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 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 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&

IR radiative relaxation of benzene derivatives and large carbon clusters in the gas phase

This thesis is in line with investigations started after the statement of the PAH hypothesis. According to this hypothesis, the carriers of ubiquitously observed emission features in the mid-infrared wavelength range in the interstellar medium regions are species similar to aromatic hydrocarbons (with an aliphatic component). These species undergo a radiative relaxation induced by the absorption of a stellar UV photon by emitting in the mid-infrared by vibrational radiative de-excitation. The radiative relaxation after electronic excitation of benzene derivatives highly excited vibrationally in gas phase is detected via an infrared spectrometer (FIREFLY) developed in the laboratory. The temporal analysis of the emission spectrum (between 2.5 and 4.5 µm) can give information on the relaxation dynamics. This analysis can eventually be extended if it is supported by a vibrational emission simulation in competition with other pathways during the radiative cascade (collision, photodissociation, photoisomerization). This method has been applied to the isomerization dynamics of toluene (partially deuterated) to cycloheptatriene (isomerization from a 6-ring to a 7-ring) while trying to characterize it (isomerization barrier, A-prefactor). Although the isomerization dynamics proved to be too fast to be fully probed, this first study allowed to see the possibilities and limitations of the analysis of the radiative emission during cooling for this type of isomerization. This method of analysis was extended, without simulation, to the study of other isomerizations of the same type in the case of phenylacetylene and aniline (both partially deuterated). Recurrent fluorescence phenomenon (or Poincaré fluorescence) in Cn carbon clusters (n= 24, 42, 60) in competition with vibrational emission has been theoretically investigated. The simulation of the radiative cooling in the experiment described above has been adapted to this context where only radiative processes are in competition. In this work, the vibrational and electronic structures of several thousands of isomers of carbon clusters are known beforehand. This allows to draw conclusions on the emission spectrum of a large set of carbon clusters. Indeed, a large number of isomers are suspected to be present in the interstellar medium (ISM) and in burning media. It turned out that the competition between recurrent fluorescencevibrational emission depends on the internal energy, the size and the family of clusters. The emission spectrum simulation of carbon clusters in ISM conditions (irradiation by a 20 000 K star or the standard radiation field of the ISM) has shown that the C60 cages emission in the near-IR ( < 4 µm) could explain astrophysical observations. The resulting abundance of C60 cages in NGC 7023 has been estimated. The emission of a large set of carbon clusters in temperature fixed conditions has been simulation (still by a molecular approach) and indicates that the spectrum is blackbody-like. The average carbon clusters emissivity is then deduced. In this work, IR radiative relaxation processes have been employed as observable for experimental investigation of isomerization mechanism in benzene derivatives. Theoretically, the recurrent fluorescence and vibrational emission in large Cn (n= 24, 42, 60) carbon clusters have been studied in interstellar and temperature fixed context.Ce travail de thèse s’inscrit dans la lignée des travaux entamés depuis l’énoncé de l’hypothèse PAH stipulant que les pics d’émissions observés et omniprésents dans l’IR moyen dans le milieu interstellaire seraient dus à des espèces semblables aux hydrocarbures aromatiques polycycliques (avec éventuellement une composante aliphatique). Ces espèces émettraient dans l’IR moyen par désexcitation radiative vibrationnelle induite par l’absorption d’un photon UV stellaire. C’est dans ce contexte que nous avons effectué un travail expérimentale et théorique. La relaxation radiative après excitation électronique des dérivés de benzène en phase gazeuse très excités vibrationnellement a pu être détecté via un spectromètre infrarouge (FIREFLY) développé dans l’équipe. L’analyse des spectres d’émission (entre 2.5 et 4.5 μm) en fonction du temps peuvent donner des informations sur la dynamique de relaxation. Cette analyse peut éventuellement être approfondie si elle est appuyée par une simulation Monte Carlo cinétique de l’émission vibrationnelle en compétition avec d’autres voies lors de la cascade de relaxation (collision, photodissociation, photoisomérisation). Cette méthode a été appliquée à la dynamique d’isomérisation du toluène (partiellement deutérée) vers le cycloheptatriène (isomérisaton d’un cycle à 6 à un cycle à 7) en essayant de la caractériser (barrière d’isomérisation, préfacteur A). Bien que la dynamique d’isomérisation s’est avérée trop rapide pour être entièrement sondée, cette première étude a permis de voir les possibilités et les limites de l’analyse de la dynamique d’isomérisation via les processus de relaxation radiative. Cette méthode d’analyse a été étendu, sans simulation, à d’autres isomérisations du même type dans le cas du phénylacétylène et l’aniline (les deux partiellement deutérés). Le phénomène de fluorescence récurrente (ou fluorescence de Poincaré) dans les agrégats de carbone Cn (n= 24, 42, 60) en compétition avec l’émission vibrationnelle a été étudié par simulation. La simulation de l’expérience décrite plus haut a été adaptée à ce contexte où seules les processus radiatifs sont en compétition. Nous avions à notre disposition les structures vibrationnelles et électroniques de plusieurs milliers d’isomères d’agrégats carbonés, ce qui nous a permis de tirer des conclusions sur le spectre d’émission d’un grand ensemble d’agrégats de carbone. Dans ce travail, les structures vibrationnelles et électroniques de plusieurs milliers d’isomères d’agrégats carbonés sont au préalable connus. Ceci permet de tirer des conclusions sur le spectre d’émission d’un grand ensemble agrégats carbonés. En effet, c’est un grand nombre d’isomères qui sont susceptés d’être présents dans le milieu interstellaire et dans les milieux en combustion. Il s’est avéré que la compétition entre la fluorescence récurrente et vibrationnelle dépend de l’énergie interne, de la taille et de la famille des agrégats. La simulation du spectre d’émission des agrégats de carbones dans les conditions du milieu interstellaire (irradiés par une étoile à 20000 K ou par le champ de radiation standard du milieu interstellaire) a montré que l’émission des cages C60 dans le proche IR (< 4 μm) pourrait expliquer le continuum d’émission dans le proche IR observés dans certaines sources astrophysiques. L’émission d’un grand nombre d’agrégats de carbone à température fixée a aussi été simulée (toujours par une approche moléculaire) et indique que le spectre d’émission est de type corps noir. L’émissivité moyenne des agrégats de carbone a alors été déduite. Dans ce travail, les processus de relaxation radiative dans l’infrarouge ont été employé en tant qu’observable pour l’étude expérimentale du mécanisme d’isomérisation dans les dérivés du benzène. Sur le plan théorique, la fluorescence récurrente et l’émission vibrationnelle dans les gros agrégats de carbone Cn (n = 24, 42, 60) ont été étudiées dans le contexte d’environnement à température fixe

Relaxation radiative dans l'infrarouge des dérivés de benzène et de gros agrégats de carbone en phase gazeuse

Ce travail de thèse s’inscrit dans la lignée des travaux entamés depuis l’énoncé de l’hypothèse PAH stipulant que les pics d’émissions observés et omniprésents dans l’IR moyen dans le milieu interstellaire seraient dus à des espèces semblables aux hydrocarbures aromatiques polycycliques (avec éventuellement une composante aliphatique). Ces espèces émettraient dans l’IR moyen par désexcitation radiative vibrationnelle induite par l’absorption d’un photon UV stellaire. C’est dans ce contexte que nous avons effectué un travail expérimentale et théorique. La relaxation radiative après excitation électronique des dérivés de benzène en phase gazeuse très excités vibrationnellement a pu être détecté via un spectromètre infrarouge (FIREFLY) développé dans l’équipe. L’analyse des spectres d’émission (entre 2.5 et 4.5 μm) en fonction du temps peuvent donner des informations sur la dynamique de relaxation. Cette analyse peut éventuellement être approfondie si elle est appuyée par une simulation Monte Carlo cinétique de l’émission vibrationnelle en compétition avec d’autres voies lors de la cascade de relaxation (collision, photodissociation, photoisomérisation). Cette méthode a été appliquée à la dynamique d’isomérisation du toluène (partiellement deutérée) vers le cycloheptatriène (isomérisaton d’un cycle à 6 à un cycle à 7) en essayant de la caractériser (barrière d’isomérisation, préfacteur A). Bien que la dynamique d’isomérisation s’est avérée trop rapide pour être entièrement sondée, cette première étude a permis de voir les possibilités et les limites de l’analyse de la dynamique d’isomérisation via les processus de relaxation radiative. Cette méthode d’analyse a été étendu, sans simulation, à d’autres isomérisations du même type dans le cas du phénylacétylène et l’aniline (les deux partiellement deutérés). Le phénomène de fluorescence récurrente (ou fluorescence de Poincaré) dans les agrégats de carbone Cn (n= 24, 42, 60) en compétition avec l’émission vibrationnelle a été étudié par simulation. La simulation de l’expérience décrite plus haut a été adaptée à ce contexte où seules les processus radiatifs sont en compétition. Nous avions à notre disposition les structures vibrationnelles et électroniques de plusieurs milliers d’isomères d’agrégats carbonés, ce qui nous a permis de tirer des conclusions sur le spectre d’émission d’un grand ensemble d’agrégats de carbone. Dans ce travail, les structures vibrationnelles et électroniques de plusieurs milliers d’isomères d’agrégats carbonés sont au préalable connus. Ceci permet de tirer des conclusions sur le spectre d’émission d’un grand ensemble agrégats carbonés. En effet, c’est un grand nombre d’isomères qui sont susceptés d’être présents dans le milieu interstellaire et dans les milieux en combustion. Il s’est avéré que la compétition entre la fluorescence récurrente et vibrationnelle dépend de l’énergie interne, de la taille et de la famille des agrégats. La simulation du spectre d’émission des agrégats de carbones dans les conditions du milieu interstellaire (irradiés par une étoile à 20000 K ou par le champ de radiation standard du milieu interstellaire) a montré que l’émission des cages C60 dans le proche IR (< 4 μm) pourrait expliquer le continuum d’émission dans le proche IR observés dans certaines sources astrophysiques. L’émission d’un grand nombre d’agrégats de carbone à température fixée a aussi été simulée (toujours par une approche moléculaire) et indique que le spectre d’émission est de type corps noir. L’émissivité moyenne des agrégats de carbone a alors été déduite. Dans ce travail, les processus de relaxation radiative dans l’infrarouge ont été employé en tant qu’observable pour l’étude expérimentale du mécanisme d’isomérisation dans les dérivés du benzène. Sur le plan théorique, la fluorescence récurrente et l’émission vibrationnelle dans les gros agrégats de carbone Cn (n = 24, 42, 60) ont été étudiées dans le contexte d’environnement à température fixe.This thesis is in line with investigations started after the statement of the PAH hypothesis. According to this hypothesis, the carriers of ubiquitously observed emission features in the mid-infrared wavelength range in the interstellar medium regions are species similar to aromatic hydrocarbons (with an aliphatic component). These species undergo a radiative relaxation induced by the absorption of a stellar UV photon by emitting in the mid-infrared by vibrational radiative de-excitation. The radiative relaxation after electronic excitation of benzene derivatives highly excited vibrationally in gas phase is detected via an infrared spectrometer (FIREFLY) developed in the laboratory. The temporal analysis of the emission spectrum (between 2.5 and 4.5 µm) can give information on the relaxation dynamics. This analysis can eventually be extended if it is supported by a vibrational emission simulation in competition with other pathways during the radiative cascade (collision, photodissociation, photoisomerization). This method has been applied to the isomerization dynamics of toluene (partially deuterated) to cycloheptatriene (isomerization from a 6-ring to a 7-ring) while trying to characterize it (isomerization barrier, A-prefactor). Although the isomerization dynamics proved to be too fast to be fully probed, this first study allowed to see the possibilities and limitations of the analysis of the radiative emission during cooling for this type of isomerization. This method of analysis was extended, without simulation, to the study of other isomerizations of the same type in the case of phenylacetylene and aniline (both partially deuterated). Recurrent fluorescence phenomenon (or Poincaré fluorescence) in Cn carbon clusters (n= 24, 42, 60) in competition with vibrational emission has been theoretically investigated. The simulation of the radiative cooling in the experiment described above has been adapted to this context where only radiative processes are in competition. In this work, the vibrational and electronic structures of several thousands of isomers of carbon clusters are known beforehand. This allows to draw conclusions on the emission spectrum of a large set of carbon clusters. Indeed, a large number of isomers are suspected to be present in the interstellar medium (ISM) and in burning media. It turned out that the competition between recurrent fluorescencevibrational emission depends on the internal energy, the size and the family of clusters. The emission spectrum simulation of carbon clusters in ISM conditions (irradiation by a 20 000 K star or the standard radiation field of the ISM) has shown that the C60 cages emission in the near-IR ( < 4 µm) could explain astrophysical observations. The resulting abundance of C60 cages in NGC 7023 has been estimated. The emission of a large set of carbon clusters in temperature fixed conditions has been simulation (still by a molecular approach) and indicates that the spectrum is blackbody-like. The average carbon clusters emissivity is then deduced. In this work, IR radiative relaxation processes have been employed as observable for experimental investigation of isomerization mechanism in benzene derivatives. Theoretically, the recurrent fluorescence and vibrational emission in large Cn (n= 24, 42, 60) carbon clusters have been studied in interstellar and temperature fixed context

Aromatic and Acetylenic C–H or C–D Stretching Bands Anharmonicity Detection of Phenylacetylene by UV Laser-Induced Vibrational Emission

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Emission spectra of fullerenes: Computational evidence for blackbody-like radiation due to structural diversity and electronic similarity

The spectral emission of hot C60 has been experimentally shown to be broad and continuous, in apparent contradiction with the discrete and narrow absorption spectrum associated with the high symmetry of buckminsterfullerene. In the present work we computationally model the emission spectrum of isolated carbon clusters, assuming a broad distribution of isomers that are likely populated under the experimental conditions. The contributions of individual structures to the global spectrum correspond to the relaxation via recurrent fluorescence and vibrational emission, electronic and vibrational structures being described by a simple but efficient density-functional-based tight-binding scheme. The model predicts a blackbody-like emission spectrum that is naturally broad and correctly accounts for the experimental measurements, except for a maximum that is quantitatively shifted with respect to Wien's displacement law. To quantify such differences, we introduce an emissivity parameter ɛ as the ratio between the spectral emittance and the corresponding exact blackbody spectrum; ɛ is numerically found to scale as (λT)−2 at leading order with increasing temperature T and for wavelengths λ>350 nm, and we provide a theoretical justification for this behavior. Our results are discussed in the light of the astrophysical detection of interstellar fullerenes, as well as in combustion environments where carbon clusters are relevant in the context of nascent soot particle formation

Radiative relaxation in isolated large carbon clusters: Vibrational emission versus recurrent fluorescence

International audienc

Formation of the methyl cation by photochemistry in a protoplanetary disk

Forty years ago it was proposed that gas phase organic chemistry in the interstellar medium was initiated by the methyl cation CH + 3 (1-3), but hitherto it has not been observed outside the Solar System (4,5). Alternative routes involving processes on grain surfaces have been invoke

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