Publications 20232024-03-27T14:06:43+01:00

Publications (2023)

Total peer-reviewed articles: 111

The Galaxy Activity, Torus, and Outflow Survey (GATOS)

Authors: Garcia-Bernete, I.; Alonso-Herrero, A.; Rigopoulou, D.; Pereira-Santaella, M.; Shimizu, T.; Davies, R.; Donnan, F. R.; Roche, P. F.; Gonzalez-Martin, O.; Ramos Almeida, C.; Bellocchi, E.; Boorman, P.; Combes, F.; Efstathiou, A.; Esparza-Arredondo, D.; Garcia-Burillo, S.; Gonzalez-Alfonso, E.; Hicks, E. K. S.; Honig, S.; Labiano, A.; Levenson, N. A.; Lopez-Rodriguez, E.; Ricci, C.; Packham, C.; Rouan, D.; Stalevski, M.; Ward, M. J.


Publication date: 2023/12/25

DOI: 10.1051/0004-6361/202348266

Abstract: We present the first results of the Galaxy Activity, Torus, and Outflow Survey (GATOS), a project aimed at understanding the properties of the dusty molecular tori and their connection to the host galaxy in nearby Seyfert galaxies. Our project expands the range of active galactic nuclei (AGN) luminosities and Eddington ratios covered by previous surveys of Seyferts conducted by the Atacama Large Millimeter Array (ALMA), allowing us to study the gas feeding and feedback cycle in a combined sample of 19 Seyferts. We used ALMA to obtain new images of the emission of molecular gas and dust using the CO(3-2) and HCO+(4-3) lines as well as their underlying continuum emission at 870 mu m with high spatial resolutions (0.1 ” similar to 7 – 13 pc) in the circumnuclear disks (CND) of ten nearby (D < 28 Mpc) Seyfert galaxies selected from an ultra-hard X-ray survey. Our new ALMA observations detect 870 mu m continuum and CO line emission from spatially resolved disks located around the AGN in all the sources. The bulk of the 870 mu m continuum flux can be accounted for by thermal emission from dust in the majority of the targets. For most of the sources, the disks show a preponderant orientation perpendicular to the AGN wind axes, as expected for dusty molecular tori. The median diameters and molecular gas masses of the tori are similar to 42 pc and similar to 6 x 10(5) M-circle dot, respectively. We also detected the emission of the 4-3 line of HCO+ in four GATOS targets. The order of magnitude differences found in the CO/HCO+ ratios within our combined sample point to a very different density radial stratification inside the dusty molecular tori of these Seyferts. We find a positive correlation between the line-of-sight gas column densities responsible for the absorption of X-rays and the molecular gas column densities derived from CO toward the AGN in our sources. Furthermore, the median values of both column densities are similar. This suggests that the neutral gas line-of-sight column densities of the dusty molecular tori imaged by ALMA significantly contribute to the obscuration of X-rays. The radial distributions of molecular gas in the CND of our combined sample show signs of nuclear-scale molecular gas deficits. We also detect molecular outflows in the sources that show the most extreme nuclear-scale gas deficits in our sample. These observations find for the first time supporting evidence that the imprint of AGN feedback is more extreme in higher luminosity and/or higher Eddington ratio Seyfert galaxies.

Missing levels in intermediate spectra

Authors: Hita-Perez, Maria; Munoz, Laura; Molina, Rafael A.


Publication date: 2023/12/15

DOI: 10.1088/1751-8121/ad0d1e

Abstract: We derive an expression for the nearest-neighbor spacing distribution P(s) of the energy levels of quantum systems with intermediate dynamics between regularity and chaos and missing levels due to random experimental errors. The expression is based on the Brody distribution, the most widely used for fitting mixed spectra as a function of one parameter. By using Monte Carlo simulations of intermediate spectra based on the beta-Hermite ensemble of random matrix theory (RMT), we evaluate the quality of the formula and its suitability for fitting purposes. Estimations of the Brody parameter and the fraction of missing levels can be obtained by a least-square two-parameter fitting of the experimental P(s). The results should be important to distinguish the origins of deviations from RMT in experimental spectra.

Reply to the ‘Comment on Perturbation theory of scattering for grazing-incidence fast-atom diffraction’ by G. A. Bocan, H. Breiss, S. Szilasi, A. Momeni, E. M. S. Casagrande, E. A. Sanchez, M. S. Gravielle and H. Khemliche, Phys. Chem. Chem. Phys., 2023, 25, DOI: 10.1039/D3CP02486E

Authors: Allison, W.; Miret-Artes, S.; Pollak, E.


Publication date: 2023/12/13

DOI: 10.1039/d3cp04559e

Abstract: In this Reply, we show that criticisms of perturbation theory for grazing-incidence fast-atom diffraction (GIFAD) are ill-founded. We show explicitly that our formulation (W. Allison, S. Miret-Artes and E. Pollak, Phys. Chem. Chem. Phys., 2022, 24, 15851) provides a similar precision in describing the observed phenomena as ab initio potentials. Since that is the main criterion to distinguish between methods, it seems reasonable to conclude that the perturbation approach using a Morse-type potential reproduces the essential aspects of the dynamics correctly. In addition we expand on the historical context and summarize the physical insights provided by our methods.

Detection of vibrationally excited C6H in the cold prestellar core TMC-1 with the QUIJOTE line survey

Authors: Cernicharo, J.; Fuentetaja, R.; Agundez, M.; Cabezas, C.; Tercero, B.; Marcelino, N.; de Vicente, P.


Publication date: 2023/12/11

DOI: 10.1051/0004-6361/202348495

Abstract: In this work, we present the detection of twelve doublets with quantum numbers of N = 12-11 to N = 17-16 of the nu(11)(mu(2)Sigma) vibrationally excited state of C6H towards TMC-1. This marks the first time that an excited vibrational state of a molecule has been detected in a cold starless core. The data are part of the QUIJOTE line survey gathered with the Yebes 40m radio telescope. The line intensities have been aptly reproduced with a rotational temperature of 6.2 +/- 0.4 K and a column density of (1.2 +/- 0.2)x10(11) cm(-2). We also analysed the ground state transitions of C6H, detecting fourteen lines with quantum numbers of J = 23/2-21/2 to J = 35/2 for each of the two 2 pi 3/2 and 2 pi 1/2 ladders. It is not possible to model the intensities of all the transitions of the ground state simultaneously using a single column density. We considered the two ladders as two different species and found that the rotational temperature is the same for both ladders, Trot(2 pi 3/2)=Trot(2 pi 1/2)=6.2 +/- 0.2, achieving a result that is comparable to that of the nu 11(2 mu sigma) state. The derived column densities are N(2 pi 3/2) = (6.2 +/- 0.3)x1012 cm-2 and N(2 pi 1/2) = (8.0 +/- 0.4)x1010 cm-2. The fraction of C6H molecules in its 2 pi 3/2, 2 pi 1/2, and nu 11(mu 2 sigma) states is 96.8%, 1.3%, and 1.9%, respectively. Finally, we report that this vibrational mode has also been detected towards the cold cores Lupus-1A and L1495B, as well as the low-mass star forming cores L1527 and L483, with fractions of C6H molecules in this mode of 3.8%, 4.1%, 14.8%, and 6%, respectively.

Astronomical CH3+ rovibrational assignments: A combined theoretical and experimental study validating observational findings in the d203-506 UV-irradiated protoplanetary disk

Authors: Changala, P. Bryan; Chen, Ning L.; Le, Hai L.; Gans, Berenger; Steenbakkers, Kim; Salomon, Thomas; Bonah, Luis; Schroetter, Ilane; Canin, Amelie; Martin-Drumel, Marie-Aline; Jacovella, Ugo; Dartois, Emmanuel; Boye-Peronne, Severine; Alcaraz, Christian; Asvany, Oskar; Brunken, Sandra; Thorwirth, Sven; Schlemmer, Stephan; Goicoechea, Javier R.; Rouille, Gael; Sidhu, Ameek; Chown, Ryan; van de Putte, Dries; Trahin, Boris; Alarcon, Felipe; Berne, Olivier; Habart, Emilie; Peeters, Els


Publication date: 2023/12/05

DOI: 10.1051/0004-6361/202347765

Abstract: Context. The methyl cation (CH3+) has recently been discovered in the interstellar medium through the detection of 7 mu m (1400 cm(-1)) features toward the d203-506 protoplanetary disk by the JWST. Line-by-line spectroscopic assignments of these features, however, were unsuccessful due to complex intramolecular perturbations preventing a determination of the excitation and abundance of the species in that source. Aims. Comprehensive rovibrational assignments guided by theoretical and experimental laboratory techniques provide insight into the excitation mechanisms and chemistry of CH3+ in d203-506. Methods. The rovibrational structure of CH3+ was studied theoretically by a combination of coupled-cluster electronic structure theory and (quasi-)variational nuclear motion calculations. Two experimental techniques were used to confirm the rovibrational structure of CH3+ : (1) infrared leak-out spectroscopy of the methyl cation, and (2) rotationally resolved photoelectron spectroscopy of the methyl radical (CH3). In (1), CH3+ ions, produced by the electron impact dissociative ionization of methane, were injected into a 22-pole ion trap where they were probed by the pulses of infrared radiation from the FELIX free electron laser. In (2), neutral CH3, produced by CH3NO2 pyrolysis in a molecular beam, was probed by pulsed-field ionization zero-kinetic-energy photoelectron spectroscopy. Results. The quantum chemical calculations performed in this study have enabled a comprehensive spectroscopic assignment of the nu(+)(2) 2 and nu(+)(4) bands of CH3+ detected by the JWST. The resulting spectroscopic constants and derived Einstein A coefficients fully reproduce both the infrared and photoelectron spectra and permit the rotational temperature of CH3+ (T = 660 +/- 80 K) in d203-506 to be derived. A beam-averaged column density of CH3+ in this protoplanetary disk is also estimated.

Identifying Physical Structures in Our Galaxy with Gaussian Mixture Models: An Unsupervised Machine Learning Technique

Authors: Tiwari, M.; Kievit, R.; Kabanovic, S.; Bonne, L.; Falasca, F.; Guevara, C.; Higgins, R.; Justen, M.; Karim, R.; Kavak, U.; Pabst, C.; Pound, M. W.; Schneider, N.; Simon, R.; Stutzki, J.; Wolfire, M.; Tielens, A. G. G. M.


Publication date: 2023/12/01

DOI: 10.3847/1538-4357/ad003c

Abstract: We explore the potential of the Gaussian mixture model (GMM), an unsupervised machine-learning method, to identify coherent physical structures in the interstellar medium. The implementation we present can be used on any kind of spatially and spectrally resolved data set. We provide a step-by-step guide to use these models on different sources and data sets. Following the guide, we run the models on NGC 1977, RCW 120, and RCW 49 using the [C ii] 158 mu m mapping observations from the SOFIA telescope. We find that the models identified six, four, and five velocity coherent physical structures in NGC 1977, RCW 120, and RCW 49, respectively, which are validated by analyzing the observed spectra toward these structures and by comparison to earlier findings. In this work we demonstrate that GMM is a powerful tool that can better automate the process of spatial and spectral analysis to interpret mapping observations.

Confining He Atoms in Diverse Ice-Phases: Examining the Stability of He Hydrate Crystals through DFT Approaches

Authors: Yanes-Rodriguez, Raquel; Prosmiti, Rita


Publication date: 2023/12/01

DOI: 10.3390/molecules28237893

Abstract: In the realm of solid water hydrostructures, helium atoms have a tendency to occupy the interstitial spaces formed within the crystal lattice of ice structures. The primary objective of this study is to examine the stability of various ice crystals when influenced by the presence of He atoms. Presenting a first attempt at a detailed computational description of the whole energy components (guest-water, water-water, guest-guest) in the complete crystal unit cells contributes to enhancing the knowledge available about these relatively unexplored helium-water systems, which could potentially benefit future experiments. For this purpose, two different ice structures were considered: the previously established He@ice II system, and the predicted (but currently nonexistent) He@ice XVII system. One of the main features of these He-filled structures is the stability conferred by the weak van der Waals dispersion forces that occur between the host lattice and the guest atoms, in addition to the hydrogen bonds established among the water molecules. Hence, it is crucial to accurately describe these interactions. Therefore, the first part of this research is devoted examining the performance and accuracy of various semi-local and non-local DFT/DFT-D functionals, in comparison with previous experimental and/or high-level computational data. Once the best-performing DFT functional has been identified, the stability of these empty and He-filled structures, including different number of He atoms within the lattices, is analysed in terms of their structural (lattice deformation), mechanical (pressure compression effects) and energetic properties (binding and saturation energies). In this manner, the potential formation of these structures under zero temperature and pressure conditions can be evaluated, while their maximum storage capacity is also determined. The obtained results reveal that, despite the weak underlying interactions, the He encapsulation has a rather notable effect on both lattice parameters and energetics, and therefore, the guest-host interactions are far from being negligible. Besides, both ice crystals are predicted to remain stable when filled with He atoms, with ice XVII exhibiting a higher capacity for accommodating a larger number of guest atoms within its interstitial spaces.

Improved Quantum-Classical Treatment of N2-N2 Inelastic Collisions: Effect of the Potentials and Complete Rate Coefficient Data Sets

Authors: Hong, Qizhen; Storchi, Loriano; Sun, Quanhua; Bartolomei, Massimiliano; Pirani, Fernando; Coletti, Cecilia


Publication date: 2023/11/26

DOI: 10.1021/acs.jctc.3c01103

Abstract: [Storchi, Loriano; Coletti, Cecilia] Univ Gabriele Annunzio Chieti Pescara, Dipartimento Farm, I-66100 Chieti, Italy; [Hong, Qizhen; Sun, Quanhua] Chinese Acad Sci, Inst Mech, State Key Lab High Temp Gas Dynam, Beijing 100190, Peoples R China; [Sun, Quanhua] Univ Chinese Acad Sci, Sch Engn Sci, Beijing 100049, Peoples R China; [Bartolomei, Massimiliano] CSIC, Inst Fis Fundamental, Madrid 28006, Spain; [Pirani, Fernando] Univ Perugia, Dipartimento Chim Biol & Biotecnol, I-06123 Perugia, Italy

Elastic and electronically inelastic scattering of electrons by the pyrazine molecule

Authors: Silva, Murilo O.; Moreira, Giseli M.; Rosado, Jaime; Blanco, Francisco; Garcia, Gustavo; Bettega, Marcio H. F.; da Costa, Romarly F.


Publication date: 2023/11/21

DOI: 10.1039/d3cp04619b

Abstract: We report on elastic and electronically inelastic integral and differential cross sections as well as ionization and total cross sections for electron collisions with the pyrazine molecule. The Schwinger multichannel method is applied in calculations carried out according to the minimal orbital basis for single configuration interactions strategy from the 1-channel up to 139-channels close-coupling level of approximation. With these calculations we have obtained integral and differential cross sections as well as excitation functions for elastic electron scattering and, also, integral and differential cross sections for electronic excitation from the ground state to the 3B1u, 3B2u, 3B3u, 1B1u, 1B2u and 1B3u excited states of pyrazine by electron impact. By summing the total ionization cross section obtained by means of the binary-encounter-Bethe model to these elastic and electronically inelastic contributions, we provided an estimate for the total cross section describing the electron-pyrazine interaction process. The independent atom model with the screening-corrected additivity rule plus interference terms method was also used in the present study to determine elastic integral and differential as well as ionization and total cross sections for electron collisions from pyrazine. The present results were, whenever possible, critically compared to the experimental and theoretical data available in the literature. In general, the overall agreement between the present results and the experiment is quite encouraging. We report on elastic and electronically inelastic integral and differential cross sections as well as ionization and total cross sections for electron collisions with the pyrazine molecule.

Energy transfer in N-component nanosystems enhanced by pulse-driven vibronic many-body entanglement

Authors: Gomez-Ruiz, Fernando J.; Acevedo, Oscar L.; Rodriguez, Ferney J.; Quiroga, Luis; Johnson, Neil F.


Publication date: 2023/11/15

DOI: 10.1038/s41598-023-46256-z

Abstract: The processing of energy by transfer and redistribution, plays a key role in the evolution of dynamical systems. At the ultrasmall and ultrafast scale of nanosystems, quantum coherence could in principle also play a role and has been reported in many pulse-driven nanosystems (e.g. quantum dots and even the microscopic Light-Harvesting Complex II (LHC-II) aggregate). Typical theoretical analyses cannot easily be scaled to describe these general N-component nanosystems; they do not treat the pulse dynamically; and they approximate memory effects. Here our aim is to shed light on what new physics might arise beyond these approximations. We adopt a purposely minimal model such that the time-dependence of the pulse is included explicitly in the Hamiltonian. This simple model generates complex dynamics: specifically, pulses of intermediate duration generate highly entangled vibronic (i.e. electronic-vibrational) states that spread multiple excitons – and hence energy – maximally within the system. Subsequent pulses can then act on such entangled states to efficiently channel subsequent energy capture. The underlying pulse-generated vibronic entanglement increases in strength and robustness as N increases.

The electronic structure of 2(5H)-thiophenone investigated by vacuum ultraviolet synchrotron radiation and theoretical calculations

Authors: Kumar, S.; Duflot, D.; Jones, N. C.; Hoffmann, S. V.; Garcia, G.; Limao-Vieira, P.


Publication date: 2023/11/01

DOI: 10.1140/epjd/s10053-023-00771-w

Abstract: The electronic state spectroscopy of 2(5H)-thiophenone, C4H4OS, has been investigated by high-resolution vacuum ultraviolet photoabsorption in the 3.76-10.69 eV energy range using synchrotron radiation, together with novel quantum chemical calculations performed at the equation of motion coupled cluster singles and doubles (EOM-CCSD) level of theory. The major electronic transitions have been assigned to valence and Rydberg character, with relevant C=O, C=C and C-C stretching vibrations across the entire absorption spectrum. Photolysis lifetimes in the Earth’s atmosphere (0-50 km altitude) have been estimated from the absolute photoabsorption cross-sections, indicating that solar photolysis can be expected to be a strong sink mechanism.

Quantum computations in heavy noble-gas hydride cations: Reference energies and new spectroscopic data

Authors: de Oca-Estevez, Maria Judit Montes; Prosmiti, Rita


Publication date: 2023/11/01

DOI: 10.1016/j.jmgm.2023.108562

Abstract: Computational quantum chemistry has become a powerful tool with a wide range of possibilities to solve chemical-physical problems. As a result of this, the interest in the applications of computational quantum chemistry has expanded considerably, and has opened up novel research opportunities. In particular, those related to the characterization of heavy-atoms complexes, as most electronic structure calculations for such systems struggle with the problem posed by the large number of electrons present in them, and consequently, the introduction of relativistic effects. The present study performed an exhaustive assess to characterized the uncommon NgH+ (Ng = Kr, Xe, and Rn) hydride cations in order to provide accurate rovibrational data of their isotopes to assist in the laboratory characterization or even their astronomical detection. Scalar relativistic effects were included, and the ground and first electronically exited states potential curves were obtained from benchmark ab initio CCSD(T)/CBS and MRCI+Q electronic structure calculations. Next, such interaction potentials, correctly extended to long-range asymptotic regions, were employed in quantum bound state calculations and molecular spectroscopic constants were determined for the most abundance 84Kr, 132Xe, and 222Rn isotopes. Our results were discussed in comparison with available experimental and previous theoretical estimates, aiming to treat accuracy issues. The new sets provide reference data that could serve for spectroscopic characterization of such low abundance and high radioactive species.

HCN emission from translucent gas and UV-illuminated cloud edges revealed by wide-field IRAM 30 m maps of the Orion B GMC

Authors: Santa-Maria, M. G.; Goicoechea, J. R.; Pety, J.; Gerin, M.; Orkisz, J. H.; Le Petit, F.; Einig, L.; Palud, P.; Magalhaes, V. de Souza; Beslic, I.; Segal, L.; Bardeau, S.; Bron, E.; Chainais, P.; Chanussot, J.; Gratier, P.; Guzman, V. V.; Hughes, A.; Languignon, D.; Levrier, F.; Lis, D. C.; Liszt, H. S.; Le Bourlot, J.; Oya, Y.; Oberg, K.; Peretto, N.; Roueff, E.; Roueff, A.; Sievers, A.; Thouvenin, P. -A.; Yamamoto, S.


Publication date: 2023/10/30

DOI: 10.1051/0004-6361/202346598

Abstract: Context. Massive stars form within dense clumps inside giant molecular clouds (GMCs). Finding appropriate chemical tracers of the dense gas (n(H-2) > several 10(4) cm(-)3 or A(V) > 8 mag) and linking their line luminosity with the star formation rate is of critical importance. Aims. Our aim is to determine the origin and physical conditions of the HCN-emitting gas and study their relation to those of other molecules. Methods. In the context of the IRAM 30m ORION-B large program, we present 5 deg(2) (similar to 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 (HNC)-C-13 lines. We compare the observed HCN line intensities with radiative transfer models including line overlap effects and electron excitation. Furthermore, we study the HCN/HNC isomeric abundance ratio with updated photochemical models. Results. We spectroscopically resolve the HCN J = 1-0 hyperfine structure (HFS) components (and partially resolved J = 2-1 and 3-2 components). We detect anomalous HFS line intensity (and line width) ratios almost everywhere in the cloud. About 70% of the total HCN J = 1-0 luminosity, L'(HCN J = 1-0) = 110Kkm s(-1) pc(-2), arises from AV < 8 mag. The HCN/CO J = 1-0 line intensity ratio, widely used as a tracer of the dense gas fraction, shows a bimodal behavior with an inflection point at AV less than or similar to 3 mag typical of translucent gas and illuminated cloud edges. We find that most of the HCN J = 1-0 emission arises from extended gas with n(H-2) less than or similar to 10(4) cm(-3), and even lower density gas if the ionization fraction is chi(e) >= 10(-5) and electron excitation dominates. This result contrasts with the prevailing view of HCN J = 1-0 emission as a tracer of dense gas and explains the low-AV branch of the HCN/CO J =1-0 intensity ratio distribution. Indeed, the highest HCN/CO ratios (similar to 0.1) at A(V) < 3 mag correspond to regions of high [C I] 492 GHz/CO J =1-0 intensity ratios (>1) characteristic of low-density photodissociation regions. The low surface brightness (. 1 Kkm s-1) and extended HCN and HCO+ J = 1-0 emission scale with IFIR – a proxy of the stellar far-ultraviolet (FUV) radiation field – in a similar way. Together with CO J = 1-0, these lines respond to increasing I-FIR up to G(0) similar or equal to 20. On the other hand, the bright HCN J = 1-0 emission (> 6 Kkm s(-1)) from dense gas in star-forming clumps weakly responds to IFIR once the FUV field becomes too intense (G(0) > 1500). In contrast, HNC J = 1-0 and [CI] 492 GHz lines weakly respond to I-FIR for all G(0). 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. Conclusions. Given the widespread and extended nature of the [CI] 492 GHz emission, as well as its spatial correlation with that of HCO+, HCN, and (CO)-C-13 J = 1-0 lines (in this order), we argue that the edges of GMCs are porous to FUV radiation from nearby massive stars. Enhanced FUV radiation favors the formation and excitation of HCN on large scales, not only in dense star-forming clumps, and it leads to a relatively low value of the dense gas mass to total luminosity ratio, alpha (HCN) = 29 M-circle dot/(K km s(-1)pc(2)) in Orion B. 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, which may be better traced by HNC than by HCN.

Attachment of Hydrogen Molecules to Atomic Ions (Na+, Cl-): Examination of an Adiabatic Separation of the H2 Rotational Motion

Authors: Garcia-Arroyo, Esther; Campos-Martinez, Jose; Bartolomei, Massimiliano; Hernandez, Marta I.; Pirani, Fernando; Halberstadt, Nadine


Publication date: 2023/10/29

DOI: 10.1002/cphc.202300424

Abstract: Interactions between molecular hydrogen and ions are of interest in cluster science, astrochemistry and hydrogen storage. In dynamical simulations, H-2 molecules are usually modelled as point particles, an approximation that can fail for anisotropic interactions. Here, we apply an adiabatic separation of the H-2 rotational motion to build effective pseudoatom-ion potentials and in turn study the properties of (H-2)(n)Na+/Cl- clusters. These interaction potentials are based on high-level ab initio calcu- lations and Improved Lennard-Jones parametrizations, while the subsequent dynamics has been performed by quantum Monte Carlo calculations. By comparisons with simulations explicitly describing the molecular rotations, it is concluded that the present adiabatic model is very adequate. Interestingly, we find differences in the cluster stabilities and coordination shells depending on the spin isomer considered (para- or ortho-H-2), especially for the anionic clusters.

Neural network-based emulation of interstellar medium models

Authors: Palud, Pierre; Einig, Lucas; Le Petit, Franck; Bron, Emeric; Chainais, Pierre; Chanussot, Jocelyn; Pety, Jerome; Thouvenin, Pierre-Antoine; Languignon, David; Beslic, Ivana; Santa-Maria, Miriam G.; Orkisz, Jan H.; Segal, Leontine E.; Zakardjian, Antoine; Bardeau, Sebastien; Gerin, Maryvonne; Goicoechea, Javier R.; Gratier, Pierre; Guzman, Viviana V.; Hughes, Annie; Levrier, Francois; Liszt, Harvey S.; Le Bourlot, Jacques; Roueff, Antoine; Sievers, Albrecht


Publication date: 2023/10/25

DOI: 10.1051/0004-6361/202347074

Abstract: 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.

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