In the interstellar medium, cosmic rays (CRs) generate a field of ultraviolet (UV) photons via excitation and subsequent radiative decay of H2 molecules. This UV field is a major agent of ionization and dissociation in the inner regions of molecular clouds that are shielded from the effects of the interstellar radiation field. In particular, the dissociation of H2, by far the most abundant molecule in interstellar clouds, leads to the production of atomic hydrogen, which then takes part in the production of a multitude of molecules, in particular complex organics on the surfaces of interstellar dust grains. Precise knowledge of the rates of CR-induced dissociation processes is thus crucial for constructing reliable chemical models. For the present paper, we have derived a new value of k diss,CR(H2) = 0.831 zeta for the rate of H2 dissociation, where zeta is the CR ionization rate of H2. This prediction contrasts a previous value from the Leiden database which overestimated the rate due to an inconsistent treatment of the H2 abundances and photodissociation cross sections. By running a series of chemical models, we show that the overestimated dissociation rate has a large effect on the results of chemical simulations, with the abundance of methanol being overestimated by over 1 order of magnitude. Hence, we strongly recommend the adoption of our new estimate, k diss,CR(H2) = 0.831 zeta in all chemical models that include this process. Our newly derived value corresponds to H2 being purely in the para form (J '' = 0). However, in the interiors of molecular clouds, the H2 ortho-to-para ratio is low and using the rate for para-H2 is an adequate approximation.

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

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

Prestellar cores are the sites of the earliest stages of star formation. Dust continuum observations are often used to identify and characterize their properties yet only a small fraction of them was observed and studied in terms of their composition and dynamical status. We explore the chemical diversity of prestellar cores and protostellar cores residing in the Orion B giant molecular cloud selected on their dust continuum emission to provide an unbiased view of their line emission properties and how they vary as function of the core parameters and environment. We make use of the large scale maps of Orion B in 25 molecular lines from which we extract information for a sample of 1001 cores selected using positions extracted from Herschel dust continuum observations. The main properties of the core sample are derived using the Principal Component Analysis and additional maps of physical parameters: column density N_H_2, far-ultraviolet (FUV) radiation field G_0 and mean volume gas density n. Additional high spectral resolution observations of C^18O(1-0) serve to evaluate the dynamical status of cores. The average line width of the cores is larger than what is typically expected for prestellar cores of closer star forming regions, which suggests that cores in Orion B are subjected to stronger turbulence affecting their stability. The first factor of the PCA analysis explaining the variation of the detected line intensities is the core column density of molecular gas. The second factor explains how the core chemical composition is strictly linked to their environment, which can be traced by the ratio of the external FUV radiation field over the core volume density, G_0/n. The third factor explaining the core chemical diversity is the mean density along the core line of sight, which is also associated with freeze-out and fractionation signatures.

Complex organic molecules (COMs) are considered essential precursors to prebiotic species in the interstellar and circumstellar medium. Despite their astrobiological relevance, many aspects of the formation of COMs remain unclear, particularly the role of ultraviolet (UV) radiation. While COMs were once expected to be efficiently destroyed under UV-irradiated conditions, detections in photodissociation regions (PDRs) have challenged this view. However, the mechanisms by which UV radiation contributes to their formation are still uncertain. Here we present moderately resolved maps of simple and complex organic molecules at the UV-illuminated edge of the Horsehead nebula, obtained by combining Atacama Large Millimeter/submillimeter Array (ALMA) and IRAM 30 m single-dish observations at similar to 15 '' resolution. For the first time in this PDR environment, we analyzed the spatial distribution of species such as (CO)-O-17, CH2CO, CH3CHO, HNCO, CH3CN, and HC3N. By incorporating previous (CO)-O-17 and (CO)-O-18 single-dish data as well as Plateau de Bure Interferometer (PdBI) maps of H2CO and CH3OH, we derived profiles of gas density, temperature, thermal pressure, and column densities of the organic species as a function of distance from the UV source. Our results show that most organic species - particularly H2CO, CH2CO, CH3CHO, HNCO, and CH3CN - exhibit enhanced column densities at the UV-illuminated edge compared to cloud interiors, possibly indicating efficient dust-grain surface chemistry driven by the diffusion of atomic C and radicals produced via photodissociation of CO and CH3OH, as supported by recent laboratory experiments. The exceptions, HC3N and CH3OH, can be attributed to inefficient formation on dust grains and ineffective nonthermal desorption into the gas phase, respectively. Additionally, contributions from gas-phase hydrocarbon photochemistry, possibly seeded by grain-surface products, cannot be ruled out. Further chemical modeling is needed to confirm the efficiency of these pathways for the studied species, which could have important implications for other cold UV-irradiated environments such as protoplanetary disks.

(Abridged) Complex organic molecules (COMs) are considered essential precursors to prebiotic species. While COMs were once expected to be efficiently destroyed under UV-irradiated conditions, detections in photodissociation regions (PDRs) have challenged this view. However, the mechanisms by which UV radiation contributes to their formation are still uncertain. Here, we present moderately resolved maps of simple and complex organic molecules at the UV-illuminated edge of the Horsehead nebula, obtained by combining ALMA and IRAM 30m single-dish observations at ∼ 15^'' resolution. We analyze the spatial distribution of species such as C^17O, CH_2CO, CH_3CHO, HNCO, CH_3CN, and HC_3N. By incorporating previous C^17O and C^18O single-dish data as well as PdBI maps of H_2CO and CH_3OH, we derive profiles of gas density, temperature, thermal pressure, and column densities of the organic species as a function of distance from the UV source. Our results show that most organic species-particularly H_2CO, CH_2CO, CH_3CHO, HNCO, and CH_3CN-exhibit enhanced column densities at the UV-illuminated edge compared to cloud interiors, possibly indicating efficient dust-grain surface chemistry driven by the diffusion of atomic C and radicals produced via photodissociation of CO and CH_3OH, as supported by recent laboratory experiments. The exceptions, HC_3N and CH_3OH, can be attributed to inefficient formation on dust grains and ineffective non-thermal desorption into the gas phase, respectively. Additionally, contributions from gas-phase hydrocarbon photochemistry, possibly seeded by grain-surface products, cannot be ruled out. Further chemical modeling is needed to confirm the efficiency of these pathways for the studied species, which could have important implications for other cold, UV-irradiated environments such as protoplanetary disks.

The ionization fraction (f_e=n_e/n_H) is a crucial parameter of interstellar gas, yet estimating it requires deep knowledge of molecular gas chemistry and observations of specific lines, such as those from isotopologs like HCO^+ and N_2H^+, which are detectable only in dense cores. Previous challenges in constraining f_e over large areas stemmed from the limitations of observational tracers and chemical models. Recent models have identified molecular line ratios that can trace f_e in different environments within molecular clouds. In this study, we analyze various molecular lines in the 3-4 mm range to derive the ionization fraction across the Orion B giant molecular cloud. We focus on dense and translucent gas, exploring variations with gas density (n) and the far-ultraviolet (FUV) radiation field (G_0). Our findings show that the ionization fraction ranges from 10^-5.5 to 10^-4 in translucent gas and 10^-8 to 10^-6 in dense gas. Notably, f_e is sensitive to G_0 in dense, UV-illuminated regions, decreasing with increasing volume density (f_e∝ n^-0.227 for dense and f_e∝ n^-0.3 for translucent gas) and increasing with G_0. In translucent gas, differing line ratios yield consistent fe values, indicating the importance of electron excitation of HCN and HNC. For dense gas, we recommend using the CN(1-0)/N_2H^+(1-0) ratio for upper limits on fe and C^18O(1-0)/HCO^+(1-0) for lower limits. In translucent environments, CCH(1-0)/HNC(1-0) effectively traces f_e. The higher fe values in translucent gas align with the C^+/CI/CO transition, while values in dense gas are adequate for coupling with the magnetic field.

We aim to develop a new method to infer the sub-beam probability density function (PDF) of H2 column densities and the dense gas mass within molecular clouds using spatially unresolved observations of molecular emission lines in the 3 mm band. We model spatially unresolved line integrated intensity measurements as the average of an emission function weighted by the sub-beam column density PDF. The emission function, which expresses the line integrated intensity as a function of the gas column density, is an empirical fit to high resolution (< 0.05 pc) multi-line observations of the Orion B molecular cloud. The column density PDF is assumed to be parametric, composed of a lognormal distribution at moderate column densities and a power law distribution at higher column densities. To estimate the sub-beam column density PDF, the emission model is combined with a Bayesian inversion algorithm (the Beetroots code), which takes account of thermal noise and calibration errors. We validate our method by demonstrating that it recovers the true column density PDF of the Orion B cloud, reproducing the observed emission line integrated intensities. We apply the method to 12CO(J=1-0), 13CO(J=1-0), C18O(J=1-0), HCN(J=1-0), HCO+(J=1-0) and N2H+(J=1-0) observations of a 700 x 700 pc2 field of view (FoV) in the nearby galaxy M51. On average, the model reproduces the observed intensities within 30

We present a theoretical framework for interpreting far-ultraviolet (FUV) fluorescent emission from molecular hydrogen (H _2 ) in high-redshift galaxies, motivated by the unique capabilities of the James Webb Space Telescope (JWST) to probe the rest-frame FUV at cosmic dawn. Using the Meudon photodissociation region code, we model the H _2 fluorescence spectrum under extreme interstellar medium (ISM) conditions in terms of high pressure (10 ^11 K cm ^−3 ) and high radiation field (10 ^6 G _0 ), combined with low metallicity ( Z = 0.1 Z _⊙ ) and high cosmic ionization rate ( ζ = 10 ^−14 s ^−1 ), characteristic of early galaxies. As a case study, we apply this framework to stacked NIRSpec spectra from the JWST Advanced Deep Extragalactic Survey for galaxies at redshifts z ≥ 7. The stacked spectrum exhibits emission features consistent in profile and wavelength with the predicted H _2 fluorescence lines, including a blueshift suggestive of an outflow of molecular gas. Although individual features remain below robust detection thresholds, this demonstration illustrates the feasibility of using FUV fluorescence modeling to guide and interpret JWST spectroscopy of the molecular ISM at high redshift. Our framework provides a foundation for future searches for molecular hydrogen emission and the study of galactic feedback processes in the early Universe.

The likely JWST detection of vibrationally excited H-3(+) emission in Orion's irradiated disk system d203-506 raises the important question of whether cosmic-ray ionization is enhanced in disks within clustered star-forming regions, or whether alternative mechanisms contribute to H-3(+) formation and excitation. We present a detailed model of the photodissociation region (PDR) component of a protoplanetary disk - comprising the outer disk surface and the photoevaporative wind - exposed to strong external far-ultraviolet (FUV) radiation. We investigate key gas-phase reactions involving excited H-2 that lead to the formation of H-3(+) in the PDR, including detailed state-to-state dynamical calculations of reactions H-2(v >= 0) + HOC+ -> H-3(+) + CO and H-2(v >= 0) + H+ -> H-2(+) + H. We also consider the effects of photoionization of vibrationally excited H-2(v >= 4), a process not previously included in PDR or disk models. We find that these FUV-driven reactions dominate the formation of H-3(+) in the PDR of strongly irradiated disks, largely independently of cosmic-ray ionization. The predicted H-3(+) abundance in the disk PDR peaks at x(H-3(+)) greater than or similar to 10(-8), coinciding with regions of enhanced HOC+ and water vapor abundances, and is linked to the strength of the external FUV field (G(0)). The predicted H-3(+) column density (less than or similar to 10(13) cm(-2)) agrees with the presence of H-3(+) in the PDR of d203-506. We also find that formation pumping, resulting from exoergic reactions between excited H-2 and HOC+, drives the vibrational excitation of H-3(+) in these regions. We expect this photochemistry to be highly active in disks where G(0) > 10(3). The H-3(+) formation pathways studied here may also be relevant in the inner disk region (near the host star), in exoplanetary ionospheres, and in the early Universe.

The likely JWST detection of vibrationally excited H 3 + emission in Orion’s irradiated disk system d203-506 raises the important question of whether cosmic-ray ionization is enhanced in disks within clustered star-forming regions, or whether alternative mechanisms contribute to H 3 + formation and excitation. We present a detailed model of the photodissociation region (PDR) component of a protoplanetary disk – comprising the outer disk surface and the photoevaporative wind – exposed to strong external far-ultraviolet (FUV) radiation. We investigate key gas-phase reactions involving excited H 2 that lead to the formation of H 3 + in the PDR, including detailed state-to-state dynamical calculations of reactions H 2 ( v ≥ 0) + HOC + → H 3 + + CO and H 2 ( v ≥ 0) + H + → H 2 + + H. We also consider the effects of photoionization of vibrationally excited H 2 ( v ≥ 4), a process not previously included in PDR or disk models. We find that these FUV-driven reactions dominate the formation of H 3 + in the PDR of strongly irradiated disks, largely independently of cosmic-ray ionization. The predicted H 3 + abundance in the disk PDR peaks at x (H 3 + ) ≳ 10 −8 , coinciding with regions of enhanced HOC + and water vapor abundances, and is linked to the strength of the external FUV field ( G 0 ). The predicted H 3 + column density (≲10 13 cm −2 ) agrees with the presence of H 3 + in the PDR of d203-506. We also find that formation pumping, resulting from exoergic reactions between excited H 2 and HOC + , drives the vibrational excitation of H 3 + in these regions. We expect this photochemistry to be highly active in disks where G 0 > 10 3 . The H 3 + formation pathways studied here may also be relevant in the inner disk region (near the host star), in exoplanetary ionospheres, and in the early Universe.

Context. In diffuse interstellar clouds, the excitation temperature derived from the lowest levels of H3+ is systematically lower than that derived from H2. The differences may be attributed to the lack of state-specific formation and destruction rates of H3+, which are needed to thermalize the two species.Aims. In this work, we aim to investigate the possible influence of rotational excitation collisions of H3+ with atomic hydrogen on its excitation temperature.Methods. We used a time-independent close-coupling method to calculate the state-to-state rate coefficients, incorporating a very accurate and full-dimensional potential energy surface recently developed for H4+. We take a symmetric top approach to describe a frozen H3+ as an equilateral triangle.Results. We derive rotational excitation collision rate coefficients of H3+ with atomic hydrogen in a temperature range corresponding to diffuse interstellar conditions up to (J, K, +/-) = (7, 6, +) and (J, K, +/-) = (6, 4, +) for its ortho and para forms. This allows us to obtain a consistent set of collisional excitation rate coefficients and to improve on a previous study that included speculations regarding these contributions.Conclusions. The new state-specific inelastic H3+ + H rate coefficients yield differences of up to 20% in the excitation temperature, and their impact increases with decreasing molecular fraction. We also confirm the impact of chemical state-to-state destruction reactions on the excitation balance of H3+, and that reactive H + H3+ collisions are also needed to account for possible further ortho to para transitions.

We present a theoretical framework for interpreting far-ultraviolet (FUV) fluorescent emission from molecular hydrogen (H_2) in high-redshift galaxies, motivated by the unique capabilities of the James Webb Space Telescope (JWST) to probe the rest frame FUV at cosmic dawn. Using the Meudon photodissociation region (PDR) code, we model the H_2 fluorescence spectrum under extreme interstellar medium (ISM) conditions in terms of high pressure (10^11 K cm^-3), high radiation field (10^6 G_0) combined with low metallicity (Z = 0.1 Z_⊙) and high cosmic ionization rate (ζ= 10^-14 s^-1), characteristic of early galaxies. As a case study, we apply this framework to stacked NIRSpec spectra from the JWST Advanced Deep Extragalactic Survey (JADES) for galaxies at redshifts z≥7. The stacked spectrum exhibits emission features consistent in profile and wavelength with the predicted H_2 fluorescence lines, including a blue shift suggestive of an outflow of molecular gas. Although individual features remain below robust detection thresholds, this demonstration illustrates the feasibility of using FUV fluorescence modeling to guide and interpret JWST spectroscopy of the molecular ISM at high redshift. Our framework provides a foundation for future searches for molecular hydrogen emission and the study of galactic feedback processes in the early universe.

Context. Observations of molecular emission lines are commonly used to derive the physical properties of cold molecular gas clouds. In external galaxies, these measurements suffer from limited spatial resolution, typically averaging a complex position–position– velocity distribution of emission over several tens of parsecs. Aims. We aim to quantify the variability in the basic parameters (peak brightness and line width) of spatially unresolved (>20 pc) line profiles that can be attributed to beam averaging. We focus on the commonly observed low-J transitions of CO isotopologues, HCN, HNC, HCO + , CS, SO and N 2 H + . Methods. We generated a sample of 1000 toy molecular cloud observations by resampling high-resolution (<0.05 pc) multiline Galactic observations of the Orion B molecular cloud. In the construction of our toy clouds, we imposed a range of density and velocity fields, characterised by their statistics and power spectra. These high-resolution molecular cloud observations were then averaged to single spatially unresolved spectra. We examined the resulting distribution of line profile parameters, and searched for potential correlations among line profile parameters and the underlying sub-beam density and velocity fields. Results. We find that unresolved line profiles’ parameters can vary significantly because of the sub-beam distribution of the emission. Emission lines that tend to be excited at higher densities show the most variability, up to a factor of two for N 2 H + ( J = 1 0). This variability in an emission line profile is related to the emission line’s covering fraction. As the spectral index of the velocity field increases, unresolved emission lines’ profiles increasingly diverge from a Gaussian shape. Conclusions. Line profile parameters exhibit non-negligible variability solely due to the sub-beam position-position-velocity distribution of the emission. This variability may exceed calibration and noise-related uncertainties.

Context. The current generation of millimeter (mm) receivers is capable of producing cubes of 800 000 pixels over 200 000 frequency channels to cover a number of square degrees over the 3 mm atmospheric window. Estimating the physical conditions of the interstellar medium (ISM) with an astrophysical model on the basis of such large datasets is challenging. Common approaches tend to converge to local minima and end up poorly reconstructing regions with a low signal-to-noise ratio (S/N) in most cases. This instrumental revolution thus calls for new scalable data analysis techniques with more advanced approaches to statistical modeling and methods. Aims. Our aim is to design a general method to reconstruct large maps of physical conditions from the rich datasets produced by new and future instruments. The requirements of the method include the ability to scale to very large maps, to be robust to varying S/N, and to escape from the local minima. In addition, we want to quantify the uncertainties associated with our reconstructions to produce reliable analyses. Methods. We present BEETROOTS, a PYTHON software that performs Bayesian reconstructions of maps of physical conditions based on observation maps and an astrophysical model. It relies on an accurate statistical model, exploits spatial regularization to guide estimations, and uses state-of-the-art algorithms. It can also assess the ability of the astrophysical model to explain the observations, providing feedback to improve ISM models. In this work, we demonstrate the power of BEETROOTS with the Meudon PDR code on synthetic data. We then apply it to estimate physical condition maps in the full Orion molecular cloud 1 (OMC-1) star-forming region based on Herschel molecular line emission maps. Results. The application to the synthetic case shows that BEETROOTS can currently analyze maps with up to ten thousand pixels, addressing large variations among the S/N values within the observations while escaping from local minima and providing consistent uncertainty quantifications. On a personal laptop, the inference runtime ranges from a few minutes for maps of 100 pixels to 28 hours for maps of 8100 pixels. Regarding OMC-1, our reconstructions of the incident UV radiation field intensity, G(0), are consistent with those obtained from FIR luminosities. This demonstrates that the considered molecular tracers are able to constrain G(0) over a wide range of environments. In addition, the obtained thermal pressures are high in all dense regions of OMC-1 and positively correlated with G(0). Finally, the Meudon PDR code successfully explains the observations and the obtained G(0) values are reasonable, which indicates that UV photons control the gas physics and chemistry across the rim of OMC-1. Conclusions. This work paves the way toward systematic and rigorous analyses of observations produced by current and future instruments. Subsequent efforts still need to be made in parallelizing the algorithm and thereby gaining two orders of magnitude for the map sizes.

Context. Emission lines such as HCN(J = 1 -> 0) are commonly used by extragalactic studies to trace high density molecular gas (nH(2) > similar to 104 cm(-3)). Recent Milky Way studies have challenged their utility as unambiguous dense gas tracers, suggesting that a large fraction of their emission in nearby clouds is excited in low density gas. Aims. We aim to develop a new method to infer the sub-beam probability density function (PDF) of H-2 column densities and the dense gas mass within molecular clouds using spatially unresolved observations of molecular emission lines in the 3 mm band. Methods. We modelled spatially unresolved line integrated intensity measurements as the average of an emission function weighted by the sub-beam column density PDF. The emission function, which expresses the line integrated intensity as a function of the gas column density, is an empirical fit to high resolution (< 0.05 pc) multi-line observations of the Orion B molecular cloud. We assumed the column density PDF to be parametric, composed of a log-normal distribution at moderate column densities and a power-law distribution at higher column densities. To estimate the sub-beam column density PDF, we combined the emission model with a Bayesian inversion algorithm (implemented in the BEETROOTS code), which takes account of thermal noise and calibration errors. Results. We validate our method by demonstrating that it recovers the true column density PDF of the Orion B cloud and reproduces the observed emission line integrated intensities within noise and calibration uncertainties. We applied the method to (CO)-C-12(J =1 -> 0), (CO)-C-13(J =1 -> 0), (CO)-O-18(J =1 -> 0), HCN(J =1 -> 0), HCO+ ( J = 1 -> 0) and N2H+(J =1 -> 0) observations of a 700 x 700 pc(2) field of view (FoV) in the nearby galaxy M51. On average, the model reproduces the observed intensities within 30%. The column density PDFs obtained for the spiral arm region within our test FoV are dominated by a power-law tail at high column densities, with slopes that are consistent with gravitational collapse. Outside the spiral arm, the column density PDFs are predominantly log-normal, consistent with supersonic isothermal turbulence setting the dynamical state of the molecular gas. We calculated the mass associated with the power-law tail of the column density PDFs and observe a strong, linear correlation between this mass and the 24 mu m surface brightness. Conclusions. Our method is a promising approach to infer the physical conditions within extragalactic molecular clouds using spectral line observations that are feasible with current millimetre facilities. Future work will extend the method to include additional physical parameters that are relevant for the dynamical state and star formation activity of molecular clouds.

The James Webb Space Telescope enabled the first detection of several rovibrational emission lines of HD in the Orion Bar, a prototypical photodissociation region. This provides an incentive to examine the physics of HD in dense and strong PDRs. Using the latest data available on HD excitation by collisional, radiative and chemical processes, our goal is to unveil HD formation and excitation processes in PDRs by comparing our state-of-the-art PDR model with observations made in the Orion Bar and discuss if and how HD can be used as a complementary tracer of physical parameters in the emitting region. We compute detailed PDR models, using an upgraded version of the Meudon PDR code, which are compared to NIRSpec data using excitation diagrams and synthetic emission spectra. The models predict that HD is mainly produced in the gas phase via the reaction D + H2 = H + HD at the front edge of the PDR and that the D/HD transition is located slightly closer to the edge than the H/H2 transition. Rovibrational levels are excited by UV pumping. In the observations, HD rovibrational emission is detected close to the H/H2 dissociation fronts of the Orion Bar and peaks where vibrationally excited H2 peaks, rather than at the maximum emission of pure rotational H2 levels. We derive an excitation temperature around Tex 480 - 710 K. Due to high continuum in the Orion Bar, fringes lead to high noise levels beyond 15 μm, no pure rotational lines of HD are detected. The comparison to PDR models shows that a range of thermal pressure P = (3-9)x10^7 K cm^-3 with no strong constraints on the intensity of the UV field are compatible with HD observations. This range of pressure is compatible with previous estimates from H2 observations with JWST. This is the first time that observations of HD emission lines in the near-infrared are used to put constraints on the thermal pressure in the PDR.

Context. The ionization fraction (f(e)=n(e)/n(H)) represents a fundamental parameter of the gas in the interstellar medium. However, estimating f(e) relies on a deep knowledge of the underlying chemistry of molecular gas as well as observations of atomic recombination lines and electron-sensitive molecular emission, such as deuterated isotopologs of HCO(+)and N2H+, which are only detectable in the dense cores. Until now, it has been challenging to constrain the ionization fraction in the interstellar gas over a large areas because of the observational limitations on these tracers and chemistry models. Aims. Recent models have provided a set of molecular lines whose ratios (intensities and column densities) can be used to trace f(e) in different environments of molecular clouds. Here, we use a set of various molecular lines typically detected in the 3-4 mm range to constrain the ionization fraction across the Orion B giant molecular cloud. In this work, we derived the ionization fraction for dense and translucent gas, and we investigated its variation with the density of the gas, n, and the strength of the far-ultraviolet radiation field, G(0), with their ratio G(0)/n. Methods. We present our results for the ionization fraction across one square degree in Orion B derived using analytical models as well as observational intensity and column density ratios of CN(1-0)/N2H+(1-0), (CO)-C-13(1-0)/HCO+(1-0), and (CO)-O-18(1-0)/HCO+(1-0) in the dense and shielded medium (A(v) >= 10 mag). We also used ratios of C2H(1-0)/HNC(1-0), C2H(1-0)/HCN(1-0), and C2H(1-0)/CN(1-0) in the translucent gas (2 mag <= A(v) <= 6 mag). Results. We find that the ionization fraction is within the range of 10(-5.5)-10(-4) for the translucent medium and 10(-8)-10(-6) for the dense medium. Our results show that the inferred f(e) values are sensitive to the value of G(0), especially in the dense, highly UV-illuminated gas. We also find that the ionization fraction in dense and translucent gas decreases with an increasing volume density (f(e) proportional to n(-0.227) for dense gas and f(e) proportional to n(-0.3) in translucent gas). It increases with G(0), which is a consequence of how sensitive the emission of selected molecular lines (e.g., CN and HCO+) is to the UV radiation field. In the case of the translucent medium, we did not find any significant difference in the ionization fraction computed from different line ratios. The range of f(e) values found in translucent gas implies that the electron excitation of HCN and HNC becomes significant in this regime. Conclusions. In dense and shielded gas, we recommend using CN(1-0)/N2H+(1-0) to derive an upper limit on the ionization fraction f(e), along with (CO)-O-18(1-0)/HCO+(1-0) to set constraints on the lower limit. In a translucent medium, C2H(1-0)/HNC(1-0) serves as a good tracer of f(e). The moderately high f(e) values found in translucent gas are consistent with the C+/CI/CO transition regime, while the values we find in the dense gas are sufficient to couple the gas with the magnetic field.

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.

We report the first interstellar identification of protonated acetylene, C2H3+, a fundamental hydrocarbon, in the z=0.89 molecular absorber toward the gravitationally lensed quasar PKS1830-211. The molecular species is identified from clear absorption features corresponding to the 2_12-1_01 (rest frequency 494.034 GHz) and 1_11-0_00 (431.316 GHz) ground-state transitions of ortho and para forms of C2H3+, respectively, in ALMA spectra toward the southwestern image of PKS1830-211, where numerous molecules, including other hydrocarbons, have already been detected. From the simple assumption of local thermodynamic equilibrium (LTE) with cosmic microwave background photons and an ortho-to-para ratio of three, we estimate a total C2H3+ column density of 2 x 10^12 cm^-2 and an abundance of 10^-10 compared to H_2. However, formation pumping could affect the population of metastable states, yielding a C2H3+ column density higher than the LTE value by a factor of a few. We explore possible routes to the formation of C2H3+, mainly connected to acetylene and methane, and find that the methane route is more likely in PDR environment. As one of the initial hydrocarbon building blocks, C2H3+ is thought to play an important role in astrochemistry, in particular in the formation of more complex organic molecules.