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

(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'.Comment: 25 pages, 10 figures, to appear in A&

PDRs4All II: JWST's NIR and MIR imaging view of the Orion Nebula

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

PDRs4All III: JWST's NIR spectroscopic view of the Orion Bar

(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.Comment: 52 pages, 30 figures, submitted to A&

PDRs4All: A JWST Early Release Science Program on Radiative Feedback from Massive Stars

22 pags., 8 figs., 1 tab.Massive stars disrupt their natal molecular cloud material through radiative and mechanical feedback processes. These processes have profound effects on the evolution of interstellar matter in our Galaxy and throughout the universe, from the era of vigorous star formation at redshifts of 1-3 to the present day. The dominant feedback processes can be probed by observations of the Photo-Dissociation Regions (PDRs) where the far-ultraviolet photons of massive stars create warm regions of gas and dust in the neutral atomic and molecular gas. PDR emission provides a unique tool to study in detail the physical and chemical processes that are relevant for most of the mass in inter-and circumstellar media including diffuse clouds, proto-planetary disks, and molecular cloud surfaces, globules, planetary nebulae, and star-forming regions. PDR emission dominates the infrared (IR) spectra of star-forming galaxies. Most of the Galactic and extragalactic observations obtained with the James Webb Space Telescope (JWST) will therefore arise in PDR emission. In this paper we present an Early Release Science program using the MIRI, NIRSpec, and NIRCam instruments dedicated to the observations of an emblematic and nearby PDR: the Orion Bar. These early JWST observations will provide template data sets designed to identify key PDR characteristics in JWST observations. These data will serve to benchmark PDR models and extend them into the JWST era. We also present the Science-Enabling products that we will provide to the community. These template data sets and Science-Enabling products will guide the preparation of future proposals on star-forming regions in our Galaxy and beyond and will facilitate data analysis and interpretation of forthcoming JWST observations.Support for JWST-ERS program ID 1288 was provided through grants from the STScI under NASA contract NAS5-03127 to STScI (K.G., D.V.D.P., M.R.), Univ. of Maryland (M.W., M.P.), Univ. of Michigan (E.B., F.A.), and Univ. of Toledo (T.S.-Y.L.). O.B. and E.H. are supported by the Programme National “Physique et Chimie du Milieu Interstellaire” (PCMI) of CNRS/INSU with INC/INP co-funded by CEA and CNES, and through APR grants 6315 and 6410 provided by CNES. E. P. and J.C. acknowledge support from the National Science and Engineering Council of Canada (NSERC) Discovery Grant program (RGPIN-2020-06434 and RGPIN-2021-04197 respectively). E.P. acknowledges support from a Western Strategic Support Accelerator Grant (ROLA ID 0000050636). J.R.G. and S.C. thank the Spanish MCINN for funding support under grant PID2019-106110GB-I00. Work by M.R. and Y.O. is carried out within the Collaborative Research Centre 956, subproject C1, funded by the Deutsche Forschungsgemeinschaft (DFG)—project ID 184018867. T.O. acknowledges support from JSPS Bilateral Program, grant No. 120219939. M.P. and M.W. acknowledge support from NASA Astrophysics Data Analysis Program award #80NSSC19K0573. C.B. is grateful for an appointment at NASA Ames Research Center through the San José State University Research Foundation (NNX17AJ88A) and acknowledges support from the Internal Scientist Funding Model (ISFM) Directed Work Package at NASA Ames titled: “Laboratory Astrophysics—The NASA Ames PAH IR Spectroscopic Database.”Peer reviewe

Photophysics and evolution of cosmic polycyclic aromatic hydrocarbons at the James Webb space telescope era

Les spectres infrarouges des galaxies et de nombreux objets de la Voie Lactée sont dominés par des bandes spectrales intenses, localisées à 3.3, 6.2, 7.7, 8.6, 11.2, et 12.7µm, nommées "bandes aromatiques infrarouges" (AIB). Elles sont attribuées au refroidissement d'une classe de molécules appelées "hydrocarbures aromatiques polycycliques" (PAH), excitées par l'absorption des photons ultraviolets (UV) interstellaires. Les PAH jouent un rôle clé dans la physique des environnements dominés par le rayonnement UV, appelées régions de photodissociation (PDR). En particulier, ils ont une contribution majeure au chauffage du gaz par effet photoélectrique (effet PE). Cependant, à ce jour aucun PAH spécifique n'a pu être identifié, ce qui rend délicat la description, dans les modèles, de leur contribution à la physique et la chimie des milieux. En particulier la contribution de l'effet PE au chauffage du gaz est prise en compte dans de nombreux codes astrophysiques. Il est néanmoins possible d'obtenir des informations globales sur les familles de PAH interstellaires, en se basant sur l'analyse des spectres AIB et de leurs variations. Ceci peut être réalisé en utilisant des méthodes d'apprentissage basées sur la séparation aveugle de sources (SAS), qui permettent d'extraire des spectres d'émission infrarouge représentatifs de populations moyennes de PAH. Le premier volet de cette thèse a consisté à améliorer les approches basées sur la SAS dans le contexte de l'arrivée du futur observatoire spatial infrarouge JWST. En effet, les méthodes existantes étaient limitées par deux aspects : leur incapacité à traiter de gros volumes de données comme celles qui seront fournies par le JWST, et le fait que leur initialisation, réalisée de manière aléatoire, implique une non-stabilité des résultats. Pour résoudre ces problèmes, j'ai proposé et testé, un protocole basé sur une méthode de SAS hybride issue d'études antérieures. Cette approche a été testée sur un jeu de données d'archives (spectres SWS du satellite ISO) qui présente des caractéristiques semblables d'un point de vue spectral à celle du JWST. Ces tests ont permis de démontrer la robustesse et la rapidité de l'approche et les perspectives à venir pour l'analyse des données du JWST. Par ailleurs, nous avons obtenu de nouvelles informations sur la nature des porteurs des AIB avec, en particulier, une première analyse du domaine à 3µm. Le deuxième volet de cette thèse a concerné l'estimation de la contribution des PAH au chauffage du gaz interstellaire par effet photoélectrique ainsi que de l'effet de leur charge sur ce dernier, dans le cas de la PDR NGC 7023 NW. Pour obtenir cette contribution, j'ai comparé l'efficacité théorique de ce processus de chauffage à une valeur haute de l'efficacité observée, appelée efficacité "réduite". La première est calculée grâce aux paramètres moléculaire des PAH obtenues par de récentes expériences de laboratoire et par calcul de chimie quantique. L'efficacité observée "réduite" est obtenu par le ratio entre l'émission du gaz et l'émission additionnée du gaz et des PAH (obtenue via l'outil PAHTAT). Le rapport entre l'efficacité réduite et théorique donne la contribution relative des PAH au chauffage du gaz par rapport aux autres grains. Cette quantité, dans NGC 7023 NW s'élève entre 70 et 80 %, ce qui montre l'importance majeure des PAH dans le chauffage du gaz ainsi qu'une description moléculaire de ce processus de chauffage. La taille des PAH influence peu ce résultat, en revanche le modèle montre qu'une connaissance plus précise de l'énergie cinétique des photoélectrons est cruciale pour avancer dans ce domaine.The infrared spectra of galaxies and numerous astrophysical objects of the Milky Way are dominated by intense spectral bands, localized at 3.3, 6.2, 7.7, 8.6, 11.2 and 12.7µm, called "aromatic infrared bands" (AIB). They are attributed to the cooling of a class of molecules called "polycyclic aromatic hydrocarbons" (PAH), excited by the absorption of interstellar UV photons. PAHs play a key role in the physics of environments dominated by UV radiation, called photodissociation regions (PDR). In particular, they are the major contributors to the gas heating through the photoelectric effect (PE effect). Furthermore, PAHs play an important role in the extinction of stellar radiation and in the gas chemistry, being a possible catalyst in the Hdollar_2dollar formation, the most abundant molecule of the universe. However, for now no specific PAH has been identified in the interstellar medium, which can yield in a difficult description of their contribution to the physics and chemistry in models. In particular, the contribution of the PE effect on dust in the gas heating is considered in several codes. It is nevertheless possible to obtain global information on interstellar PAH families, based on the analysis of AIB spectra and their variations. This can be achieved using learning methods such as blind signal separation (BSS), which allows to extract infrared emission spectra, representative of mean populations of PAHs. The first part of this thesis consisted in upgrading the methods based on BSS in the context of the arrival of the James Webb Space Telescope (JWST). Indeed, current methods were limited in two aspects: their inability to handle large volumes of data such as those that will be provided by JWST, and the fact that their random initialization implied an instability of the results. In order to fix these issues, I proposed and developed an approach based on a hybrid BSS method. This approach has been tested on an archival data set (spectra from the SWS instrument onboard the ISO satellite) which present similar spectral characteristics as the coming JWST data. These tests allowed to show the robustness and rapidity of this approach and the perspectives for the analysis of the incoming JWST data. Moreover, we obtained new information on the nature of AIB carriers with, in particular a first analysis of the 3µm domain. The second part of this work was focused on the estimation of the PAH contribution to the interstellar gas heating by photoelectric effect as well as the effect of the charge in this process, in the case of the PDR NGC 7023 NW. In order to derive this contribution, I compared the theoritical efficiency of this heating process with an upper value of the observed efficiency, called "reduced" efficiency. the first one is computed using the molecular parameter of PAHs obtained by recent laboratory experiments and quantum chemistry computations. The "reduced" efficiency is derived using the ratio between the gas cooling emission and the sum of the gas cooling emission and the PAH cooling emission. The ratio between the "reduced" and theoritical efficiencies allows to derive the contribution of PAHs to the gas heating relatively to that of other types of grains. This quantity, in NGC 7023 NW, is comprised between 70 and 80%, which shows the major influence of PAHs in the gas heating as well as the molecular description of this heating process. The PAH size has only a little impact on this result, however the model shows that a more profound knowledge of the kinetic energy of photoelectrons is crucial to move forward in this area

Photophysique et évolution des hydrocarbures aromatiques polycycliques cosmiques à l'ère du télescope spatial James Webb

The infrared spectra of galaxies and numerous astrophysical objects of the Milky Way are dominated by intense spectral bands, localized at 3.3, 6.2, 7.7, 8.6, 11.2 and 12.7µm, called "aromatic infrared bands" (AIB). They are attributed to the cooling of a class of molecules called "polycyclic aromatic hydrocarbons" (PAH), excited by the absorption of interstellar UV photons. PAHs play a key role in the physics of environments dominated by UV radiation, called photodissociation regions (PDR). In particular, they are the major contributors to the gas heating through the photoelectric effect (PE effect). Furthermore, PAHs play an important role in the extinction of stellar radiation and in the gas chemistry, being a possible catalyst in the Hdollar_2dollar formation, the most abundant molecule of the universe. However, for now no specific PAH has been identified in the interstellar medium, which can yield in a difficult description of their contribution to the physics and chemistry in models. In particular, the contribution of the PE effect on dust in the gas heating is considered in several codes. It is nevertheless possible to obtain global information on interstellar PAH families, based on the analysis of AIB spectra and their variations. This can be achieved using learning methods such as blind signal separation (BSS), which allows to extract infrared emission spectra, representative of mean populations of PAHs. The first part of this thesis consisted in upgrading the methods based on BSS in the context of the arrival of the James Webb Space Telescope (JWST). Indeed, current methods were limited in two aspects: their inability to handle large volumes of data such as those that will be provided by JWST, and the fact that their random initialization implied an instability of the results. In order to fix these issues, I proposed and developed an approach based on a hybrid BSS method. This approach has been tested on an archival data set (spectra from the SWS instrument onboard the ISO satellite) which present similar spectral characteristics as the coming JWST data. These tests allowed to show the robustness and rapidity of this approach and the perspectives for the analysis of the incoming JWST data. Moreover, we obtained new information on the nature of AIB carriers with, in particular a first analysis of the 3µm domain. The second part of this work was focused on the estimation of the PAH contribution to the interstellar gas heating by photoelectric effect as well as the effect of the charge in this process, in the case of the PDR NGC 7023 NW. In order to derive this contribution, I compared the theoritical efficiency of this heating process with an upper value of the observed efficiency, called "reduced" efficiency. the first one is computed using the molecular parameter of PAHs obtained by recent laboratory experiments and quantum chemistry computations. The "reduced" efficiency is derived using the ratio between the gas cooling emission and the sum of the gas cooling emission and the PAH cooling emission. The ratio between the "reduced" and theoritical efficiencies allows to derive the contribution of PAHs to the gas heating relatively to that of other types of grains. This quantity, in NGC 7023 NW, is comprised between 70 and 80%, which shows the major influence of PAHs in the gas heating as well as the molecular description of this heating process. The PAH size has only a little impact on this result, however the model shows that a more profound knowledge of the kinetic energy of photoelectrons is crucial to move forward in this area.Les spectres infrarouges des galaxies et de nombreux objets de la Voie Lactée sont dominés par des bandes spectrales intenses, localisées à 3.3, 6.2, 7.7, 8.6, 11.2, et 12.7µm, nommées "bandes aromatiques infrarouges" (AIB). Elles sont attribuées au refroidissement d'une classe de molécules appelées "hydrocarbures aromatiques polycycliques" (PAH), excitées par l'absorption des photons ultraviolets (UV) interstellaires. Les PAH jouent un rôle clé dans la physique des environnements dominés par le rayonnement UV, appelées régions de photodissociation (PDR). En particulier, ils ont une contribution majeure au chauffage du gaz par effet photoélectrique (effet PE). Cependant, à ce jour aucun PAH spécifique n'a pu être identifié, ce qui rend délicat la description, dans les modèles, de leur contribution à la physique et la chimie des milieux. En particulier la contribution de l'effet PE au chauffage du gaz est prise en compte dans de nombreux codes astrophysiques. Il est néanmoins possible d'obtenir des informations globales sur les familles de PAH interstellaires, en se basant sur l'analyse des spectres AIB et de leurs variations. Ceci peut être réalisé en utilisant des méthodes d'apprentissage basées sur la séparation aveugle de sources (SAS), qui permettent d'extraire des spectres d'émission infrarouge représentatifs de populations moyennes de PAH. Le premier volet de cette thèse a consisté à améliorer les approches basées sur la SAS dans le contexte de l'arrivée du futur observatoire spatial infrarouge JWST. En effet, les méthodes existantes étaient limitées par deux aspects : leur incapacité à traiter de gros volumes de données comme celles qui seront fournies par le JWST, et le fait que leur initialisation, réalisée de manière aléatoire, implique une non-stabilité des résultats. Pour résoudre ces problèmes, j'ai proposé et testé, un protocole basé sur une méthode de SAS hybride issue d'études antérieures. Cette approche a été testée sur un jeu de données d'archives (spectres SWS du satellite ISO) qui présente des caractéristiques semblables d'un point de vue spectral à celle du JWST. Ces tests ont permis de démontrer la robustesse et la rapidité de l'approche et les perspectives à venir pour l'analyse des données du JWST. Par ailleurs, nous avons obtenu de nouvelles informations sur la nature des porteurs des AIB avec, en particulier, une première analyse du domaine à 3µm. Le deuxième volet de cette thèse a concerné l'estimation de la contribution des PAH au chauffage du gaz interstellaire par effet photoélectrique ainsi que de l'effet de leur charge sur ce dernier, dans le cas de la PDR NGC 7023 NW. Pour obtenir cette contribution, j'ai comparé l'efficacité théorique de ce processus de chauffage à une valeur haute de l'efficacité observée, appelée efficacité "réduite". La première est calculée grâce aux paramètres moléculaire des PAH obtenues par de récentes expériences de laboratoire et par calcul de chimie quantique. L'efficacité observée "réduite" est obtenu par le ratio entre l'émission du gaz et l'émission additionnée du gaz et des PAH (obtenue via l'outil PAHTAT). Le rapport entre l'efficacité réduite et théorique donne la contribution relative des PAH au chauffage du gaz par rapport aux autres grains. Cette quantité, dans NGC 7023 NW s'élève entre 70 et 80 %, ce qui montre l'importance majeure des PAH dans le chauffage du gaz ainsi qu'une description moléculaire de ce processus de chauffage. La taille des PAH influence peu ce résultat, en revanche le modèle montre qu'une connaissance plus précise de l'énergie cinétique des photoélectrons est cruciale pour avancer dans ce domaine

Learning mid-IR emission spectra of polycyclic aromatic hydrocarbon populations from observations

International audienceContext. The James Webb Space Telescope (JWST) will deliver an unprecedented quantity of high-quality spectral data over the 0.6-28 μm range. It will combine sensitivity, spectral resolution, and spatial resolution. Specific tools are required to provide efficient scientific analysis of such large data sets.Aims: Our aim is to illustrate the potential of unsupervised learning methods to get insights into chemical variations in the populations that carry the aromatic infrared bands (AIBs), more specifically polycyclic aromatic hydrocarbon (PAH) species and carbonaceous very small grains (VSGs).Methods: We present a method based on linear fitting and blind signal separation (BSS) for extracting representative spectra for a spectral data set. The method is fast and robust, which ensures its applicability to JWST spectral cubes. We tested this method on a sample of ISO-SWS data, which resemble most closely the JWST spectra in terms of spectral resolution and coverage.Results: Four representative spectra were extracted. Their main characteristics appear consistent with previous studies with populations dominated by cationic PAHs, neutral PAHs, evaporating VSGs, and large ionized PAHs, known as the PAHx population. In addition, the 3 μm range, which is considered here for the first time in a BSS method, reveals the presence of aliphatics connected to neutral PAHs. Each representative spectrum is found to carry second-order spectral signatures (e.g., small bands), which are connected with the underlying chemical diversity of populations. However, the precise attribution of theses signatures remains limited by the combined small size and heterogeneity of the sample of astronomical spectra available in this study.Conclusions: The upcoming JWST data will allow us to overcome this limitation. The large data sets of hyperspectral images provided by JWST analysed with the proposed method, which is fast and robust, will open promising perspectives for our understanding of the chemical evolution of the AIB carriers

Learning mid-IR emission spectra of PAHs populations from observations

International audienceObservations of the mid-infrared (mid-IR, 3-15 µm) spectra of photo-dissociation regions reveal ubiquitous, broad and intense emission bands, the aromatic infrared bands (AIBs), attributed to polycyclic aromatic hydrocarbons (PAHs). Studies of the AIBs showed spectral variations (e.g. in the band positions) between different astrophysical objects, or even within single object, thanks to hyperspectral images. The James Webb Space Telescope (JWST) will allow to get further spectral and spatial details compared to former space observatories. This will come with large data sets, which will require specific tools in order to perform efficient scientific analysis. We propose in this study a method based on blind signal separation to reduce the analysis of such large data set to that of a small number of elementary spectra, spectrally representative of the data set and physically interpretable as the spectra of populations of mid-IR emitters. The robustness and fastness of the method are improved compared to former algorithms. It is tested on a ISO-SWS data set, which approaches the best the characteristics of JWST data, from which four elementary spectra are extracted, attributed to cationic, neutral PAHs, evaporating very small grains and large and ionized PAHs

PDRs4All II: JWST's NIR and MIR imaging view of the Orion Nebula

International audienceThe 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

A far-ultraviolet–driven photoevaporation flow observed in a protoplanetary disk

International audienceMost low-mass stars form in stellar clusters that also contain massive stars, which are sources of far-ultraviolet (FUV) radiation. Theoretical models predict that this FUV radiation produces photodissociation regions (PDRs) on the surfaces of protoplanetary disks around low-mass stars, which affects planet formation within the disks. We report James Webb Space Telescope and Atacama Large Millimeter Array observations of a FUV-irradiated protoplanetary disk in the Orion Nebula. Emission lines are detected from the PDR; modeling their kinematics and excitation allowed us to constrain the physical conditions within the gas. We quantified the mass-loss rate induced by the FUV irradiation and found that it is sufficient to remove gas from the disk in less than a million years. This is rapid enough to affect giant planet formation in the disk