Rovibrational Lines Research Articles

Systematic Study of the Inner Structure of Molecular Tori in Nearby U/LIRGs Using Velocity Decomposition of CO Rovibrational Absorption Lines*

Abstract Determining the inner structure of the molecular torus around an active galactic nucleus is essential for understanding its formation mechanism. However, spatially resolving the torus is difficult because of its small size. To probe the clump conditions in the torus, we therefore perform the systematic velocity-decomposition analyses of the gaseous 12CO rovibrational absorption lines (v = 0 → 1, ΔJ = ±1) at λ ∼ 4.67 μm observed toward four (ultra)luminous infrared galaxies using the high-resolution (R ∼ 5000–10,000) spectroscopy from the Subaru Telescope. We find that each transition has two to five distinct velocity components with different line-of-sight (LOS) velocities (V LOS ∼ −240 to +100 km s−1) and dispersions (σ V ∼ 15–190 km s−1), i.e., the components (a), (b), ⋯, beginning with the broadest one in each target, indicating that the tori have clumpy structures. By assuming a hydrostatic disk ( σ V ∝ R rot − 0.5 ), we find that the tori has dynamic inner structures, with the innermost component (a) outflowing with velocity ∣V LOS∣ ∼ 160–240 km s−1, and the outer components (b) and (c) outflowing more slowly or infalling with ∣V LOS∣ ≲ 100 km s−1. In addition, we find that the innermost component (a) can be attributed to collisionally excited hot (≳530 K) and dense ( log ( n H 2 / cm − 3 ) ≳ 6 ) clumps, based on the level populations. Conversely, the outer component (b) can be attributed to cold (∼30–140 K) clumps radiatively excited by a far-infrared-to-submillimeter background with a brightness temperature higher than ∼20–400 K. These observational results demonstrate the clumpy and dynamic structure of tori in the presence of background radiation.

Pulling Back the Curtain on Shocks and Star Formation in NGC 1266 with Gemini-NIFS

We present Gemini near-infrared integral field spectrograph K-band observations of the central 400 pc of NGC 1266, a nearby (D ≈ 30 Mpc) post-starburst galaxy with a powerful multiphase outflow and a shocked interstellar medium. We detect seven H2 rovibrational emission lines excited thermally to T ∼ 2000 K, and weak Brγ emission, consistent with a fast continuous shock (or C-shock). With these bright H2 lines, we observe the spatial structure of the shock with an unambiguous tracer for the first time. The Brγ emission is concentrated in the central ≲100 pc, indicating that any remaining star formation in NGC 1266 is in the nucleus, while the surrounding cold molecular gas has little ongoing star formation. Though it is unclear what fraction of this Brγ emission is from star formation or the active galactic nuclei (AGN), assuming it is entirely due to star formation we measure an instantaneous star formation rate of 0.7 M ⊙ yr−1, though the star formation rate may be significantly higher in the presence of additional extinction. NGC 1266 provides a unique laboratory to study the complex interactions between AGN, outflows, shocks, and star formation, all of which are necessary to unravel the evolution of the post-starburst phase.

ExoMol line lists – LXII. Ro-vibrational energy levels and line strengths for the propadienediylidene (C3) in its ground electronic state

ABSTRACT Improved opacities are needed for modelling the atmospheres and evolution of cool carbon-rich stars and extra-solar planets; in particular, contributions made by the astrophysically important propadienediylidene (${\mathrm{C}}_{3}$) molecule need, at a minimum, to be determined using a line list which includes all significant transitions in the energy range of interest. We report variational calculations giving ro-vibrational energy levels and corresponding line strengths for $^{12}{\mathrm{C}}_3$, $^{12}{\mathrm{C}}^{13}{\mathrm{C}}^{12}{\mathrm{C}}$, and $^{12}{\mathrm{C}}^{12}{\mathrm{C}}^{13}{\mathrm{C}}$. In the $^{12}{\mathrm{C}}_3$ case, we obtain 2166 503 ro-vibrational state energies $\leqslant$2000 cm−1 for the electronic $\tilde{X}{\, }^{1}{\Sigma _{\rm g}}^{+}$ ground state. Comparison with experiment indicates a maximum error of $\pm 0.03$ ${\rm cm}^{-1}$ in calculated positions of lines involving an upper state energy $\lessapprox$4000 cm−1. For lines with upper state energies $\gtrapprox$4000 cm−1 to have comparable line-position accuracies, conical intersections would need to be accounted for in an adopted potential energy surface. Line lists and associated opacities are provided in the ExoMol data base (http://www.exomol.com).

High resolution laser diode spectroscopy of the hot bands of C2HD in the first overtone region of C-H stretching

The frequencies of C2HD hot bands ro-vibrational lines of the first overtone of C-H stretching were measured using tunable diode lasers. To expand the measuring range to the region of the large rotational numbers, a heated gas cell was used. Measurements were made in the range R1–R36 and P2–P14 for the transition (2,0,0,1,0) ← (0,0,0,1,0), and in the range R1–R18 for the transition (2,0,0,0,1) ← (0,0,0,0,1). The rotational parameters of the upper and lower states for the band (2,0,0,1,0) ← (0,0,0,1,0) were determined. The doublet structure of ro-vibrational lines was studied. For the transition (2,0,0,1,0) ← (0,0,0,1,0), a sharp increase in the splitting value was detected at J > 25.

First Ab Initio Line Lists for Triatomic Sulfur-Containing Molecules: S2O and S3.

We present in this paper the first room temperature rovibrational line lists for the main isotopologues of disulfur monoxide (S2O) and thiozone (S3) variationally computed from newly developed ab initio potential energy and dipole moment surfaces (PESs and DMSs). S2O and S3 are supposed to be potential candidates for astronomical detection, especially in the Venusian atmosphere where sulfur chemistry plays a major role. Contrary to other stable sulfur-containing species like SO2 or H2S, there is much less experimental data for the short-lived reactive S2O and S3 molecules. Indeed, the infrared spectra of S2O and S3 are quite complicated to analyze without consistent theoretical predictions because they contain both high J (∼100) and a lot of hot band transitions, even at T = 296 K. In this work, we have constructed PESs based on the single-reference coupled cluster approach [CCSD(T)] and including corrections due to the scalar relativistic effects, DBOC, and highly excited Slater determinants. The structure of the excited electronic states of S3 was also discussed. The nuclear-motion Eckart-Watson Hamiltonian expressed in terms of normal-mode irreducible tensor operators was used to compute the energy levels. For line intensity calculation, ab initio DMSs were computed at the CCSD(T)/aug-cc-pV5Z level of the theory. Rotation-vibration patterns in the fundamental bands of S2O and S3 have been simulated taking into account the recent Fourier-transform spectra of S2O recorded at the SOLEIL synchrotron. The present work aims at providing line intensities of S2O and S3 that are missing in the literature as well as new spectroscopic support for atmospheric applications.

Rovibrational Line Lists of Triplet and Singlet Methylene.

The methylene molecule (CH2) is a short-lived radical with lacking data on its spectral line intensities. Although the lifetime of CH2 is extremely short under Earth's conditions, it exists in a free form in interstellar media. CH2 is an important intermediate species in chemical reactions associated with the formation and destruction of complex hydrocarbons. We present the first rovibrational line lists of CH2 in its ground triplet and first excited singlet electronic state. To this end, our previously developed accurate ab initio potential energy surface (PES) was used for the ground electronic triplet state [Egorov et al. J. Comp. Chem. 2024. V. 45. (2). P. 83] while a new PES for the singlet state was constructed in this work using the single-reference coupled cluster approach [CCSD(T)] combined with the extrapolation to the complete basis set (CBS) limit based on the correlation-consistent orbital basis sets with the core-valence electron correlation effects [aug-cc-pCVXZ, X = T, Q, 5, and 6]. In addition, the contributions to the correlation energy from highly excited Slater determinants [CC(n), n = 3-5] were included as well as the scalar relativistic effects and DBOC. The most accurate description of the infrared band origins of singlet CH2 was thus achieved for the energy range where the impact of the nonadiabatic coupling due to the Renner-Teller effect can be neglected. To obtain the probabilities of the rovibrational transitions, new ab initio DMSs were constructed both for the triplet and singlet CH2 using the CCSD(T)/aug-cc-pCVQZ approach. Finally, the absorption spectra of triplet and singlet methylene were predicted from the variationally computed line lists.

JWST study of the DG Tau B disk-wind candidate

Context. The origin of outflows and their impact on protoplanetary disk evolution and planet-formation processes are still crucial open questions. DG Tau B is a Class I protostar associated with a structured disk and a rotating conical CO outflow and was recently identified by ALMA as one of the best CO disk wind candidate. It is therefore a perfect target for studying these questions. Aims. We aim to map and study any outflow component intermediate between the axial jet and the CO outflow in order to constrain the origin (irradiated or shocked disk wind or swept-up material) of the redshifted molecular outflows in DG Tau B. Methods. We analyzed observations obtained with James Webb Space Telescope NIRSpec-IFU and NIRCam, supplemented with IFU data from the SINFONI/VLT instrument. We investigated the morphology, kinematics, and excitation conditions of the ro-vibrational H2 emission lines and their relation with the atomic jet and CO outflow. We focus our analysis on the redshifted outflow lobe. Results. We observe a global layered structure of the redshifted outflows in DG Tau B, with the atomic jet inside the H2 cavity, which in turn is nested inside the CO conical outflow. We also find temperature, velocity, and collimation to be increasing towards the axis of the flow. The redshifted H2 emission traces a narrow conical cavity (semi-opening angle of 9.4°) that is wider than the axial jet but nested just inside the CO outflow. Both the jet and the H2 cavity originate from the innermost regions of the disk (r0 < 6 au). The redshifted H2 cavity flows with a constant vertical velocity of Vz = 22.5 ± 0.8 km s−1, twice faster than the conical CO flow. The excitation conditions imply a hot H2 gas (Tex ≃ 2200 K) with an average mass flux of Ṁ(H2) = 3 × 10−11 M⊙ yr−1, which is significantly lower than the jet and CO values. Conclusions. The global layered H2-CO structure in temperature, velocity, and collimation in the DG Tau B redshifted lobe is consistent with a magneto-hydrodynamic disk wind scenario. The hot H2 could trace the inner, dense photodissociation layer in the wind. An H2 launching region at disk radii of 0.2−0.4 au combined with a large ejection efficiency (ξ ≃ 1) would account for the mass flux and kinematics. Alternatively, the near-IR ro-vibrational H2 could be emitted in the interaction layer driven by successive jet bow shocks into an outer disk wind or envelope. Further constraints on both scenarios will be obtained from the analysis of MIRI observations.

Radiative and mechanical energies in galaxies

Context. Atomic and molecular lines emitted from galaxies are fundamental tracers of the medium responsible for the emission and contain valuable information regarding the energy budget and the strength of the different feedback mechanisms. Aims. The goal of this work is to provide a new framework for the interpretation of atomic and molecular lines originating from extragalactic sources and a robust method to deduce the mechanical and radiative energy budget from a set of observations. Methods. Atomic and molecular lines detected in a given object are assumed to result from the combination of distributions of shocks and photo-dissociation regions (PDRs) within the observational beam. The emission of individual structures is computed using the Paris-Durham shock code and the Meudon PDR code over a wide range of parameters. The total emission is then calculated assuming probability distribution functions for shocks and PDRs. A distance between the observational dataset and the model is finally defined based on the ratios of the observed to the predicted intensities. Results. As a test case scenario, we consider the radio galaxy 3C 326 N. The dataset is composed of 12 rotational and ro-vibrational lines of H2 and the fine structure lines of C+ and O. Our interpretative framework shows that both shocks and PDRs are required to explain the line fluxes. Surprisingly, viable solutions are obtained at low density only (nH < 100 cm−3), indicating that most of the emission originates from diffuse interstellar matter. The optimal solution, obtained for nH = 10 cm−3, corresponds to a distribution of low-velocity shocks (between 5 and 20 km s−1) propagating in PDR environments illuminated by a UV radiation field ten times larger than that in the solar neighborhood. This solution implies that at least 4% of the total mass carried by the PDRs is shocked. The H2 0-0 S(0) 28 μm, [CII] 158 μm, and [OI] 63 μm lines originate from the PDR components, while all the other H2 lines are mostly emitted by shocks. The total solid angles sustained by PDRs and shocks imply that the radiative and mechanical energies reprocessed by these structures are LUV = 6.3 × 109 L⊙ and LK = 3.9 × 108 L⊙, respectively, in remarkable agreement with the values of the IR luminosity deduced from the fit of the spectral energy distribution (SED) of 3C 326 N, and consistent with a small fraction of the active galactic nuclei (AGN) jet kinetic power dissipated in the interstellar medium (≈1%). Conclusions. This work shows that the radiative and mechanical energy budget of galaxies can be derived from the sole observations of atomic and molecular lines. It reveals the unexpected importance of the diffuse medium for 3C 326 N, in contrast to previous studies. A last-minute comparison of the model to new JWST data obtained in 3C 326 N shows a striking agreement and demonstrates the ability of the model to make accurate predictions. This framework opens new prospects for the prediction and interpretation of extragalactic observations, in particular in the context of JWST observations.

On the Interpretation of Mid-infrared Absorption Lines of Gas-phase H2O as Observed by JWST/MIRI

Rovibrational absorption lines of H2O in the 5–8 μm wavelength range selectively probe gas against the mid-infrared continuum-emitting background of the inner regions of young stellar objects and active galactic nuclei and deliver important information about these warm, dust-obscured environments. JWST/Mid-Infrared Instrument (MIRI) detects these lines in many lines of sight at a moderate spectral resolving power of R ∼ 3500 (full width at half-maximum of 85 km s−1). Based on our analysis of high-resolution SOFIA/EXES observations, we find that the interpretation of JWST/MIRI absorption spectra can be severely hampered by the blending of individual transitions and the lost information on the intrinsic line width or the partial coverage of the background continuum source. In this paper, we point out problems such as degeneracy that arise in deriving physical properties from an insufficiently resolved spectrum. This can lead to differences in the column density by 2 orders of magnitude. We emphasize the importance of weighting optically thin and weak lines in spectral analyses and provide recipes for breaking down the coupled parameters. We also provide an online tool to generate the H2O absorption line spectra that can be compared to observations.

High resolution water emission from nitromethane combustion under high pressure mono- and bipropellant conditions

Optical emission spectroscopy was used to measure rovibrationally resolved spectra of water formed during nitromethane combustion at elevated pressures in both oxidizing and inert atmospheres. Complementary kinetic models were used to predict flame temperatures, product distributions, and product formation rates, and results were compared with experimental observations. Experiments were carried out at pressures of 27.4 bar and 34.2 bar in inert atmospheres (referred to as monopropellant conditions) and air (referred to as bipropellant conditions). Dispersed emission shows many rovibrational transitions with the strongest occurring between 13,000 and 14,500 cm−1. These lines are primarily assigned to the relaxation from water's (3,0,1) vibrationally excited state to its vibrational ground state. Weaker progressions in this same region are assigned to water relaxation from the (1,0,3) and (2,2,1) vibrationally excited states. Data collected under mono- and bipropellant conditions showed very similar relative intensities of individual rovibrational lines. The spectra were fit using a simulated temperature of 2,500 K ± 500 K for both mono- and bipropellant conditions. Ex situ FTIR spectra of NM exhaust confirms the presence of H2O in both mono- and bipropellant combustion. Interestingly, analyses of these same spectra also show significant amounts of CO in monopropellant exhaust, but no detected CO in bipropellant exhaust. These latter findings show higher CO/CO2 concentration ratios compared to simulations, motivating the need for refined models describing nitromethane monopropellant combustion.

High resolution far-infrared synchrotron spectroscopy of 2-furfural conformers: Fundamental and hot bands.

In the continuity of a previous jet-cooled rovibrational study of trans and cis conformers of 2-furfural in the mid-infrared region (700-1750cm-1) [Chawananon et al., Molecules 28 (10), 4165 (2023)], the present work investigates the far-infrared spectroscopy of 2-furfural using a long path absorption cell coupled to a high-resolution Fourier transform spectrometer and synchrotron radiation at the AILES beamline of the SOLEIL synchrotron. Guided by anharmonic calculations, vibrational energy levels and excited-state rotational constants are sufficiently predictive for a complete assignment of all fundamental and combination bands up to 700cm-1, as well as the rovibrational analysis of 4 (1) low-frequency modes of trans-(cis-)2-furfural. A global rovibrational simulation, including far-infrared rovibrational lines and microwave and millimeter-wave rotational lines assigned in a previous study [Motiyenko et al., J. Mol. Spectrosc., 244, 9 (2007)] provides a reliable set of ground- and excited-state rotational parameters involving ring torsion, bending, and ring puckering modes of 2-furfural. In a second step, a rovibrational analysis of several hot band sequences, mainly involving the lowest frequency ring CHO torsion mode, is carried out. Reliable values of some anharmonic coefficients are obtained experimentally and could serve as a benchmark for validating advanced anharmonic calculations related to these large amplitude motions of flexible molecules.

Shockingly Bright Warm Carbon Monoxide Molecular Features in the Supernova Remnant Cassiopeia A Revealed by JWST

We present JWST NIRCam (F356W and F444W filters) and MIRI (F770W) images and NIRSpec Integral Field Unit (IFU) spectroscopy of the young Galactic supernova remnant Cassiopeia A (Cas A) to probe the physical conditions for molecular CO formation and destruction in supernova ejecta. We obtained the data as part of a JWST survey of Cas A. The NIRCam and MIRI images map the spatial distributions of synchrotron radiation, Ar-rich ejecta, and CO on both large and small scales, revealing remarkably complex structures. The CO emission is stronger at the outer layers than the Ar ejecta, which indicates the re-formation of CO molecules behind the reverse shock. NIRSpec-IFU spectra (3–5.5 μm) were obtained toward two representative knots in the NE and S fields that show very different nucleosynthesis characteristics. Both regions are dominated by the bright fundamental rovibrational band of CO in the two R and P branches, with strong [Ar vi] and relatively weaker, variable strength ejecta lines of [Si ix], [Ca iv], [Ca v], and [Mg iv]. The NIRSpec-IFU data resolve individual ejecta knots and filaments spatially and in velocity space. The fundamental CO band in the JWST spectra reveals unique shapes of CO, showing a few tens of sinusoidal patterns of rovibrational lines with pseudocontinuum underneath, which is attributed to the high-velocity widths of CO lines. Our results with LTE modeling of CO emission indicate a temperature of ∼1080 K and provide unique insight into the correlations between dust, molecules, and highly ionized ejecta in supernovae and have strong ramifications for modeling dust formation that is led by CO cooling in the early Universe.

JWST/NIRSpec and MIRI observations of an expanding, jet-driven bubble of warm H2 in the radio galaxy 3C 326 N

The physical link between AGN activity and the suppression of star formation in their host galaxies is one of the major open questions of the AGN feedback scenario. The Spitzer space mission revealed a subset of powerful nearby radio galaxies with unusually bright line emission from warm ($T 100$ K) molecular hydrogen, while typical star-formation tracers such as polycyclic aromatic hydrocarbons (PAHs) or a dust continuum have been exceptionally faint or undetected. Here, we present JWST NIRSpec and MIRI MRS IFU observations of one of the best studied galaxies of this class, 3C 326 N at z=0.09. We identified a total of 19 lines of the S, O, and Q series of ro-vibrational H$_2$ emission with NIRSpec at a 0.11 spatial resolution, probing a small quantity odot $) of gas at temperatures of $T 1000$ K. We also mapped the rotational mid-infrared lines of H$_2$ 0--0 S(3), S(5), and S(6) at a spatial resolution of 0.4 with MIRI/MRS, probing most of the $2 odot $ of warm H$_2$ in this galaxy. The CO band heads show a stellar component consistent with a 'slow-rotator' that is typical of a massive ($3 $\,M$_ galaxy, offering a reliable systemic redshift of $z=0.08979 0.0003$. The extended line emission shows a bipolar bubble expanding through the molecular disk at velocities of up to $, delineated by several bright clumps along the northern outer rim, potentially coming from gas fragmentation. Throughout the disk, H$_2$ is very broadly dispersed, with an FWHM of $ 100-1300$ km $ and complex, dual-component Gaussian line profiles. The extended FeII lambda 1.644 and Paalpha follow the same morphology, however NeIII lambda 15.56 is more symmetric about the nucleus. We show that most of the gas (with the exception of NeIII lambda 15.56) is predominantly heated by shocks driven by the radio jets into the gas, both for the ro-vibrational and rotational H$_2$ lines. In addition, the accompanying line broadening is sufficient to suppress star formation in the molecular gas. We also compared the morphology and kinematics of the rotational and ro-vibrational lines, finding the latter to be a good proxy to the global morphology and kinematic properties of the former in strongly turbulent environments. This demonstrates the potential of using the higher frequency ro-vibrational lines in studying turbulent molecular gas. Provided they are bright enough, they would allow us to examine turbulence in galaxies during the early phases of cosmic history, while most rotational lines are red-shifted out of the MIRI bandpass for $z ge1.5$

Jet-cooled ethylene cavity ring-down spectroscopy between 5880 and 6200 cm−1

Absorption spectra of jet-cooled ethylene (ethene, C2H4) are recorded at three different rotational temperatures (6/8 K, 12 K, 38 K) using cavity ring-down spectroscopy (CRDS) in the 5880–6200 cm−1 spectral region. Rotational cooling is used to determine the various vibrational band centers by simplifying drastically the rotational band structure. A line-by-line assignment, based on a direct comparison with the TheoReTS variational line list and a systematic use of lower state combination difference (LSCD), is performed. Experimental line lists including line position and line integrated absorption cross sections are drawn up. The 6/8 K, 12 K and 38 K spectra contain 668, 1553 and 1679 absorption lines respectively. Overall, 320 rovibrational lines are assigned across 20 interacting vibrational bands. Among the 20 vibrational cold bands identified in this work, 14 had never been observed before. Line intensities are in the range of 10−24 - 10−20 cm/molecule. A direct comparison between our work and the TheoReTS and ExoMol theoretical line lists, as well as with the recent experimental work of Ben Fathallah et al. (2024) is provided.

Shear Coaxial Methane–Oxygen Injector Mixing and Combustion Examined by Laser Absorption Tomography

Mixing length scales for methane–oxygen shear-coaxial single-element rocket injectors were experimentally assessed using laser absorption tomography. The laser spectroscopy technique enables quantitative and spatially resolved measurements of carbon monoxide (CO) and temperature within the near-field region, probing rovibrational absorption lines of CO near 5 μm. Multiple injector designs were examined with differing oxidizer post recess depths to compare mixing characteristics. All tests were performed at an oxidizer-to-fuel ratio of 3 with a total propellant flow rate of 0.350 g/s. Planar measurements were taken at 16 axial positions, which collectively captured the first 57 mm of each flame. A tomographic inversion method was applied to obtain radially resolved distributions of temperature and CO mole fraction for each axial position, from which two-dimensional images of the thermochemical structure were generated. Characteristic mixing parameters are defined and extracted from features within the species and temperature profiles to visualize the spatial evolution of CO production. Increasing oxidizer post recess improved mixing due to enhanced shear-induced turbulence and associated radial diffusion of species and temperature; however, the enhancement was nonlinear. This work establishes the first use of laser absorption tomography to directly measure mixing length scales associated with temperature and species profiles in coaxial flames.

PDRs4All

Context. JWST has taken the sharpest and most sensitive infrared (IR) spectral imaging observations ever of the Orion Bar photodis-sociation region (PDR), which is part of the nearest massive star-forming region the Orion Nebula, and often considered to be the ‘prototypical’ strongly illuminated PDR. Aims. We investigate the impact of radiative feedback from massive stars on their natal cloud and focus on the transition from the H II region to the atomic PDR – crossing the ionisation front (IF) –, and the subsequent transition to the molecular PDR – crossing the dissociation front (DF). Given the prevalence of PDRs in the interstellar medium and their dominant contribution to IR radiation, understanding the response of the PDR gas to far-ultraviolet (FUV) photons and the associated physical and chemical processes is fundamental to our understanding of star and planet formation and for the interpretation of any unresolved PDR as seen by JWST. Methods. We used 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 programme. We constructed a 3″ × 25″’ spatio-spectral mosaic covering 0.97– 5.27 μm at a spectral resolution R of ~2700 and an angular resolution of 0.075″–0.173″. To study the properties of key regions captured in this mosaic, we extracted five template spectra in apertures centred on the three H2 dissociation fronts, the atomic PDR, and the H II region. This wealth of detailed spatial-spectral information was analysed in terms of variations in the physical conditions-incident UV field, density, and temperature – of the PDR gas. Results. The NIRSpec data reveal a forest of lines including, but not limited to, He I, H I , and C I recombination lines; ionic lines (e.g. Fe III and Fe II); O I and N I fluorescence lines; aromatic infrared bands (AIBs, including aromatic CH, aliphatic CH, and their CD counterparts); pure rotational and ro-vibrational lines from H2; and ro-vibrational lines from HD, CO, and CH+, with most of them having been 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. In addition, we observed numerous smaller-scale structures whose typical size decreases with distance from θ1 Ori C and IR lines from C I , if solely arising from radiative recombination and cascade, reveal very high gas temperatures (a few 1000 K) consistent with the hot irradiated surface of small-scale dense clumps inside the PDR. The morphology of the Bar, in particular that of the H2 lines, reveals multiple prominent filaments that 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. We attribute the different characteristics of the H2 filaments to their varying depth into the PDR and, in some cases, not reaching the C+/C/CO transition. These observations thus reveal what local conditions are required to drive the physical and chemical processes needed to explain the different characteristics of the DFs and the photochemical evolution of the AIB carriers. Conclusions. 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.

GOALS-JWST: Mid-infrared Molecular Gas Excitation Probes the Local Conditions of Nuclear Star Clusters and the Active Galactic Nucleus in the LIRG VV 114

The enormous increase in mid-IR sensitivity and spatial and spectral resolution provided by the JWST spectrographs enables, for the first time, detailed extragalactic studies of molecular vibrational bands. This opens an entirely new window for the study of the molecular interstellar medium in luminous infrared galaxies (LIRGs). We present a detailed analysis of rovibrational bands of gas-phase CO, H2O, C2H2, and HCN toward the heavily obscured eastern nucleus of the LIRG VV 114, as observed by NIRSpec and the medium resolution spectrograph on the Mid-InfraRed Instrument (MIRI MRS). Spectra extracted from apertures of 130 pc in radius show a clear dichotomy between the obscured active galactic nucleus (AGN) and two intense starburst regions. We detect the 2.3 μm CO bandheads, characteristic of cool stellar atmospheres, in the star-forming regions, but not toward the AGN. Surprisingly, at 4.7 μm, we find highly excited CO (T ex ≈ 700–800 K out to at least rotational level J = 27) toward the star-forming regions, but only cooler gas (T ex ≈ 200 K) toward the AGN. We conclude that only mid-infrared pumping through the rovibrational lines can account for the equilibrium conditions found for CO and H2O in the deeply embedded starbursts. Here, the CO bands probe regions with an intense local radiation field inside dusty young massive star clusters or near the most massive young stars. The lack of high-excitation molecular gas toward the AGN is attributed to geometric dilution of the intense radiation from the bright point source. An overview of the relevant excitation and radiative transfer physics is provided in an appendix.

NLTE modelling of water-rich exoplanet atmospheres. Cooling and heating rates

The hydrogen and water molecules respond very differently to the collisional-radiative processes taking place in planetary atmospheres. Naturally, the question arises whether H2O-rich atmospheres are more (or less) resilient to long-term mass loss than H2-dominated ones if they radiate away the incident stellar energy more (or less) efficiently. If confirmed, the finding would have implications on our understanding of the evolution of sub-Neptune exoplanets. As a key step towards answering this question, we present a non-local thermodynamic equilibrium (NLTE) model of H2O for the atmospheric region where the gas accelerates to escape the planet and conditions relevant to close-in sub-Neptunes. Our exploratory calculations for isothermal gas composed of H2, H2O and e− reveal that: (1) In the pressure region ∼10−2–10−4 dyn cm−2 where the stellar extreme-ultraviolet (XUV) photons are typically deposited in the atmosphere, H2O is in rotational LTE but vibrational NLTE. Vibrational LTE is facilitated by high H2O abundances and fractional ionizations, and we report critical densities for the LTE-NLTE transition; (2) Vibrational cooling may locally dominate over rotational cooling, partly because of the comparatively small opacities of ro-vibrational lines; (3) Even low H2O abundances notably enhance the cooling, foreseeably offsetting some of the stellar heating; (4) Heating due to the deposition of stellar infrared (IR) photons is significant at pressures ≳0.1 dyn cm−2. We estimate the contribution of H2O excitation to the internal energy of the gas and speculate on the photodissociation from the excited vibrational states. Ultimately, our findings motivate the consideration of NLTE in the mass loss rate calculations of H2O-rich atmospheres.

Improved Model of the Effective Dipole Moments and Absolute Ro-Vibrational Line Strengths of the XY2 Asymmetric Top Molecules in the X 2B1 Doublet Electronic States

Improved Model of the Effective Dipole Moments and Absolute Ro-Vibrational Line Strengths of the XY2 Asymmetric Top Molecules in the X 2B1 Doublet Electronic States

Spectral profile of ro-vibrational transitions of HCl broadened by He, Ar and SF6: Testing the β-correction to the Hartmann-Tran profile and the speed dependent (complex) hard collision model

The β-correction to the Hartmann-Tran (HT) profile, recently introduced to model the spectral shape of a molecular transition strongly affected by the Dicke narrowing effect (Konefal et al., JQSRT 242 (2020) 106,784), the HT profile (Ngo et al., JQSRT 129 (2013) 89), and the speed dependent (complex) hard collision model (SDcHC, in which the velocity changing collision rate is characterized by a complex number) are tested using spectra of two rovibrational lines of HCl. The R(5) and R(9) lines of the fundamental band of HCl broadened by He, Ar and SF6 have been recorded with a difference-frequency laser spectrometer for total pressures ranging from 12 to 930 mbar. These lines, measured in large pressure ranges with different collision-partners provide a meaningful test of the above-mentioned line-shape models. The results confirm that non-Voigt effects are significant for HCl broadened by Ar and SF6 and mainly due to the large influence of the Dicke narrowing effect. For HCl-SF6 and HCl-Ar, especially for the R(9) line, using the β-correction together with the HT profile (and with the speed-dependent hard collision model, SDHC) significantly improves the fit residuals while it has no effect on (or tends to deteriorate) the quality of the fit for HCl-He and for the R(5) line of HCl-SF6, for which the influence of velocity changing collisions is smaller. Except for HCl-He, the SDcHC model leads to better quality of fit compared to the HT profile. The results also show that numerical correlations between refined line-shape parameters of the HT profile are important and can lead to ill-determined parameters while they are more properly determined with the SDcHC model.