Context. The Galactic center (GC) is a unique and extreme astrophysical laboratory for studying the interplay between gas, stars, and a supermassive black hole (SMBH). In particular, the circumnuclear disk (CND) and its central cavity (CC) present two contrasting environments in terms of gas content, density, and stellar activity, making them ideal regions in which to study the multiphase structure and chemical composition of the interstellar medium (ISM). Aims. We aim to determine the properties (temperature, density, abundances, and spatial distribution) of the various phases of the ISM in the central parsec of the GC, with particular attention in this paper to the ionized medium. Methods. We used newly obtained observations from the Mid-Infrared Instrument (MIRI) equipped with the Medium Resolution Spectrometer (MRS) aboard the James Webb Space Telescope (JWST) to extract spectra covering the entire spectral range from 5 to 27 mu m in the CND and in the CC. We used the photoionization code CLOUDY to generate synthetic spectra with the same spectral range and resolution, simulating a wide range of gas phases and abundances. We then determined the contribution of each phase to the spectra. Once the abundances and contribution from each phase of the gas were determined, we identified four dominant phases and performed a spatial analysis to determine their contribution to each spaxel of the datacubes. Results. We find that in both the CND and the CC, the bulk of the emission originates from warm ionized gas with temperatures of between 10(4) and 10(4.8) K. In the CND, molecular gas contributes significantly to the flux and is spatially structured, while the CC shows minimal molecular gas content, as is expected from these regions. Coronal gas is detected in both regions at the interface between molecular and warm ionized gas. The hottest coronal phase appears faint and patchy in the CC, and has an elongated morphology in the CND. Abundance fitting (in solar-normalized logarithmic units) is primarily constrained by abundances: we measure a robust depletion of Fe relative to alpha elements with log(Fe/alpha) = -0.78 +/- 0.20 (CC) and -0.84 +/- 0.26 (CND), while CNO is only mildly enhanced relative to alpha, log(CNO/alpha) = 0.27 +/- 0.20 (CC) and 0.05 +/- 0.26 (CND). Absolute abundances are supersolar but more degenerate; the best-fitting models yield (log alpha, log CNO, log Fe) = (1.4, 1.4, 0.4) in the CC and (2.0, 1.8, 1.2) in the CND. Conclusions. The observed abundance pattern (enhanced CNO and alpha elements with suppressed Fe) indicates a chemically young environment, recently enriched by core-collapse supernovae and stellar winds, with a limited contribution from older Type Ia supernovae. This favors a scenario of massive, recent star formation rather than cumulative long-term enrichment. Additionally, the projected orientation of the newly identified CND elongated hot coronal feature, perpendicular to the direction toward the SMBH, suggests the action of a large-scale shock possibly resulting from past energetic outflows.

Evaluation of the Voigt function, a convolution of a Lorentzian and a Gaussian profile, is essential in various fields such as spectroscopy, atmospheric science, and astrophysics. Efficient computation of the function is crucial, especially in applications where the function may be called for an enormous number of times. In this paper, we present a highly efficient novel algorithm and its Fortran90 implementation for the practical evaluation of the Voigt function with accuracy in the order of 1.0e-6. The algorithm uses improved fits based on Chebyshev subinterval polynomial approximation for functions in two variables. The algorithm significantly outperforms widely-used competitive algorithms in the literature, in terms of computational speed, making it highly suitable for real-time applications and large-scale data processing tasks. The substantial improvement in efficiency positions the present algorithm and computer code as a valuable tool in relevant scientific domains. The algorithm has been adopted and implemented in the Meudon PDR code at Paris Observatory and is recommended for similar applications and simulation packages.

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.

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. Molecular hydrogen (H-2) is the most abundant molecule in the interstellar medium. Because of its excited form in irradiated regions, it is a useful tool for studying photodissociation regions (PDRs), where radiative feedback from massive stars on molecular clouds is dominant. The James Webb Space Telescope (JWST), with its high spatial resolution, sensitivity, and wavelength coverage, provides unique access to the detection of most of the H-2 rotational and rovibrational lines, as well as the analysis of their spatial morphology. Aims. Our goal is to use H-2 line emission detected with JWST in the Horsehead nebula to constrain the physical parameters (e.g., extinction, gas temperature, and thermal pressure) throughout the PDR and its geometry. Methods. We used spectro-imaging data acquired using both the NIRSpec and MIRI-MRS instruments on board JWST to study the H-2 spatial distribution at very small scales (down to 0.1('')). From the H-2 line ratios, we constrained the extinction throughout the PDR. We then studied the excitation of H-2 levels in detail and used this analysis to derive the physical parameters. Results. We detect hundreds of H-2 rotational and rovibrational lines in the Horsehead nebula. The H-2 morphology reveals a spatial separation between H-2 lines (similar to 0.5('')) across the PDR interface. Far-ultraviolet (FUV)-pumped lines (v = 0 J(u) > 6, v > 0) peak closer to the edge of the PDR than thermalized lines. From H-2 lines arising from the same upper level, we estimated the value of extinction throughout the PDR. We find that A(V) increases from the edge of the PDR to the second and third H-2 filaments. We find A(V)=0.3 +/- 1.3 in the first filament and A(V)=6.1 +/- 1.4 in the second and third filaments. We then studied the H-2 excitation in different regions across the PDR. The excitation diagrams were fit by two excitation temperatures. As the first levels of H-2 are thermalized, the colder temperature corresponds to the gas temperature. The second, hotter component corresponds to the FUV-pumped levels. In each filament, we derive a gas temperature of T similar to 500 K. The temperature profile shows that the observed gas temperature remains nearly constant throughout the PDR, with a slight decrease in each of the dissociation fronts. The spatial distribution of H-2 reveals that most of the H-2 column density is concentrated in the second and third filaments. The column density in the first filament is approximately N(H-2)=(3.8 +/- 0.8) x 10(19) cm(-2), while in the second and third filaments it is N(H-2)=(1.9 +/- 0.4) x 10(20) cm(-2), about five times higher. The ortho-to-para ratio (OPR) is far from equilibrium, varying from 2-2.5 at the edge of each dissociation front to 1.3-1.5 deeper into the PDR. We observe a clear spatial separation between the para and ortho rovibrational levels, as well as between 0-0 S(2) and 0-0 S(1), indicating that efficient ortho-para conversion and preferential ortho self-shielding are driving the spatial variations of the OPR. Finally, we derive a thermal pressure in the first filament of about P-gas >= 6 x 10(6) K cm(-3), which is approximately ten times higher than that of the ionized gas. We highlight that template stationary 1D PDR models cannot account for the intrinsic 2D structure and the very high temperature observed in the Horsehead nebula. We argue that the highly excited, over-pressurized H-2 gas at the edge of the PDR interface could originate from mixing between the cold and hot phases induced by photo-evaporation of the cloud. Conclusions. The analysis of H-2 lines detected with JWST provides unique access to the geometry and physical conditions in the Horsehead nebula at very small scales and reveals, for the first time, the possible importance of dynamical effects at the edge of the PDR. This study nevertheless highlights the need for extended modeling of these dynamical effects.

Context. The Galactic center (GC) is a unique and extreme astrophysical laboratory for studying the interplay between gas, stars, and a supermassive black hole (SMBH). In particular, the circumnuclear disk (CND) and its central cavity (CC) present two contrasting environments in terms of gas content, density, and stellar activity, making them ideal regions in which to study the multiphase structure and chemical composition of the interstellar medium (ISM). Aims. We aim to determine the properties (temperature, density, abundances, and spatial distribution) of the various phases of the ISM in the central parsec of the GC, with particular attention in this paper to the ionized medium. Methods. We used newly obtained observations from the Mid-Infrared Instrument (MIRI) equipped with the Medium Resolution Spectrometer (MRS) aboard the James Webb Space Telescope (JWST) to extract spectra covering the entire spectral range from 5 to 27 µm in the CND and in the CC. We used the photoionization code CLOUDY to generate synthetic spectra with the same spectral range and resolution, simulating a wide range of gas phases and abundances. We then determined the contribution of each phase to the spectra. Once the abundances and contribution from each phase of the gas were determined, we identified four dominant phases and performed a spatial analysis to determine their contribution to each spaxel of the datacubes. Results. We find that in both the CND and the CC, the bulk of the emission originates from warm ionized gas with temperatures of between 10 4 and 10 4 . 8 K. In the CND, molecular gas contributes significantly to the flux and is spatially structured, while the CC shows minimal molecular gas content, as is expected from these regions. Coronal gas is detected in both regions at the interface between molecular and warm ionized gas. The hottest coronal phase appears faint and patchy in the CC, and has an elongated morphology in the CND. Abundance fitting (in solar-normalized logarithmic units) is primarily constrained by abundances: we measure a robust depletion of Fe relative to α elements with log(Fe /α ) = −0 . 78 ± 0 . 20 (CC) and −0 . 84 ± 0 . 26 (CND), while CNO is only mildly enhanced relative to α , log(CNO /α ) = 0 . 27 ± 0 . 20 (CC) and 0 . 05 ± 0 . 26 (CND). Absolute abundances are supersolar but more degenerate; the best-fitting models yield (log α, log CNO , log Fe) = (1 . 4 , 1 . 4 , 0 . 4) in the CC and (2 . 0 , 1 . 8 , 1 . 2) in the CND. Conclusions. The observed abundance pattern (enhanced CNO and α elements with suppressed Fe) indicates a chemically young environment, recently enriched by core-collapse supernovae and stellar winds, with a limited contribution from older Type Ia supernovae. This favors a scenario of massive, recent star formation rather than cumulative long-term enrichment. Additionally, the projected orientation of the newly identified CND elongated hot coronal feature, perpendicular to the direction toward the SMBH, suggests the action of a large-scale shock possibly resulting from past energetic outflows.

H2 is the most abundant molecule in the interstellar medium and is a useful tool to study photodissociation regions, where radiative feedback from massive stars on molecular clouds is dominant. The James Webb Space Telescope, with its high spatial resolution, sensitivity, and wavelength coverage provides unique access to the detection of most of H2 lines and the analysis of its spatial morphology. Our goal is to use H2 line emission detected with the JWST in the Horsehead nebula to constrain the physical parameters (e.g., extinction, gas temperature, thermal pressure) throughout the PDR and its geometry. The study of H2 morphology reveals that FUV-pumped lines peak closer to the edge of the PDR than thermalized lines. From H2 lines, we estimate the value of extinction throughout the PDR. We find that AV is increasing from the edge of the PDR to the second and third H2 filaments. Then, we study the H2 excitation in different regions across the PDR. The temperature profile shows that the observed gas temperature is quite constant throughout the PDR, with a slight decline in each of the dissociation fronts. This study also reveals that the OPR is far from equilibrium. We observe a spatial separation of para and ortho rovibrational levels, indicating that efficient ortho-para conversion and preferential ortho self-shielding are driving the spatial variations of the OPR. Finally, we derive a thermal pressure in the first filament around P > 6x10$^6$ K cm$^{-3}$, about ten times higher than that of the ionized gas. We highlight that template stationary 1D PDR models cannot account for the intrinsic 2D structure and the very high temperature observed in the Horsehead nebula. We argue the highly excited, over-pressurized H2 gas at the edge of the PDR interface could originate from the mixing between the cold and hot phase induced by the photo-evaporation of the cloud.

Context. Recent spectroscopic measurements have revealed absorption from higher rotational levels in C_2 than previous observations. These improvements are accompanied by the availability of updated radiative and collisional data. Aims. We revisit the density and radiation field intensity diagnostics provided by the observations of many rotational levels of inter- stellar C_2 and extensive molecular information. Methods. We built an excitation model of C2 without spatial structure, including levels up to J= 34 where updated radiative and collisional excitation data are introduced as well as excitation by chemical formation. Results. We confirm the importance of the recent collisional excitation rate coefficients of C_2 by molecular H_2. We show that the new higher level observations cannot be explained by the standard balance between collisional excitation and radiative transitions. We propose that chemical excitation at formation provides a plausible mechanism to explain the observed high excitation of C_2. In addition, it allows us to lift the degeneracy of the density over radiation field strength parameter in the excitation model. Conclusions. A 0D model remains limited and it is highly desirable to use a full Photon Dominated Region (PDR) model, which includes all excitation processes introduced here and full chemical and thermal balance.

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.

One of the main problems in astrochemistry is determining the amount of sulfur in volatiles and refractories in the interstellar medium. The detection of the main sulfur reservoirs (icy H$_2$S and atomic gas) has been challenging, and estimates are based on the reliability of models to account for the abundances of species containing less than 1% of the total sulfur. The high sensitivity of the James Webb Space Telescope provides an unprecedented opportunity to estimate the sulfur abundance through the observation of the [S I] 25.249 $\mu$m line. We used the [S III] 18.7 $\mu$m, [S IV] 10.5 $\mu$m, and [S l] 25.249 $\mu$m lines to estimate the amount of sulfur in the ionized and molecular gas along the Orion Bar. For the theoretical part, we used an upgraded version of the Meudon photodissociation region (PDR) code to model the observations. New inelastic collision rates of neutral atomic sulfur with ortho- and para- molecular hydrogen were calculated to predict the line intensities. The [S III] 18.7 $\mu$m and [S IV] 10.5 $\mu$m lines are detected over the imaged region with a shallow increase (by a factor of 4) toward the HII region. We estimate a moderate sulfur depletion, by a factor of $\sim$2, in the ionized gas. The corrugated interface between the molecular and atomic phases gives rise to several edge-on dissociation fronts we refer to as DF1, DF2, and DF3. The [S l] 25.249 $\mu$m line is only detected toward DF2 and DF3, the dissociation fronts located farthest from the HII region. The detailed modeling of DF3 using the Meudon PDR code shows that the emission of the [S l] 25.249 $\mu$m line is coming from warm ($>$ 40 K) molecular gas located at A$_{\rm V}$ $\sim$ 1$-$5 mag from the ionization front. Moreover, the intensity of the [S l] 25.249 $\mu$m line is only accounted for if we assume the presence of undepleted sulfur.

Context. Dense and cold molecular cores and filaments are surrounded by an envelope of translucent gas. Some of the low-J emission lines of CO and HCO+ isotopologues are more sensitive to the conditions either in the translucent environment or in the dense and cold one because their intensities result from a complex interplay of radiative transfer and chemical properties of these heterogeneous lines of sight (LoSs). Aims. We extend our previous single-zone modeling with a more realistic approach that introduces multiple layers to take account of possibly varying conditions along the LoS. We used the IRAM-30m data from the ORION-B large program toward the Horsehead nebula in order to demonstrate our method's capability and effectiveness. Methods. We propose a cloud model composed of three homogeneous slabs of gas along each LoS, representing an outer envelope and a more shielded inner layer. We used the non-LTE radiative transfer code RADEX to model the line profiles from the kinetic temperature (T-kin), the volume density (n(H2)), kinematics, and chemical properties of the different layers. We then used a fast and robust maximum likelihood estimator to simultaneously fit the observed lines of the CO and HCO+ isotopologues. To limit the variance on the estimates, we propose a simple chemical model by constraining the column densities. Results. A single-layer model cannot reproduce the spectral line asymmetries that result from a combination of different radial velocities and absorption effects among layers. A minimal heterogeneous model (three layers only) is sufficient for the Horsehead application, as it provides good fits of the seven fitted lines over a large part of the studied field of view. The decomposition of the intensity into three layers allowed us to discuss the distribution of the estimated physical or chemical properties along the LoS. About 80% of the (CO)-C-12 integrated intensity comes from the outer envelope, while similar to 55% of the integrated intensity of the (1 - 0) and (2 - 1) lines of (CO)-O-18 comes from the inner layer. For the lines of the (CO)-C-13 and the HCO+ isotopologues, integrated intensities are more equally distributed over the cloud layers. The estimated column density ratio N((CO)-C-13)/N((CO)-O-18) in the envelope increases with decreasing visual extinction, and it reaches 25 in the pillar outskirts. While the inferred T-kin of the envelope varies from 25 to 40 K, that of the inner layer drops to similar to 15 K in the western dense core. The estimated n(H2) in the inner layer is similar to 3 x 10(4) cm(-3) toward the filament, and it increases by a factor of ten toward dense cores. Conclusions. Our proposed method correctly retrieves the physical and chemical properties of the Horsehead nebula. It also offers promising prospects for less supervised model fits of wider-field datasets.

The gas-phase abundance of carbon, xC = [C /H] gas = xC + + xC0 + xCO + : : :, and its depletion factors are essential parameters for understanding the gas and solid compositions that are ultimately incorporated into (exo)planets. The majority of protoplanetary disks are born in clusters and, as a result, are exposed to external far-ultraviolet (FUV) radiation. These FUV photons potentially a ffect the disk's evolution, chemical composition, and line excitation. We present the first detection of the [C i] 609 -m fine-structure (3P1-3P0) line of neutral carbon (C0), achieved with ALMA, toward one of these disks, d203-506, in the Orion Nebula Cluster. We also report the detection of [C i] forbidden and C i permitted lines (from electronically excited states up to -10 eV) observed with JWST in the near-infrared (NIR). These lines trace the irradiated outer disk and photo-evaporative wind. Contrary to the common belief that these NIR lines are C+ recombination lines, we find that they are dominated by FUV-pumping of C0 followed by fluorescence cascades. They trace the transition from atomic to molecular gas, and their intensities scale with G0. The lack of outstanding NIR O i fluorescent emission, however, implies a sharper attenuation of external FUV radiation with E & 12 eV (. Lyman-fi). This is related to a lower e ffective FUV dust absorption cross section compared to that of interstellar grains, implying a more prominent role for FUV shielding by the C0 photoionization continuum. The [C i] 609 -m line intensity is proportional to N(C0) and can be used to infer xC. We derive xC ' 1.4 10 4. This implies that there is no major depletion of volatile carbon compared to xC measured in the natal cloud, hinting at a young disk. We also show that external FUV radiation impacts the outer disk and wind by vertically shifting the water freeze-out depth, which likely results in less e fficient grain growth and settling. This shift leads to nearly solar gas-phase C /O abundance ratios in these irradiated layers.

Context . Robust radiative transfer techniques are requisite for efficiently extracting the physical and chemical information from molecular rotational lines. Aims . We study several hypotheses that enable robust estimations of the column densities and physical conditions when fitting one or two transitions per molecular species. We study the extent to which simplifying assumptions aimed at reducing the complexity of the problem introduce estimation biases and how to detect them. Methods . We focus on the CO and HCO + isotopologues and analyze maps of a 50 square arcminutes field. We used the RADEX escape probability model to solve the statistical equilibrium equations and compute the emerging line profiles, assuming that all species coexist. Depending on the considered set of species, we also fixed the abundance ratio between some species and explored different values. We proposed a maximum likelihood estimator to infer the physical conditions and considered the effect of both the thermal noise and calibration uncertainty. We analyzed any potential biases induced by model misspecifications by comparing the results on the actual data for several sets of species and confirmed with Monte Carlo simulations. The variance of the estimations and the efficiency of the estimator were studied based on the Cramér-Rao lower bound. Results . Column densities can be estimated with 30% accuracy, while the best estimations of the volume density are found to be within a factor of two. Under the chosen model framework, the peak 12 CO (1 – 0) is useful for constraining the kinetic temperature. The thermal pressure is better and more robustly estimated than the volume density and kinetic temperature separately. Analyzing CO and HCO + isotopologues and fitting the full line profile are recommended practices with respect to detecting possible biases. Conclusions . Combining a non-local thermodynamic equilibrium model with a rigorous analysis of the accuracy allows us to obtain an efficient estimator and identify where the model is misspecified. We note that other combinations of molecular lines could be studied in the future.

The triatomic hydrogen ion H3+ is one of the most important species for the gas phase chemistry of the interstellar medium. Observations of H3+ are used to constrain important physical and chemical parameters of interstellar environments. However, the temperatures inferred from the two lowest rotational states of H3+ in diffuse lines of sight - typically the only ones observable - appear consistently lower than the temperatures derived from H2 observations in the same sightlines. All previous attempts at modelling the temperatures of H3+ in the diffuse interstellar medium failed to reproduce the observational results. Here we present new studies, comparing an independent master equation for H3+ level populations to results from the Meudon PDR code for photon dominated regions. We show that the populations of the lowest rotational states of H3+ are strongly affected by the formation reaction and that H3+ ions experience incomplete thermalisation before their destruction by free electrons. Furthermore, we find that for quantitative analysis more than two levels of H3+ have to be considered and that it is crucial to include radiative transitions as well as collisions with H2. Our models of typical diffuse interstellar sightlines show very good agreement with observational data, and thus they may finally resolve the perceived temperature difference attributed to these two fundamental species.

Here we report the main photodissociation processes involving molecular species, and introduce the shielding and self-shielding concepts. Particular focus will be given to the implementation of the photodissociation and self-shielding. We discuss the known uncertainties (e.g., branching ratio) and provide instructive quantitative examples.

Context. Observations of ionic, atomic, or molecular lines are performed to improve our understanding of the interstellar medium (ISM). However, the potential of a line to constrain the physical conditions of the ISM is difficult to assess quantitatively, because of the complexity of the ISM physics. The situation is even more complex when trying to assess which combinations of lines are the most useful. Therefore, observation campaigns usually try to observe as many lines as possible for as much time as possible. Aims. We have searched for a quantitative statistical criterion to evaluate the full constraining power of a (combination of) tracer(s) with respect to physical conditions. Our goal with such a criterion is twofold. First, we want to improve our understanding of the statistical relationships between ISM tracers and physical conditions. Secondly, by exploiting this criterion, we aim to propose a method that helps observers to make their observation proposals; for example, by choosing to observe the lines with the highest constraining power given limited resources and time. Methods. We propose an approach based on information theory, in particular the concepts of conditional differential entropy and mutual information. The best (combination of) tracer(s) is obtained by comparing the mutual information between a physical parameter and different sets of lines. The presented analysis is independent of the choice of the estimation algorithm (e.g., neural network or chi(2) minimization). We applied this method to simulations of radio molecular lines emitted by a photodissociation region similar to the Horsehead Nebula. In this simulated data, we considered the noise properties of a state-of-the-art single dish telescope such as the IRAM 30m telescope. We searched for the best lines to constrain the visual extinction, A(V)(tot), or the ultraviolet illumination field, G(0). We ran this search for different gas regimes, namely translucent gas, filamentary gas, and dense cores. Results. The most informative lines change with the physical regime (e.g., cloud extinction). However, the determination of the optimal (combination of) line(s) to constrain a physical parameter such as the visual extinction depends not only on the radiative transfer of the lines and chemistry of the associated species, but also on the achieved mean signal-to-noise ratio. The short integration time of the CO isotopologue J = 1 - 0 lines already yields much information on the total column density for a large range of (A(V)(tot), G(0)) space. The best set of lines to constrain the visual extinction does not necessarily combine the most informative individual lines. Precise constraints on the radiation field are more difficult to achieve with molecular lines. They require spectral lines emitted at the cloud surface (e.g., [CII] and [CI] lines). Conclusions. This approach allows one to better explore the knowledge provided by ISM codes, and to guide future observation campaigns.

Radiation is one of the key drivers of chemistry and an essential component of astrophysics in general. We present a variety of photo processes involving atomic species and how to implement them into chemical networks, both with simplified prescriptions or with a detailed frequency-dependent approach.

We present 5 deg^2 (~250 pc^2) HCN, HNC, HCO+, and CO J=1-0 maps of the Orion B GMC, complemented with existing wide-field [CI] 492 GHz maps, as well as new pointed observations of rotationally excited HCN, HNC, H13CN, and HN13C lines. We detect anomalous HCN J=1-0 hyperfine structure line emission almost everywhere in the cloud. About 70% of the total HCN J=1-0 luminosity arises from gas at A_V < 8 mag. The HCN/CO J=1-0 line intensity ratio shows a bimodal behavior with an inflection point at A_V < 3 mag typical of translucent gas and UV-illuminated cloud edges. We find that most of the HCN J=1-0 emission arises from extended gas with n(H2) ~< 10^4 cm^-3, even lower density gas if the ionization fraction is > 10^-5 and electron excitation dominates. This result explains the low-A_V branch of the HCN/CO J=1-0 intensity ratio distribution. Indeed, the highest HCN/CO ratios (~0.1) at A_V < 3 mag correspond to regions of high [CI] 492 GHz/CO J=1-0 intensity ratios (>1) characteristic of low-density PDRs. Enhanced FUV radiation favors the formation and excitation of HCN on large scales, not only in dense star-forming clumps. The low surface brightness HCN and HCO+ J=1-0 emission scale with I_FIR (a proxy of the stellar FUV radiation field) in a similar way. Together with CO J=1-0, these lines respond to increasing I_FIR up to G0~20. On the other hand, the bright HCN J=1-0 emission from dense gas in star-forming clumps weakly responds to I_FIR once the FUV radiation field becomes too intense (G0>1500). The different power law scalings (produced by different chemistries, densities, and line excitation regimes) in a single but spatially resolved GMC resemble the variety of Kennicutt-Schmidt law indexes found in galaxy averages. As a corollary for extragalactic studies, we conclude that high HCN/CO J=1-0 line intensity ratios do not always imply the presence of dense gas.

Context. The availability of large bandwidth receivers for millimeter radio telescopes allows the acquisition of position-position-frequency data cubes over a wide field of view and a broad frequency coverage. These cubes contain much information on the physical, chemical, and kinematical properties of the emitting gas. However, their large size coupled with inhomogenous signal-to-noise ratio (SNR) are major challenges for consistent analysis and interpretation.Aims. We search for a denoising method of the low SNR regions of the studied data cubes that would allow to recover the low SNR emission without distorting the signals with high SNR.Methods. We perform an in-depth data analysis of the 13 CO and C 17 O (1 -- 0) data cubes obtained as part of the ORION-B large program performed at the IRAM 30m telescope. We analyse the statistical properties of the noise and the evolution of the correlation of the signal in a given frequency channel with that of the adjacent channels. This allows us to propose significant improvements of typical autoassociative neural networks, often used to denoise hyperspectral Earth remote sensing data. Applying this method to the 13 CO (1 -- 0) cube, we compare the denoised data with those derived with the multiple Gaussian fitting algorithm ROHSA, considered as the state of the art procedure for data line cubes.Results. The nature of astronomical spectral data cubes is distinct from that of the hyperspectral data usually studied in the Earth remote sensing literature because the observed intensities become statistically independent beyond a short channel separation. This lack of redundancy in data has led us to adapt the method, notably by taking into account the sparsity of the signal along the spectral axis. The application of the proposed algorithm leads to an increase of the SNR in voxels with weak signal, while preserving the spectral shape of the data in high SNR voxels.Conclusions. The proposed algorithm that combines a detailed analysis of the noise statistics with an innovative autoencoder architecture is a promising path to denoise radio-astronomy line data cubes. In the future, exploring whether a better use of the spatial correlations of the noise may further improve the denoising performances seems a promising avenue. In addition,

Context. The interpretation of observations of atomic and molecular tracers in the galactic and extragalactic interstellar medium (ISM) requires comparisons with state-of-the-art astrophysical models to infer some physical conditions. Usually, ISM models are too timeconsuming for such inference procedures, as they call for numerous model evaluations. As a result, they are often replaced by an interpolation of a grid of precomputed models.Aims. We propose a new general method to derive faster, lighter, and more accurate approximations of the model from a grid of precomputed models for use in inference procedures.Methods. These emulators are defined with artificial neural networks (ANNs) with adapted architectures and are fitted using regression strategies instead of interpolation methods. The specificities inherent in ISM models need to be addressed to design and train adequate ANNs. Indeed, such models often predict numerous observables (e.g., line intensities) from just a few input physical parameters and can yield outliers due to numerical instabilities or physical bistabilities and multistabilities. We propose applying five strategies to address these characteristics: (1) an outlier removal procedure; (2) a clustering method that yields homogeneous subsets of lines that are simpler to predict with different ANNs; (3) a dimension reduction technique that enables us to adequately size the network architecture; (4) the physical inputs are augmented with a polynomial transform to ease the learning of nonlinearities; and (5) a dense architecture to ease the learning of simpler relations between line intensities and physical parameters.Results. We compare the proposed ANNs with four standard classes of interpolation methods, nearest-neighbor, linear, spline, and radial basis function (RBF), to emulate a representative ISM numerical model known as the Meudon PDR code. Combinations of the proposed strategies produce networks that outperform all interpolation methods in terms of accuracy by a factor of 2 in terms of the average error (reaching 4.5% on the Meudon PDR code) and a factor of 3 for the worst-case errors (33%). These networks are also 1000 times faster than accurate interpolation methods and require ten to forty times less memory.Conclusions. This work will enable efficient inferences on wide-field multiline observations of the ISM.