociation region of the Orion Bar from the JWST Early Release Science PDRs4All program. It is found that the astronomical 6.2 μ m PAH emission band, including its blue side, is well reproduced by PAHs with edge defects, when taking into account the effects of polarization in the computations, and without a need for PAHs that contain nitrogen. Small neutral PAHs with edge defects explain the blue wing of the 3.3 μ m band. A low number of edge defects is required to reproduce the 8.6 and 11.2 μ m band profiles, while the 11.0 + 11.2/12.7 μ m band intensity ratio is a measure for the number of edge defects. A blind database fit to the Orion Bar spectrum reproduces the 6–15 μ m region with an error of 9.9% and shows a clear delineation of charge, with the 6–10 μ m PAH bands being carried by PAH cations and the 10–15 μ m region by predominantly neutral PAHs. The contribution of anions is negligible. Armchair PAHs fit the 12.7 μ m band, simultaneously producing a very weak broad emission feature centered at 3.225 μ m. Zigzag PAHs fit the 11.2 μ m band. It is concluded that PAHs with a low number of edge defects, in addition to armchair and zigzag PAHs, all contribute to the observed interstellar infrared emission.

ABSTRACT Radiolysis of α-pinene by 61.3 MeV 84Kr15 + ions was analysed with the scope to simulate the effects of heavy ion cosmic ray bombardment on chiral molecules in the interstellar medium. The α-pinene ice samples were irradiated at 10 K and their chemical evolution was monitored by mid-infrared Fourier transform (FTIR) spectroscopy to characterize the reaction products and to determine the extent of racemization. The integrated band strengths have been obtained for all the neutral α-pinene vibrational bands using the experimental band integrated absorbances and the theoretical absolute intensities calculated along the column densities. In the current heavy ion bombardment experiments, small molecules were formed and the precursor, α-pinene, was destroyed instead of being racemized. Twelve hydrocarbons were produced (final fluence of 2.0 × 1012 ions cm−2): methane (CH4), acetylene (C2H2), ethylene (C2H4), propylene (C3H6), propane (C3H8), n-butane (C4H10), butene (C4H8), propyne (C3H4), benzene (C6H6), ethane (C2H6), vinylacetylene (C4H4), and 2-methyl-1,3-butadiene or isoprene (C5H8). The highest formation cross-section (∼ 40 × 10−15 cm2) was observed for the C3H4 and the lowest was for C3H8 (∼ 3 × 10−15 cm2). The radiochemical yields for these molecules follow the same trends as those of their cross-sections. The atom budget calculation confirms that all the expected products have been generated during the radiolysis and supports the conclusion that the proposed A values are accurate. The α-pinene sputtering yield for this ion beam was found to be Y0 = 1.84 × 106 molecules per impact.

Mid-infrared emission features probe the properties of ionized gas, and hot or warm molecular gas. The Orion Bar is a frequently studied photodissociation region (PDR) containing large amounts of gas under these conditions, and was observed with the MIRI IFU aboard JWST as part of the "PDRs4All" program. The resulting IR spectroscopic images of high angular resolution (0.2") reveal a rich observational inventory of mid-IR emission lines, and spatially resolve the substructure of the PDR, with a mosaic cutting perpendicularly across the ionization front and three dissociation fronts. We extracted five spectra that represent the ionized, atomic, and molecular gas layers, and measured the most prominent gas emission lines. An initial analysis summarizes the physical conditions of the gas and the potential of these data. We identified around 100 lines, report an additional 18 lines that remain unidentified, and measured the line intensities and central wavelengths. The H I recombination lines originating from the ionized gas layer bordering the PDR, have intensity ratios that are well matched by emissivity coefficients from H recombination theory, but deviate up to 10% due contamination by He I lines. We report the observed emission lines of various ionization stages of Ne, P, S, Cl, Ar, Fe, and Ni, and show how certain line ratios vary between the five regions. We observe the pure-rotational H$_2$ lines in the vibrational ground state from 0-0 S(1) to 0-0 S(8), and in the first vibrationally excited state from 1-1 S(5) to 1-1 S(9). We derive H$_2$ excitation diagrams, and approximate the excitation with one thermal (~700 K) component representative of an average gas temperature, and one non-thermal component (~2700 K) probing the effect of UV pumping. We compare these results to an existing model for the Orion Bar PDR and highlight the differences with the observations.

The JWST has captured the most detailed and sharpest infrared images ever taken of the inner region of the Orion Nebula, the nearest massive star formation region, and a prototypical highly irradiated dense photo-dissociation region (PDR). We investigate the fundamental interaction of far-ultraviolet photons with molecular clouds. The transitions across the ionization front (IF), dissociation front (DF), and the molecular cloud are studied at high-angular resolution. These transitions are relevant to understanding the effects of radiative feedback from massive stars and the dominant physical and chemical processes that lead to the IR emission that JWST will detect in many Galactic and extragalactic environments. Due to the proximity of the Orion Nebula and the unprecedented angular resolution of JWST, these data reveal that the molecular cloud borders are hyper structured at small angular scales of 0.1-1" (0.0002-0.002 pc or 40-400 au at 414 pc). A diverse set of features are observed such as ridges, waves, globules and photoevaporated protoplanetary disks. At the PDR atomic to molecular transition, several bright features are detected that are associated with the highly irradiated surroundings of the dense molecular condensations and embedded young star. Toward the Orion Bar PDR, a highly sculpted interface is detected with sharp edges and density increases near the IF and DF. This was predicted by previous modeling studies, but the fronts were unresolved in most tracers. A complex, structured, and folded DF surface was traced by the H2 lines. This dataset was used to revisit the commonly adopted 2D PDR structure of the Orion Bar. JWST provides us with a complete view of the PDR, all the way from the PDR edge to the substructured dense region, and this allowed us to determine, in detail, where the emission of the atomic and molecular lines, aromatic bands, and dust originate.

(Abridged) We investigate the impact of radiative feedback from massive stars on their natal cloud and focus on the transition from the HII region to the atomic PDR (crossing the ionisation front (IF)), and the subsequent transition to the molecular PDR (crossing the dissociation front (DF)). We use high-resolution near-IR integral field spectroscopic data from NIRSpec on JWST to observe the Orion Bar PDR as part of the PDRs4All JWST Early Release Science Program. The NIRSpec data reveal a forest of lines including, but not limited to, HeI, HI, and CI recombination lines, ionic lines, OI and NI fluorescence lines, Aromatic Infrared Bands (AIBs including aromatic CH, aliphatic CH, and their CD counterparts), CO2 ice, pure rotational and ro-vibrational lines from H2, and ro-vibrational lines HD, CO, and CH+, most of them detected for the first time towards a PDR. Their spatial distribution resolves the H and He ionisation structure in the Huygens region, gives insight into the geometry of the Bar, and confirms the large-scale stratification of PDRs. We observe numerous smaller scale structures whose typical size decreases with distance from Ori C and IR lines from CI, if solely arising from radiative recombination and cascade, reveal very high gas temperatures consistent with the hot irradiated surface of small-scale dense clumps deep inside the PDR. The H2 lines reveal multiple, prominent filaments which exhibit different characteristics. This leaves the impression of a "terraced" transition from the predominantly atomic surface region to the CO-rich molecular zone deeper in. This study showcases the discovery space created by JWST to further our understanding of the impact radiation from young stars has on their natal molecular cloud and proto-planetary disk, which touches on star- and planet formation as well as galaxy evolution.

The NASA Raman Spectroscopic Database (Ramdb) was developed to provide a publicly accessible, user-friendly database for spectra relevant to the planetary science community. This paper describes the first set of spectra made available in version 1.00 of Ramdb, the methods used to obtain and process the Raman spectra, and provides a walkthrough of the database website that is located at www.astrochemistry.org/ramdb. Ramdb presently offers 112 laboratory and theoretical Raman spectra of samples relevant to planetary exploration and space science. Laboratory spectra were measured at multiple laser excitation wavelengths, namely: 405, 532, and 785 nm. Spectral data for six projects (amino acids, polycyclic aromatic hydrocarbons, carbon allotropes, minerals, analogs, and planetary studies) are provided as both raw and processed, tagged with key spectroscopic parameters, and, where applicable, accompanied by microscope images of the samples. The contents of Ramdb will be continuously expanded with a wide range of samples relevant to Astrophysics, Planetary Science, Exobiology, and Earth Science, guided by ongoing and future space exploration missions.

(Abridged) 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 $\mu$m. The most sensitive, highest-resolution infrared spectral imaging data ever taken of the prototypical PDR, the Orion Bar, have been captured by JWST. 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, the atomic PDR, and the HII region. We use JWST NIRSpec IFU and MIRI MRS observations of the Orion Bar from the JWST Early Release Science Program, PDRs4All (ID: 1288). We extract five template spectra to represent the morphology and environment of the Orion Bar PDR. 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. While the spectra are dominated by the well-known AIBs at 3.3, 6.2, 7.7, 8.6, 11.2, and 12.7 $\mu$m, a wealth of weaker features and sub-components are present. We report trends in the widths and relative strengths of AIBs across the five template spectra. These trends yield valuable insight into the photochemical evolution of PAHs, such as the evolution responsible for the shift of 11.2 $\mu$m AIB emission from class B$_{11.2}$ in the molecular PDR to class A$_{11.2}$ in the PDR surface layers. This photochemical evolution is driven by the increased importance of FUV processing in the PDR surface layers, resulting in a "weeding out" of the weakest links of the PAH family in these layers. For now, these JWST observations are consistent with a model in which the underlying PAH family is composed of a few species: the so-called 'grandPAHs'.

The effects of cosmic-ray bombardment of chiral molecules in the interstellar medium are simulated in the laboratory by performing radiolysis experiments of pure a-pinene ices at four different temperatures. The identification and significance of a-pinene have not been fully understood because of the insufficient amount of spectral information of these compounds at low temperatures. A comparison of the temperature dependence of the mid-infrared spectra of pure a-pinene ices before and after irradiation its irradiation by 61.3 MeV 84Kr15+ ions is performed. Mid-infrared Fourier transform (FTIR) spectroscopy was used to follow the changes in the chemicals, which allowed us to characterize the reaction products. This is the first time that the temperature dependence of a-pinene's radiolysis is determined; measurements occurred at 10, 50, 100 and 130 K. The spectra of non-irradiated samples are compared with those of samples irradiated by heavy ions. The new complex organic molecules (COMs) formed by radiolysis at different temperatures are non-chiral and contain up to six carbon atoms (e.g. benzene).