Showing 1–50 of 113 results for author: Faure, A

Chemistry and ro-vibrational excitation of CH$^+$ in the planetary nebula NGC 7027

Authors: Milan Sil, Alexandre Faure, Helmut Wiesemeyer, Pierre Hily-Blant, Tomás González-Lezana, Josh Forer, Jérôme Loreau, François Lique

Abstract: Small carbon hydride cations, such as the methylidyne ion (CH$^+$), play an important role in the chemistry of the interstellar medium (ISM). They participate in gas-phase reaction networks leading to the formation of hydrocarbon species that act as precursors to more complex organic molecules. CH$^+$ is a highly reactive ion that is rapidly destroyed by H, H$_2$, and free electrons, making its ex… ▽ More Small carbon hydride cations, such as the methylidyne ion (CH$^+$), play an important role in the chemistry of the interstellar medium (ISM). They participate in gas-phase reaction networks leading to the formation of hydrocarbon species that act as precursors to more complex organic molecules. CH$^+$ is a highly reactive ion that is rapidly destroyed by H, H$_2$, and free electrons, making its excitation challenging to model. Its level populations depend not only on radiative and inelastic processes but also on chemical formation and destruction rates, a mechanism known as chemical pumping. We investigate this effect using a new set of ab initio state-resolved ro-vibrational (reactive and inelastic) collision data to model the observed CH$^+$ emission. Multiple rotational and ro-vibrational transitions of CH$^+$ detected toward the planetary nebula NGC 7027 are analyzed. The chemical structure of CH$^+$ is modeled with the CLOUDY code using updated reaction rates, providing the temperature and density structure across the nebula. A non-local thermodynamic equilibrium (NLTE) analysis is performed using CLOUDY and the single-zone RADEX code with a comprehensive set of spectroscopic and collisional data. In addition, chemical formation and destruction processes are implemented in RADEX and explored via Markov Chain Monte Carlo sampling. The CLOUDY model reproduces the observed CH$^+$ line fluxes within a factor of 1.3 on average. It indicates that rotational and ro-vibrational lines arise from physically distinct regions, primarily differing in temperature. RADEX models show that chemical pumping significantly enhances populations above ($\upsilon = 0, J = 1$), strongly increasing ro-vibrational emission, especially in the $\upsilon =2 \to 1$ band. Single-zone models remain limited, highlighting the need for full 1D modeling including all excitation processes. △ Less

Submitted 30 April, 2026; v1 submitted 9 April, 2026; originally announced April 2026.

Comments: accepted for publication in A&A

Journal ref: A&A 709, A271 (2026)

CORSIKA 8: A General Framework for Particle Cascade Simulations

Authors: J. M. Alameddine, J. Albrecht, A. A Alves Jr., J. Ammerman-Yebra, L. Arrabito, D. Baack, A. Coleman, C. Deaconu, H. Dembinski, D. Elsässer, R. Engel, A. Faure, A. Ferrari, C. Gaudu, C. Glaser, M. Gottowik, D. Heck, T. Huege, K. H. Kampert, N. Karastathis, J. Lazar, L. Nellen, D. Parello, T Pierog, R. Prechelt , et al. (12 additional authors not shown)

Abstract: The simulation of extensive air showers and particle cascades in general is a cornerstone of modern astroparticle physics. For more than two decades, CORSIKA, currently in version 7, has been one of the most widely used tools for this purpose. However, its architecture reflects design constraints of an earlier computing era, as well as increasingly limiting extensibility, maintainability, and adap… ▽ More The simulation of extensive air showers and particle cascades in general is a cornerstone of modern astroparticle physics. For more than two decades, CORSIKA, currently in version 7, has been one of the most widely used tools for this purpose. However, its architecture reflects design constraints of an earlier computing era, as well as increasingly limiting extensibility, maintainability, and adaptability to modern experimental requirements. CORSIKA 8 is a complete redesign of the original CORSIKA code, implemented in modern C++ and based on contemporary software engineering principles. It introduces a modular and extensible simulation framework with explicit handling of units, flexible geometry, and environment descriptions. In this paper, we present the design philosophy and core architecture of CORSIKA 8, describe the implementation of electromagnetic and hadronic shower physics, and validate air shower simulations against CORSIKA 7. The results demonstrate good agreement at the few-percent level for key observables, confirming the physics fidelity of CORSIKA 8. We also showcase new use cases that were beyond the capabilities of version 7, such as the simulation of cross-media showers and particle cascades in ice, including radio-signal propagation △ Less

Submitted 8 July, 2026; v1 submitted 2 April, 2026; originally announced April 2026.

Comments: 29 pages, 21 figures

Journal ref: Astroparticle Physics 182 (2026) 103265

Ortho-Para Chemistry of H2CO in the Protoplanetary Disk TW Hya

Authors: M. Gaillard, A. Faure, P. Hily-Blant, R. Le Gal, S. Lee, H. Nomura, K. Furuya

Abstract: The spatial distribution of the chemical reservoirs in protoplanetary disks is key to elucidate the composition of planets, especially habitable ones. However, the partitioning of the main elements among the refractory and volatile phases is still elusive. Key parameters such as the carbon-to-oxygen C/O elemental ratio and the ionization fraction remain poorly constrained, with the latter potentia… ▽ More The spatial distribution of the chemical reservoirs in protoplanetary disks is key to elucidate the composition of planets, especially habitable ones. However, the partitioning of the main elements among the refractory and volatile phases is still elusive. Key parameters such as the carbon-to-oxygen C/O elemental ratio and the ionization fraction remain poorly constrained, with the latter potentially orders of magnitude lower than in the interstellar medium. Moreover, the thermal structure of the gas is also poorly known, despite its deep influence on gas-phase chemistry. In this context, ortho-to-para ratios could provide selective and sensitive probes. Recent ALMA observations have measured the spatially resolved column density of ortho-and para-H2CO in the transition disk orbiting TW Hya and derived the radial profile of the ortho-to-para ratio. Yet, current disk models do not include the nuclear-spin-resolved chemistry required to interpret these observations. The present work aims to fill this gap, by combining a parametric disk physical model of TW Hya with the UGAN network, updated to include a comprehensive description of the nuclear-spin-resolved chemistry of formaldehyde. This new model reproduces the observed column density of H2CO to within a factor of 2, as well as the measured ortho-to-para ratio which varies from 1.5 in the outer disk to 3 inside 90au. In particular the low value of this ratio beyond 90au is well explained by our model. However, the statistical value of 3 measured below 70au cannot be reproduced, suggesting that additional processes involving ices may be involved. Our parameter space exploration shows that the abundance of H2CO is highly sensitive to the C/O elemental ratio and to the cosmic-ray ionization rate. Future observations of ortho-and para-H2CO, based on well selected rotational transitions, in a large sample of disks, appear highly desirable. △ Less

Submitted 13 March, 2026; originally announced March 2026.

Comments: Accepted in ACS Earth and Space Chemistry special issue Eric Herbst Festschrift 2026

Anatomy of the Class I protostar L1489 IRS with NOEMA - II. A disk replenished by a massive streamer

Authors: M. Tanious, R. Le Gal, A. Faure, S. Maret, A. López-Sepulcre, P. Hily-Blant

Abstract: (abridged) Streamers are newly identified channels that transport mass from large, molecular-cloud scales down to small, protoplanetary-disk scales. To better understand their impact on planet formation, it is essential to study their physical and chemical properties. In this framework, we aim to characterize the longest streamer identified in carbon chain emission within the Class I system L1489… ▽ More (abridged) Streamers are newly identified channels that transport mass from large, molecular-cloud scales down to small, protoplanetary-disk scales. To better understand their impact on planet formation, it is essential to study their physical and chemical properties. In this framework, we aim to characterize the longest streamer identified in carbon chain emission within the Class I system L1489 IRS, connecting the nearby prestellar core L1489 to the young stellar object (YSO). We observed multiple transitions of C$_2$H, ortho-c-C$_3$H$_2$, and HC$_3$N in L1489 IRS with NOEMA and IRAM-30m at 3mm and 2mm. Using a variety of radiative transfer methods, including a hyperfine structure (HFS) fitting, rotational diagrams, and proposing a new self-consistent Markov chain Monte Carlo (MCMC) approach combined with the non-LTE RADEX code, we derived the column densities and abundances of those molecules, as well as the H$_2$ number density along the streamer. This enabled us to estimate its mass, infall rate, and its impact on the {star+disk} system's mass. We found lower limits on the streamer mass of $\geq(4.67-18.3)\times10^{-3}$ $M_\odot$ (i.e., $\geq0.65-2.57$ times the current disk mass) and an infall rate of $\geq(1.94-7.57)\times10^{-7}$ $M_\odot$ yr$^{-1}$, where the ranges correspond to the different molecular tracers. These values are consistent with those derived in similar Class I objects. This suggests that the disk could be fully replenished by streamer material. Given its mass, the streamer is likely at the origin of the external warped disk seen in this system, as predicted by numerical simulations. Moreover, the first investigations based on the C$_2$H/c-C$_3$H$_2$ and HC$_3$N/c-C$_3$H$_2$ abundance ratios suggest that the streamer chemistry may be inherited from the core. These results suggest, for the first time, that the chemical composition of a Class I object is [...] △ Less

Submitted 15 October, 2025; originally announced October 2025.

Comments: 18 pages, 12 figures, 9 tables. Accepted for publication in A&A

VIRAL: Vision-grounded Integration for Reward design And Learning

Authors: Valentin Cuzin-Rambaud, Emilien Komlenovic, Alexandre Faure, Bruno Yun

Abstract: The alignment between humans and machines is a critical challenge in artificial intelligence today. Reinforcement learning, which aims to maximize a reward function, is particularly vulnerable to the risks associated with poorly designed reward functions. Recent advancements has shown that Large Language Models (LLMs) for reward generation can outperform human performance in this context. We intro… ▽ More The alignment between humans and machines is a critical challenge in artificial intelligence today. Reinforcement learning, which aims to maximize a reward function, is particularly vulnerable to the risks associated with poorly designed reward functions. Recent advancements has shown that Large Language Models (LLMs) for reward generation can outperform human performance in this context. We introduce VIRAL, a pipeline for generating and refining reward functions through the use of multi-modal LLMs. VIRAL autonomously creates and interactively improves reward functions based on a given environment and a goal prompt or annotated image. The refinement process can incorporate human feedback or be guided by a description generated by a video LLM, which explains the agent's policy in video form. We evaluated VIRAL in five Gymnasium environments, demonstrating that it accelerates the learning of new behaviors while ensuring improved alignment with user intent. The source-code and demo video are available at: https://github.com/VIRAL-UCBL1/VIRAL and https://youtu.be/Hqo82CxVT38. △ Less

Submitted 28 October, 2025; v1 submitted 28 May, 2025; originally announced May 2025.

Future Circular Collider Feasibility Study Report: Volume 2, Accelerators, Technical Infrastructure and Safety

Authors: M. Benedikt, F. Zimmermann, B. Auchmann, W. Bartmann, J. P. Burnet, C. Carli, A. Chancé, P. Craievich, M. Giovannozzi, C. Grojean, J. Gutleber, K. Hanke, A. Henriques, P. Janot, C. Lourenço, M. Mangano, T. Otto, J. Poole, S. Rajagopalan, T. Raubenheimer, E. Todesco, L. Ulrici, T. Watson, G. Wilkinson, A. Abada , et al. (1439 additional authors not shown)

Abstract: In response to the 2020 Update of the European Strategy for Particle Physics, the Future Circular Collider (FCC) Feasibility Study was launched as an international collaboration hosted by CERN. This report describes the FCC integrated programme, which consists of two stages: an electron-positron collider (FCC-ee) in the first phase, serving as a high-luminosity Higgs, top, and electroweak factory;… ▽ More In response to the 2020 Update of the European Strategy for Particle Physics, the Future Circular Collider (FCC) Feasibility Study was launched as an international collaboration hosted by CERN. This report describes the FCC integrated programme, which consists of two stages: an electron-positron collider (FCC-ee) in the first phase, serving as a high-luminosity Higgs, top, and electroweak factory; followed by a proton-proton collider (FCC-hh) at the energy frontier in the second phase. FCC-ee is designed to operate at four key centre-of-mass energies: the Z pole, the WW production threshold, the ZH production peak, and the top/anti-top production threshold - delivering the highest possible luminosities to four experiments. Over 15 years of operation, FCC-ee will produce more than 6 trillion Z bosons, 200 million WW pairs, nearly 3 million Higgs bosons, and 2 million top anti-top pairs. Precise energy calibration at the Z pole and WW threshold will be achieved through frequent resonant depolarisation of pilot bunches. The sequence of operation modes remains flexible. FCC-hh will operate at a centre-of-mass energy of approximately 85 TeV - nearly an order of magnitude higher than the LHC - and is designed to deliver 5 to 10 times the integrated luminosity of the HL-LHC. Its mass reach for direct discovery extends to several tens of TeV. In addition to proton-proton collisions, FCC-hh is capable of supporting ion-ion, ion-proton, and lepton-hadron collision modes. This second volume of the Feasibility Study Report presents the complete design of the FCC-ee collider, its operation and staging strategy, the full-energy booster and injector complex, required accelerator technologies, safety concepts, and technical infrastructure. It also includes the design of the FCC-hh hadron collider, development of high-field magnets, hadron injector options, and key technical systems for FCC-hh. △ Less

Submitted 25 April, 2025; originally announced May 2025.

Comments: 627 pages. Please address any comment or request to fcc.secretariat@cern.ch

Report number: CERN-FCC-ACC-2025-0004

Future Circular Collider Feasibility Study Report: Volume 3, Civil Engineering, Implementation and Sustainability

Authors: M. Benedikt, F. Zimmermann, B. Auchmann, W. Bartmann, J. P. Burnet, C. Carli, A. Chancé, P. Craievich, M. Giovannozzi, C. Grojean, J. Gutleber, K. Hanke, A. Henriques, P. Janot, C. Lourenço, M. Mangano, T. Otto, J. Poole, S. Rajagopalan, T. Raubenheimer, E. Todesco, L. Ulrici, T. Watson, G. Wilkinson, P. Azzi , et al. (1439 additional authors not shown)

Abstract: Volume 3 of the FCC Feasibility Report presents studies related to civil engineering, the development of a project implementation scenario, and environmental and sustainability aspects. The report details the iterative improvements made to the civil engineering concepts since 2018, taking into account subsurface conditions, accelerator and experiment requirements, and territorial considerations. I… ▽ More Volume 3 of the FCC Feasibility Report presents studies related to civil engineering, the development of a project implementation scenario, and environmental and sustainability aspects. The report details the iterative improvements made to the civil engineering concepts since 2018, taking into account subsurface conditions, accelerator and experiment requirements, and territorial considerations. It outlines a technically feasible and economically viable civil engineering configuration that serves as the baseline for detailed subsurface investigations, construction design, cost estimation, and project implementation planning. Additionally, the report highlights ongoing subsurface investigations in key areas to support the development of an improved 3D subsurface model of the region. The report describes development of the project scenario based on the 'avoid-reduce-compensate' iterative optimisation approach. The reference scenario balances optimal physics performance with territorial compatibility, implementation risks, and costs. Environmental field investigations covering almost 600 hectares of terrain - including numerous urban, economic, social, and technical aspects - confirmed the project's technical feasibility and contributed to the preparation of essential input documents for the formal project authorisation phase. The summary also highlights the initiation of public dialogue as part of the authorisation process. The results of a comprehensive socio-economic impact assessment, which included significant environmental effects, are presented. Even under the most conservative and stringent conditions, a positive benefit-cost ratio for the FCC-ee is obtained. Finally, the report provides a concise summary of the studies conducted to document the current state of the environment. △ Less

Submitted 25 April, 2025; originally announced May 2025.

Comments: 357 pages. Please address any comment or request to fcc.secretariat@cern.ch

Report number: CERN-FCC-ACC-2025-0003

Future Circular Collider Feasibility Study Report: Volume 1, Physics, Experiments, Detectors

Authors: M. Benedikt, F. Zimmermann, B. Auchmann, W. Bartmann, J. P. Burnet, C. Carli, A. Chancé, P. Craievich, M. Giovannozzi, C. Grojean, J. Gutleber, K. Hanke, A. Henriques, P. Janot, C. Lourenço, M. Mangano, T. Otto, J. Poole, S. Rajagopalan, T. Raubenheimer, E. Todesco, L. Ulrici, T. Watson, G. Wilkinson, P. Azzi , et al. (1439 additional authors not shown)

Abstract: Volume 1 of the FCC Feasibility Report presents an overview of the physics case, experimental programme, and detector concepts for the Future Circular Collider (FCC). This volume outlines how FCC would address some of the most profound open questions in particle physics, from precision studies of the Higgs and EW bosons and of the top quark, to the exploration of physics beyond the Standard Model.… ▽ More Volume 1 of the FCC Feasibility Report presents an overview of the physics case, experimental programme, and detector concepts for the Future Circular Collider (FCC). This volume outlines how FCC would address some of the most profound open questions in particle physics, from precision studies of the Higgs and EW bosons and of the top quark, to the exploration of physics beyond the Standard Model. The report reviews the experimental opportunities offered by the staged implementation of FCC, beginning with an electron-positron collider (FCC-ee), operating at several centre-of-mass energies, followed by a hadron collider (FCC-hh). Benchmark examples are given of the expected physics performance, in terms of precision and sensitivity to new phenomena, of each collider stage. Detector requirements and conceptual designs for FCC-ee experiments are discussed, as are the specific demands that the physics programme imposes on the accelerator in the domains of the calibration of the collision energy, and the interface region between the accelerator and the detector. The report also highlights advances in detector, software and computing technologies, as well as the theoretical tools /reconstruction techniques that will enable the precision measurements and discovery potential of the FCC experimental programme. This volume reflects the outcome of a global collaborative effort involving hundreds of scientists and institutions, aided by a dedicated community-building coordination, and provides a targeted assessment of the scientific opportunities and experimental foundations of the FCC programme. △ Less

Submitted 25 April, 2025; originally announced May 2025.

Comments: 290 pages. Please address any comment or request to fcc.secretariat@cern.ch

Report number: CERN-FCC-PHYS-2025-0002

Experimental observation of quantum interferences in CO-H$_2$ rotational energy transfer at room temperature

Authors: Hamza Labiad, Alexandre Faure, Ian R. Sims

Abstract: Using time-resolved infrared-vacuum-ultraviolet double-resonance spectroscopy, experimental room temperature measurements of state-to-state rate coefficients for rotational energy transfer within the X $^1Σ^+(v=2)$ vibrational state of CO due to H$_2$ collisions have been compared to accurate 4-D close-coupling quantum calculations. Theoretically predicted quantum interferences in the CO-H$_2$ col… ▽ More Using time-resolved infrared-vacuum-ultraviolet double-resonance spectroscopy, experimental room temperature measurements of state-to-state rate coefficients for rotational energy transfer within the X $^1Σ^+(v=2)$ vibrational state of CO due to H$_2$ collisions have been compared to accurate 4-D close-coupling quantum calculations. Theoretically predicted quantum interferences in the CO-H$_2$ collisional system are experimentally observed at room temperature, and excellent agreement between theory and experiment is observed. These results provide a valuable benchmark for validating the anisotropic part of the potential energy surface, thereby supporting the theoretical modeling of CO emission in warm astrophysical environments such as photodissociation regions. △ Less

Submitted 9 March, 2026; v1 submitted 6 April, 2025; originally announced April 2025.

Journal ref: Physical Review A 112, 062816 (2025)

Chemistry and ro-vibrational excitation of HeH$^+$ in the Planetary Nebula NGC 7027

Authors: Milan Sil, Alexandre Faure, Helmut Wiesemeyer, Pierre Hily-Blant, Jérôme Loreau, K. D. Pérez, Roman Čurík, François Lique

Abstract: HeH$^+$ belongs to the class of "reactive" ions that can be destroyed so quickly that chemical formation and destruction rates may compete with inelastic rates and should be considered when solving the statistical equilibrium equations. This so-called chemical "pumping" or "excitation" effect is investigated here for the first time in HeH$^+$. The chemical evolution of HeH$^+$ in NGC 7027 is model… ▽ More HeH$^+$ belongs to the class of "reactive" ions that can be destroyed so quickly that chemical formation and destruction rates may compete with inelastic rates and should be considered when solving the statistical equilibrium equations. This so-called chemical "pumping" or "excitation" effect is investigated here for the first time in HeH$^+$. The chemical evolution of HeH$^+$ in NGC 7027 is modeled with the CLOUDY photoionization code using updated reaction rate coefficients. The non-LTE analysis of the three observed HeH$^+$ emission lines is then performed with the CLOUDY and RADEX codes using an extensive set of spectroscopic and inelastic collisional data suitable for the specific high-temperature environment of NGC 7027. In a second approach, chemical formation and destruction rates of HeH$^+$ are implemented in RADEX. This code is combined with MCMC sampling (performed on the RADEX-parameters space) to extract the best-fit HeH$^+$ column density and physical conditions from the observed line fluxes. The CLOUDY and RADEX non-LTE results are found to be in good agreement, and the $\upsilon=1-0 \ P(2)/P(1)$ line ratio is better than 20%. Agreement to better than a factor of 2.3 is obtained when including the reaction between He($2^3S$) and H as an additional source of HeH$^+$. The RADEX/MCMC model with chemical pumping is found to reproduce both the observed line fluxes and the line ratio to 20%. Our results suggest that additional HeH$^+$ lines must be detected in NGC 7027 to better constrain the physical conditions via non-LTE models. Uncertainties in collisional (reactive and inelastic) data of HeH$^+$ have been largely reduced in this work. The three observed lines are not sensitive to chemical pumping while excited "short-lived" levels are significantly overpopulated with respect to a non-LTE model neglecting chemical excitation. △ Less

Submitted 21 February, 2025; originally announced February 2025.

Comments: 12 pages, 5 figures, accepted for publication in A&A

Journal ref: A&A 695, A244 (2025)

PDRs4All XI. Detection of infrared CH$^+$ and CH$_3^+$ rovibrational emission in the Orion Bar and disk d203-506: evidence of chemical pumping

Authors: Marion Zannese, Benoît Tabone, Emilie Habart, Emmanuel Dartois, Javier R. Goicoechea, Laurent Coudert, Bérenger Gans, Marie-Aline Martin-Drumel, Ugo Jacovella, Alexandre Faure, Benjamin Godard, Alexander G. G. M. Tielens, R. Le Gal, John H. Black, Silvia Vicente, Olivier Berné, Els Peeters, Dries Van De Putte, Ryan Chown, Ameek Sidhu, Ilane Schroetter, Amélie Canin, Olga Kannavou

Abstract: The methylidyne cation (CH$^+$) and the methyl cation (CH$_3^+$) are building blocks of organic molecules, yet their coupled formation and excitation mechanisms remain mainly unprobed. The James Webb Space Telescope (JWST), with its high spatial resolution and good spectral resolution, provides unique access to the detection of these molecules. Our goal is to use the first detection of CH$^+$ and… ▽ More The methylidyne cation (CH$^+$) and the methyl cation (CH$_3^+$) are building blocks of organic molecules, yet their coupled formation and excitation mechanisms remain mainly unprobed. The James Webb Space Telescope (JWST), with its high spatial resolution and good spectral resolution, provides unique access to the detection of these molecules. Our goal is to use the first detection of CH$^+$ and CH$_3^+$ rovibrational emission in the Orion Bar and in the protoplanetary disk d203-506, irradiated by the Trapezium cluster, to probe their formation and excitation mechanisms and constrain the physico-chemical conditions. We use spectro-imaging acquired using both the NIRSpec and MIRI-MRS instruments to study the CH$^+$ and CH$_3^+$ spatial distribution at very small scales, and compare it to excited H$_2$ emission. CH$^+$ and CH$_3^+$ emissions originate from the same region as highly excited H$_2$. Our comparison between the Bar and d203-506 reveals that both CH$^+$ and CH$_3^+$ excitation and/or formation are highly dependent on gas density. The excitation temperature of the observed CH$^+$ and CH$_3^+$ rovibrational lines is around $T$ ~ 1500 K in the Bar and $T$ ~ 800 K in d203-506. Moreover, the column densities derived from the rovibrational emission are less than 0.1 % of the total known (CH$^+$) and expected (CH$_3^+$) column densities. These results show that CH$^+$ and CH$_3^+$ level populations strongly deviate from ETL. CH$^+$ rovibrational emission can be explained by chemical formation pumping with excited H$_2$ via C$^+$ + H$_2^*$ = CH$^+$ + H. These results support a gas phase formation pathway of CH$^+$ and CH$_3^+$ via successive hydrogen abstraction reactions. However, we do not find any evidence of CH$_2^+$ emission in the JWST spectrum. Finally, observed CH$^+$ intensities coupled with chemical formation pumping model provide a diagnostic tool to trace the local density. △ Less

Submitted 12 February, 2025; originally announced February 2025.

Comments: Accepted on the 11/02/2025 in A&A

Journal ref: A&A 696, A99 (2025)

Dissociative recombination of N$_2$H$^+$: Isotopic effects

Authors: J. Zs. Mezei, A. Orban, S. Demes, M. Ayouz, A. Faure, P. Hily-Blant, Ioan F. Schneider

Abstract: The investigation of the isotopic ratio of interstellar nitrogen -- $^{14}$N versus $^{15}$N -- is done, for explaining its variations observed for N2H+ in different interstellar and Solar environments. The goal is to produce cross sections and rate coefficients for electron impact dissociative recombination for different isotopologues of N$_2$H$^+$, since it was envisioned as a novel source that… ▽ More The investigation of the isotopic ratio of interstellar nitrogen -- $^{14}$N versus $^{15}$N -- is done, for explaining its variations observed for N2H+ in different interstellar and Solar environments. The goal is to produce cross sections and rate coefficients for electron impact dissociative recombination for different isotopologues of N$_2$H$^+$, since it was envisioned as a novel source that can lead to nitrogen fractionation. We calculate dissociative recombination cross sections and rate coefficients using the normal mode approximation combined with the R-matrix theory and vibronic frame transformation within the multichannel quantum defect theory for eight isotopologues containing both $^{14}$N, $^{15}$N and H, D. Our calculations show that the relative differences respective to the main isotopologue ($^{14}$N$_2$H$^+$) is below 1% for the hydrogen containing isotopologues, but reaches almost 30% for the heaviest deuterated isotopologue, leading us to the conclusion that according to the present status of the theory, dissociative recombination is not responsible for the peculiar $^{14}$N/$^{15}$N isotopic ratios of N$_2$H$^+$ observed in the different interstellar molecular clouds. △ Less

Submitted 30 January, 2025; originally announced January 2025.

Comments: 7 pages, 8 figures, 2 tables. arXiv admin note: text overlap with arXiv:2305.17976

Rotational excitation cross sections for chloronium based on a new 5D interaction potential with molecular hydrogen

Authors: Sándor Demes, Dariusz Kędziera, Alexandre Faure, François Lique

Abstract: Chloronium (H$_2$Cl$^+$) is an important intermediate of Cl-chemistry in space. The accurate knowledge of its collisional properties allows a better interpretation of the corresponding observations in interstellar clouds and therefore a better estimation of its abundance in these environments. While the ro-vibrational spectroscopy of H$_2$Cl$^+$ is well known, the studies of its collisional excita… ▽ More Chloronium (H$_2$Cl$^+$) is an important intermediate of Cl-chemistry in space. The accurate knowledge of its collisional properties allows a better interpretation of the corresponding observations in interstellar clouds and therefore a better estimation of its abundance in these environments. While the ro-vibrational spectroscopy of H$_2$Cl$^+$ is well known, the studies of its collisional excitation are rather limited and these are available for the interaction with helium atoms only. We provide a new 5-dimensional rigid-rotor potential energy surface for the interaction of H$_2$Cl$^+$ with H$_2$, calculated from explicitly correlated coupled cluster ab initio theory, which was fitted then with a set of analytical functions, allowing to perform scattering calculations using accurate quantum theories. We analyze the collision-energy-dependence of the rotational state-to-state cross sections and the temperature dependence of the corresponding thermal rate coefficients, with a particular attention on the collisional propensity rules. When comparing our results for collisions with H$_2$ with those obtained with He as a colliding partner, we found very significant differences with non-linear scaling trends, which proves again that He is not a suitable proxy for collisions between hydride molecules and molecular hydrogen, the most abundant gas particle in the interstellar medium. △ Less

Submitted 30 December, 2024; originally announced December 2024.

Comments: Accepted in Journal of Physical Chemistry A (December 12, 2024)

Revisiting rotationally excited CH at radio wavelengths: A case study towards W51

Authors: Arshia M. Jacob, Meera Nandakumar, Nirupam Roy, Karl M. Menten, David A. Neufeld, Alexandre Faure, Maitraiyee Tiwari, Thushara G. S. Pillai, Timothy Robishaw, Carlos A. Duran

Abstract: Ever since they were first detected in the interstellar medium, the radio wavelength (3.3 GHz) hyperfine-structure splitting transitions in the rotational ground state of CH have been observed to show anomalous excitation. Astonishingly, this behaviour has been uniformly observed towards a variety of different sources probing a wide range of physical conditions. While the observed level inversion… ▽ More Ever since they were first detected in the interstellar medium, the radio wavelength (3.3 GHz) hyperfine-structure splitting transitions in the rotational ground state of CH have been observed to show anomalous excitation. Astonishingly, this behaviour has been uniformly observed towards a variety of different sources probing a wide range of physical conditions. While the observed level inversion can be explained globally by a pumping scheme involving collisions, a description of the extent of 'over-excitation' observed in individual sources requires the inclusion of radiative processes, involving transitions at higher rotational levels. Therefore, a complete description of the excitation mechanism in the CH ground state, observed towards individual sources entails observational constraints from the rotationally excited levels of CH and in particular that of its first rotationally excited state. Given the limited detections of these lines, the objective of this work is to characterise the physical and excitation properties of the rotationally excited lines of CH near 700 MHz, and investigate their influence on the pumping mechanisms of the ground-state lines of CH. This work presents the first interferometric search for the rotationally excited lines of CH near 700 MHz carried out using the uGMRT array and jointly models the physical and excitation conditions traced by lines from both the ground and first rotationally excited states of CH. △ Less

Submitted 12 November, 2024; originally announced November 2024.

Comments: Accepted for publication in A&A 18 Pages + Appendix, 22 Figures, 4 Tables

Journal ref: A&A 692, A164 (2024)

Fate and detectability of rare gas hydride ions in nova ejecta: A case study with nova templates

Authors: Milan Sil, Ankan Das, Ramkrishna Das, Ruchi Pandey, Alexandre Faure, Helmut Wiesemeyer, Pierre Hily-Blant, François Lique, Paola Caselli

Abstract: HeH$^+$ was the first heteronuclear molecule to form in the metal-free Universe after the Big Bang. The molecule gained significant attention following its first circumstellar detection in the young and dense planetary nebula NGC 7027. We target some hydride ions associated with the noble gases (HeH$^+$, ArH$^+$, and NeH$^+$) to investigate their formation in harsh environments like the nova outbu… ▽ More HeH$^+$ was the first heteronuclear molecule to form in the metal-free Universe after the Big Bang. The molecule gained significant attention following its first circumstellar detection in the young and dense planetary nebula NGC 7027. We target some hydride ions associated with the noble gases (HeH$^+$, ArH$^+$, and NeH$^+$) to investigate their formation in harsh environments like the nova outburst region. We use a photoionization modeling (based on previously published best-fit physical parameters) of the moderately fast ONe type nova, QU Vulpeculae 1984, and the CO type novae, RS Ophiuchi and V1716 Scorpii. Our steady-state modeling reveals a convincing amount of HeH$^+$, especially in the dense clump of RS Ophiuchi and V1716 Scorpii. The calculated upper limit on the surface brightness of HeH$^+$ transitions suggests that the James Webb Space Telescope (JWST) could detect some of them, particularly in sources like RS Ophiuchi and V1716 Scorpii, which have similar physical and chemical conditions and evolution. It must be clearly noted that the sources studied are used as templates, and not as targets for observations. The detection of these lines could be useful for determining the physical conditions in similar types of systems and for validating our predictions based on new electron-impact ro-vibrational collisional data at temperatures of up to 20,000 K. △ Less

Submitted 18 December, 2024; v1 submitted 8 November, 2024; originally announced November 2024.

Comments: 15 pages, 12 figures, published in Astronomy & Astrophysics (A&A) Journal

Journal ref: A&A 692, A264 (2024)

Simulating radio emission from particle cascades with CORSIKA 8

Authors: J. M. Alameddine, J. Albrecht, J. Ammerman-Yebra, L. Arrabito, A. A. Alves Jr., D. Baack, A. Coleman, H. Dembinski, D. Elsässer, R. Engel, A. Faure, A. Ferrari, C. Gaudu, C. Glaser, M. Gottowik, D. Heck, T. Huege, K. H. Kampert, N. Karastathis, L. Nellen, T. Pierog, R. Prechelt, M. Reininghaus, W. Rhode, F. Riehn , et al. (5 additional authors not shown)

Abstract: CORSIKA 8 is a new framework for simulations of particle cascades in air and dense media implemented in modern C++17, based on past experience with existing codes, in particular CORSIKA 7. The flexible and modular structure of the project allows the development of independent modules that can produce a fully customizable particle shower simulation. The radio module in particular is designed to tre… ▽ More CORSIKA 8 is a new framework for simulations of particle cascades in air and dense media implemented in modern C++17, based on past experience with existing codes, in particular CORSIKA 7. The flexible and modular structure of the project allows the development of independent modules that can produce a fully customizable particle shower simulation. The radio module in particular is designed to treat the electric field calculation and its propagation through complex media to each observer location in an autonomous and flexible way. It already allows for the simultaneous simulation of the radio emission calculated with two independent time-domain formalisms, the "Endpoint formalism" as previously implemented in CoREAS and the "ZHS" algorithm as ported from ZHAireS. The design acts as the baseline interface for current and future development for the simulation of radio emission from particle showers in standard and complex scenarios, such as cross-media showers penetrating from air into ice. In this work, we present the design and implementation of the radio module in CORSIKA 8, along with validation studies and a direct comparison of the radio emission from air showers simulated with CORSIKA 8, CORSIKA 7 and ZHAireS. We also present the impact of simulation details such as the step size of simulated particle tracks on radio-emission simulations and perform a direct comparison of the "Endpoints" and "ZHS" formalisms for the same underlying air showers. Finally, we present an in-depth comparison of CORSIKA 8 and CORSIKA 7 for optimum simulation settings and discuss the relevance of observed differences in light of reconstruction efforts for the energy and mass of cosmic rays. △ Less

Submitted 24 September, 2024; originally announced September 2024.

Comments: 17 pages

Journal ref: Astropart. Phys. 166, 103072 (2025)

Cosmic-ray induced sputtering of interstellar formaldehyde ices

Authors: M. Faure, A. Bacmann, A. Faure, E. Quirico, P. Boduch, A. Domaracka, H. Rothard

Abstract: H2CO is a ubiquitous molecule in the ISM and in the gas phase of prestellar cores, and is likely present in ice mantles, but its main desorption mechanism is unknown. In this paper our aim is to quantify the desorption efficiency of H2CO upon cosmic-ray impact in order to determine whether cosmic-ray induced sputtering could account for the H2CO abundance observed in prestellar cores. Using a heav… ▽ More H2CO is a ubiquitous molecule in the ISM and in the gas phase of prestellar cores, and is likely present in ice mantles, but its main desorption mechanism is unknown. In this paper our aim is to quantify the desorption efficiency of H2CO upon cosmic-ray impact in order to determine whether cosmic-ray induced sputtering could account for the H2CO abundance observed in prestellar cores. Using a heavy-ion beam as a cosmic-ray analogue at the GANIL accelerator, we irradiated pure H2CO ice films at 10 K under high vacuum conditions and monitored the ice film evolution with infrared spectroscopy and the composition of the sputtered species in the gas phase using mass spectrometry. We derived both the effective and intact sputtering yield of pure H2CO ices. We find that H2CO easily polymerises under heavy-ion irradiation in the ice, and is also radiolysed into CO and CO2. In the gas phase, the dominant sputtered species is CO and intact H2CO is only a minor species. We determine an intact sputtering yield for pure H2CO ices $2.5\times 10^3$ molecules ion$^{-1}$ for an electronic stopping power of $S_e\sim2830$ eV ($10^{15}$ molecules cm$^{-2}$)$^{-1}$. The corresponding cosmic-ray sputtering rate is $Γ_\mathrm{CRD}=1.5\times 10^{18}ζ$ molecules cm$^{-2}$ s$^{-1}$, where $ζ$ is the rate of cosmic-ray ionisation of molecular hydrogen in the ISM. In the frame of a simple steady-state chemical model of freeze-out and non-thermal desorption, we find that this experimental cosmic-ray sputtering rate is too low (by an order of magnitude) to account for the observed H2CO gas-phase abundance we derived in the prestellar core L1689B. We find however that this abundance can be reproduced if we assume that H2CO diluted in CO or CO2 ices co-desorbs at the same sputtering rate as pure CO or pure CO2 ices. △ Less

Submitted 9 September, 2024; v1 submitted 3 September, 2024; originally announced September 2024.

Comments: accepted by Astronomy and Astrophysics. 17 pages, 17 figures. Abridged abstract

Journal ref: A&A 693, A30 (2025)

Rotational transitions induced by collisions of HD$^{+}$ ions with low energy electrons

Authors: O. Motapon, N. Pop, F. Argoubi, J. Zs. Mezei, M. D. Epée Epée, A. Faure, M. Telmini, J. Tennyson, I. F. Schneider

Abstract: A series of Multichannel Quantum Defect Theory-based computations have been performed, in order to produce the cross sections of rotational transitions (excitations $N_{i}^{+}-2 \rightarrow$ $N_{i}^{+}$, de-excitations $N_{i}^{+}$ $\rightarrow$ $N_{i}^{+}-2$, with $N_{i}^{+}=2$ to $10$) and of their competitive process, the dissociative recombination, induced by collisions of HD$^+$ ions with elec… ▽ More A series of Multichannel Quantum Defect Theory-based computations have been performed, in order to produce the cross sections of rotational transitions (excitations $N_{i}^{+}-2 \rightarrow$ $N_{i}^{+}$, de-excitations $N_{i}^{+}$ $\rightarrow$ $N_{i}^{+}-2$, with $N_{i}^{+}=2$ to $10$) and of their competitive process, the dissociative recombination, induced by collisions of HD$^+$ ions with electrons in the energy range $10^{-5}$ to 0.3 eV. Maxwell anisotropic rate coefficients, obtained from these cross sections in the conditions of the Heidelberg Test Storage Ring (TSR) experiments ($k_{B}T_{t}=2.8$ meV and $k_{B}T_{l}=45$ $μ$eV), have been reported for those processes in the same electronic energy range. Maxwell isotropic rate coefficients have been as well presented for electronic temperatures up to a few hundreds of Kelvins. Very good overall agreement is found between our results for rotational transitions and the former theoretical computations as well as with experiment. Furthermore, owing to the full rotational computations performed, the accuracy of the resulting dissociative recombination cross sections is considerably improved. △ Less

Submitted 10 May, 2024; originally announced May 2024.

Comments: 9 pages, 8 figures, 1 table. arXiv admin note: text overlap with arXiv:1107.5267

Journal ref: Physical Review A 90, 012706 (2014)

Dissociative recombination of the CH$^+$ molecular ion at low energy

Authors: K. Chakrabarti, J. Zs Mezei, O. Motapon, A. Faure, O. Dulieu, K. Hassouni, I. F. Schneider

Abstract: The reactive collisions of the CH$^+$ molecular ion with electrons is studied in the framework of the multichannel quantum defect theory, taking into account the contribution of the core-excited Rydberg states. In addition to the $X^1Σ^+$ ground state of the ion, we also consider the contribution to the dynamics of the $a^3Π$ and $A^1Π$ excited states of CH$^+$. Our results - in the case of the di… ▽ More The reactive collisions of the CH$^+$ molecular ion with electrons is studied in the framework of the multichannel quantum defect theory, taking into account the contribution of the core-excited Rydberg states. In addition to the $X^1Σ^+$ ground state of the ion, we also consider the contribution to the dynamics of the $a^3Π$ and $A^1Π$ excited states of CH$^+$. Our results - in the case of the dissociative recombination in good agreement with the storage ring measurements - rely on decisive improvements - complete account of the ionisation channels and accurate evaluation of the reaction matrix - of a previously used model. △ Less

Submitted 30 April, 2024; originally announced April 2024.

Comments: 8 pages, 8 figures

Journal ref: J. Phys. B: At. Mol. Opt. Phys. 51 (2018) 104002 (8pp)

Maser Activity of Organic Molecules toward Sgr B2(N)

Authors: Ci Xue, Anthony Remijan, Alexandre Faure, Emmanuel Momjian, Todd R. Hunter, Ryan A. Loomis, Eric Herbst, Brett McGuire

Abstract: At centimeter wavelengths, single-dish observations have suggested that the Sagittarius (Sgr) B2 molecular cloud at the Galactic Center hosts weak maser emission from several organic molecules, including CH$_2$NH, HNCNH, and HCOOCH$_3$. However, the lack of spatial distribution information of these new maser species has prevented us from assessing the excitation conditions of the maser emission as… ▽ More At centimeter wavelengths, single-dish observations have suggested that the Sagittarius (Sgr) B2 molecular cloud at the Galactic Center hosts weak maser emission from several organic molecules, including CH$_2$NH, HNCNH, and HCOOCH$_3$. However, the lack of spatial distribution information of these new maser species has prevented us from assessing the excitation conditions of the maser emission as well as their pumping mechanisms. Here, we present a mapping study toward Sgr B2 North (N) to locate the region where the complex maser emission originates. We report the first detection of the Class I methanol (CH$_3$OH) maser at 84 GHz and the first interferometric map of the methanimine (CH$_2$NH) maser at 5.29 GHz toward this region. In addition, we present a tool for modeling and fitting the unsaturated molecular maser signals with non-LTE radiative transfer models and Bayesian analysis using the Markov-Chain Monte Carlo approach. These enable us to quantitatively assess the observed spectral profiles. The results suggest a two-chain-clump model for explaining the intense CH$_3$OH Class I maser emission toward a region with low continuum background radiation. By comparing the spatial origin and extent of maser emission from several molecular species, we find that the 5.29 GHz CH$_2$NH maser has a close spatial relationship with the 84 GHz CH$_3$OH Class I masers. This relationship serves as observational evidence to suggest a similar collisional pumping mechanism for these maser transitions. △ Less

Submitted 19 April, 2024; originally announced April 2024.

Comments: 17 figures

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

Authors: A. García Muñoz, A. Asensio Ramos, A. Faure

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

Submitted 3 April, 2024; originally announced April 2024.

Comments: Accepted for publication in Icarus

Anatomy of the Class I protostar L1489 IRS with NOEMA -- I. Disk, streamers, outflow(s) and bubbles at 3mm

Authors: M. Tanious, R. Le Gal, R. Neri, A. Faure, A. Gupta, C. J. Law, J. Huang, N. Cuello, J. P. Williams, F. Ménard

Abstract: Over the past few years, chemical studies have revealed multiple structures in the vicinity of young stellar objects (YSOs). It has become evident that specific physical conditions are associated with the emission of particular molecular lines, allowing us to use molecular probes of the YSO physics. Consequently, chemical surveys are now necessary to fully constrain the origin of the observed stru… ▽ More Over the past few years, chemical studies have revealed multiple structures in the vicinity of young stellar objects (YSOs). It has become evident that specific physical conditions are associated with the emission of particular molecular lines, allowing us to use molecular probes of the YSO physics. Consequently, chemical surveys are now necessary to fully constrain the origin of the observed structures. Several surveys have been conducted to explore the chemistry of YSOs, focusing on Class 0 and Class II objects. However, our knowledge of intermediate objects, that are Class I objects, remains limited. To bridge the gap and establish the relationship between observed structures and molecular line emission at the Class I evolutionary stage, we investigate the spatial distribution of key molecular gas species in the low-mass Class I protostar L1489 IRS (IRAS 04016+2610), a source part of the ChemYSO survey. We performed a 3mm line survey at high spatial and high spectral resolution using the NOEMA interferometer and the IRAM-30m telescope. We present here the ten brightest lines of our survey, in which we identified a new ~ 3 000 au long streamer in HC3N, C2H, and c-C3H2 emission, likely associated with more localized accretion shocks probed in SO. In addition, two ~ 10 000 au bubbles are seen with the dense molecular tracers HCO+, CS, and HCN around the YSO. Additionally, potential indicators of a second outflow appear in CS and HCN emission, but its nature remains to be confirmed. The late infall identified at large scales may originate from the nearby prestellar core L1489 and is likely responsible for the formation of an external warped disk in this system. The detection of a potential second outflow could be the direct evidence of a binary system. Finally, we hypothesize that the bubbles may result from the magnetic pressure as observed in numerical simulations. △ Less

Submitted 27 March, 2024; originally announced March 2024.

Comments: 16 pages, 12 figures, accepted for publication in Astronomy & Astrophysics

Journal ref: A&A 687, A92 (2024)

H2CO and CS in diffuse clouds: Excitation and abundance

Authors: Maryvonne Gerin, Harvey Liszt, Jerome Pety, Alexandre Faure

Abstract: To provide constraints on the chemical processes responsible for the observed columns of organic species, we used NOEMA to observe the sight line toward NRAO150 in the 2mm spectral window. We targeted the low excitation lines of o-H2CO 2(1,1)-1(1,0) and p-H2CO 2(0,2)-1(0,1) as well as the nearby transitions of CS(3-2) and c-C3H2. We combined these data with previous observations to determine the e… ▽ More To provide constraints on the chemical processes responsible for the observed columns of organic species, we used NOEMA to observe the sight line toward NRAO150 in the 2mm spectral window. We targeted the low excitation lines of o-H2CO 2(1,1)-1(1,0) and p-H2CO 2(0,2)-1(0,1) as well as the nearby transitions of CS(3-2) and c-C3H2. We combined these data with previous observations to determine the excitation conditions, column densities, and abundances relative to H2 in the different velocity components. We performed non-LTE radiative transfer calculations including collision cross sections with ortho and para H2 and with electrons. New collision cross sections with electrons were computed for ortho and para formaldehyde. The c-C3H2 line profiles are very similar to those of HCO+ and CCH, while the CS absorption features are narrower and mostly concentrated in two main velocity components at V = -17 and -10 km/s. H2CO absorption lines present an intermediate pattern with absorption in all velocity components but larger opacities in the two main velocity components. The ortho-to-para ratios of H2CO and c-C3H2 are consistent with the statistical value of 3. While the excitation temperature of all c-C3H2 velocity components is consistent with the CMB, the two strong components detected in CS show a clear excess over the CMB indicating that CS resides at higher densities than other species along this particular sightline, n(H2) ~ 2500 cm-3 while n(H2) < 500 cm-3 for the other velocity components. We detected faint absorption from o-H213CO and C34S allowing us to derive isotopic ratios: o-H2CO/o-H213CO = 61 and C32S/C34S = 24. The excitation of the 4.8GHz line of formaldehyde is sensitive to the electron fraction and its excitation temperature is predicted to be lower than the CMB at low and moderate electron fractions, x(e)< 6E-5, and to rise above the CMB at high electron fractions, > 1e-4. △ Less

Submitted 11 March, 2024; originally announced March 2024.

Comments: 13 pages, abridged abstract

Journal ref: A&A 686, A49 (2024)

Protonated acetylene in the z=0.89 molecular absorber toward PKS1830-211

Authors: S. Muller, R. Le Gal, E. Roueff, J. H. Black, A. Faure, M. Guelin, A. Omont, M. Gerin, F. Combes, S. Aalto

Abstract: 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… ▽ More 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. △ Less

Submitted 18 January, 2024; originally announced January 2024.

Comments: Accepted for publication in A&A

Identification of Ammonium Salts on Comet 67P/C-G Surface from Infrared VIRTIS/Rosetta Data Based on Laboratory Experiments. Implications and Perspectives

Authors: Olivier Poch, Istiqomah Istiqomah, Eric Quirico, Pierre Beck, Bernard Schmitt, Patrice Theulé, Alexandre Faure, Pierre Hily-Blant, Lydie Bonal, Andrea Raponi, Mauro Ciarniello, Batiste Rousseau, Sandra Potin, Olivier Brissaud, Laurène Flandinet, Gianrico Filacchione, Antoine Pommerol, Nicolas Thomas, David Kappel, Vito Mennella, Lyuba Moroz, Vassilissa Vinogradoff, Gabriele Arnold, Stéphane Erard, Dominique Bockelée-Morvan , et al. (7 additional authors not shown)

Abstract: The nucleus of comet 67P/Churyumov-Gerasimenko exhibits a broad spectral reflectance feature around 3.2 $μ$m, which is omnipresent in all spectra of the surface, and whose attribution has remained elusive since its discovery. Based on laboratory experiments, we have shown that most of this absorption feature is due to ammonium (NH4+) salts mixed with the dark surface material. The depth of the ban… ▽ More The nucleus of comet 67P/Churyumov-Gerasimenko exhibits a broad spectral reflectance feature around 3.2 $μ$m, which is omnipresent in all spectra of the surface, and whose attribution has remained elusive since its discovery. Based on laboratory experiments, we have shown that most of this absorption feature is due to ammonium (NH4+) salts mixed with the dark surface material. The depth of the band is compatible with semi-volatile ammonium salts being a major reservoir of nitrogen in the comet, which could dominate over refractory organic matter and volatile species. These salts may thus represent the long-sought reservoir of nitrogen in comets, possibly bringing their nitrogen-to-carbon ratio in agreement with the solar value. Moreover, the reflectance spectra of several asteroids are compatible with the presence of NH4+ salts at their surfaces. The presence of such salts, and other NH4+-bearing compounds on asteroids, comets, and possibly in proto-stellar environments, suggests that NH4+ may be a tracer of the incorporation and transformation of nitrogen in ices, minerals and organics, at different phases of the formation of the Solar System. △ Less

Submitted 20 November, 2023; originally announced November 2023.

Journal ref: European Conference on Laboratory Astrophysics ECLA2020, Sep 2021, Anacapri, Italy. pp.271 - 279

Bending relaxation of H$_2$O by collision with para- and ortho-H$_2$

Authors: Ricardo Manuel Garcia-Vazquez, Alexandre Faure, Thierry Stoecklin

Abstract: We extend our recent theoretical work on the bending relaxation of H$_2$O in collisions with H$_2$ by including the three water modes of vibration coupled with rotation, as well as the rotation of H$_2$. Our full quantum close-coupling method (excluding the H$_2$ vibration) is combined with a high-accuracy nine-dimensional potential energy surface. The collisions of para-H$_2$O and ortho-H$_2$O wi… ▽ More We extend our recent theoretical work on the bending relaxation of H$_2$O in collisions with H$_2$ by including the three water modes of vibration coupled with rotation, as well as the rotation of H$_2$. Our full quantum close-coupling method (excluding the H$_2$ vibration) is combined with a high-accuracy nine-dimensional potential energy surface. The collisions of para-H$_2$O and ortho-H$_2$O with the two spin modifications of H$_2$ are considered and compared for several initial states of H$_2$O. The convergence of the results as a function of the size of the rotational basis set of the two colliders is discussed. In particular, near-resonant energy transfer between H$_2$O and H$_2$ is found to control the vibrational relaxation process, with a dominant contribution of transitions with delta j2 = +2, +4. Finally, the calculated value of the H$_2$O bending relaxation rate coefficient at 295 K is found to be in excellent agreement with its experimental estimate. △ Less

Submitted 15 November, 2023; originally announced November 2023.

Chasing Gravitational Waves with the Cherenkov Telescope Array

Authors: Jarred Gershon Green, Alessandro Carosi, Lara Nava, Barbara Patricelli, Fabian Schüssler, Monica Seglar-Arroyo, Cta Consortium, :, Kazuki Abe, Shotaro Abe, Atreya Acharyya, Remi Adam, Arnau Aguasca-Cabot, Ivan Agudo, Jorge Alfaro, Nuria Alvarez-Crespo, Rafael Alves Batista, Jean-Philippe Amans, Elena Amato, Filippo Ambrosino, Ekrem Oguzhan Angüner, Lucio Angelo Antonelli, Carla Aramo, Cornelia Arcaro, Luisa Arrabito , et al. (545 additional authors not shown)

Abstract: The detection of gravitational waves from a binary neutron star merger by Advanced LIGO and Advanced Virgo (GW170817), along with the discovery of the electromagnetic counterparts of this gravitational wave event, ushered in a new era of multimessenger astronomy, providing the first direct evidence that BNS mergers are progenitors of short gamma-ray bursts (GRBs). Such events may also produce very… ▽ More The detection of gravitational waves from a binary neutron star merger by Advanced LIGO and Advanced Virgo (GW170817), along with the discovery of the electromagnetic counterparts of this gravitational wave event, ushered in a new era of multimessenger astronomy, providing the first direct evidence that BNS mergers are progenitors of short gamma-ray bursts (GRBs). Such events may also produce very-high-energy (VHE, > 100GeV) photons which have yet to be detected in coincidence with a gravitational wave signal. The Cherenkov Telescope Array (CTA) is a next-generation VHE observatory which aims to be indispensable in this search, with an unparalleled sensitivity and ability to slew anywhere on the sky within a few tens of seconds. New observing modes and follow-up strategies are being developed for CTA to rapidly cover localization areas of gravitational wave events that are typically larger than the CTA field of view. This work will evaluate and provide estimations on the expected number of of gravitational wave events that will be observable with CTA, considering both on- and off-axis emission. In addition, we will present and discuss the prospects of potential follow-up strategies with CTA. △ Less

Submitted 5 February, 2024; v1 submitted 11 October, 2023; originally announced October 2023.

Comments: Presented at the 38th International Cosmic Ray Conference (ICRC 2023), 2023 (arXiv:2309.08219)

Report number: CTA-ICRC/2023/30

Prospects for $γ$-ray observations of the Perseus galaxy cluster with the Cherenkov Telescope Array

Authors: The Cherenkov Telescope Array Consortium, :, K. Abe, S. Abe, F. Acero, A. Acharyya, R. Adam, A. Aguasca-Cabot, I. Agudo, A. Aguirre-Santaella, J. Alfaro, R. Alfaro, N. Alvarez-Crespo, R. Alves Batista, J. -P. Amans, E. Amato, E. O. Angüner, L. A. Antonelli, C. Aramo, M. Araya, C. Arcaro, L. Arrabito, K. Asano, Y. Ascasíbar, J. Aschersleben , et al. (542 additional authors not shown)

Abstract: Galaxy clusters are expected to be dark matter (DM) reservoirs and storage rooms for the cosmic-ray protons (CRp) that accumulate along the cluster's formation history. Accordingly, they are excellent targets to search for signals of DM annihilation and decay at gamma-ray energies and are predicted to be sources of large-scale gamma-ray emission due to hadronic interactions in the intracluster med… ▽ More Galaxy clusters are expected to be dark matter (DM) reservoirs and storage rooms for the cosmic-ray protons (CRp) that accumulate along the cluster's formation history. Accordingly, they are excellent targets to search for signals of DM annihilation and decay at gamma-ray energies and are predicted to be sources of large-scale gamma-ray emission due to hadronic interactions in the intracluster medium. We estimate the sensitivity of the Cherenkov Telescope Array (CTA) to detect diffuse gamma-ray emission from the Perseus galaxy cluster. We perform a detailed spatial and spectral modelling of the expected signal for the DM and the CRp components. For each, we compute the expected CTA sensitivity. The observing strategy of Perseus is also discussed. In the absence of a diffuse signal (non-detection), CTA should constrain the CRp to thermal energy ratio within the radius $R_{500}$ down to about $X_{500}<3\times 10^{-3}$, for a spatial CRp distribution that follows the thermal gas and a CRp spectral index $α_{\rm CRp}=2.3$. Under the optimistic assumption of a pure hadronic origin of the Perseus radio mini-halo and depending on the assumed magnetic field profile, CTA should measure $α_{\rm CRp}$ down to about $Δα_{\rm CRp}\simeq 0.1$ and the CRp spatial distribution with 10% precision. Regarding DM, CTA should improve the current ground-based gamma-ray DM limits from clusters observations on the velocity-averaged annihilation cross-section by a factor of up to $\sim 5$, depending on the modelling of DM halo substructure. In the case of decay of DM particles, CTA will explore a new region of the parameter space, reaching models with $τ_χ>10^{27}$s for DM masses above 1 TeV. These constraints will provide unprecedented sensitivity to the physics of both CRp acceleration and transport at cluster scale and to TeV DM particle models, especially in the decay scenario. △ Less

Submitted 7 September, 2023; originally announced September 2023.

Comments: 93 pages (including author list, appendix and references), 143 figures. Submitted to JCAP

Journal ref: JCAP10(2024)004

Hyperfine collisional excitation of ammonia by molecular hydrogen

Authors: J. Loreau, A. Faure, F. Lique, S. Demes, P. J. Dagdigian

Abstract: Ammonia is one of the most widely observed molecules in space, and many observations are able to resolve the hyperfine structure due to the electric quadrupole moment of the nitrogen nucleus. The observed spectra often display anomalies in the satellite components of the lines, which indicate substantial deviations from the local thermodynamic equilibrium. The interpretation of the spectra thus re… ▽ More Ammonia is one of the most widely observed molecules in space, and many observations are able to resolve the hyperfine structure due to the electric quadrupole moment of the nitrogen nucleus. The observed spectra often display anomalies in the satellite components of the lines, which indicate substantial deviations from the local thermodynamic equilibrium. The interpretation of the spectra thus requires the knowledge of the rate coefficients for the hyperfine excitation of NH$_3$ induced by collisions with H$_2$ molecules, the dominant collider in the cold interstellar medium. In this paper we present the first such calculations using a recoupling approach. The rate coefficients are obtained for all hyperfine levels within rotation-inversion levels up to $j=4$ and temperatures up to 100 K by means of quantum scattering close-coupling calculations on an accurate, five-dimensional, potential energy surface. We show that the rate coefficients depart significantly from those obtained with the statistical approach and that they do not conform to any simple propensity rules. Finally, we perform radiative transfer calculations to illustrate the impact of our new rate coefficients by modelling the hyperfine line intensities of the inversion transition in ground state para-NH$_3$ ($j_k=1_1$) and of the rotational transition $1_0\rightarrow 0_0$ in ortho-NH$_3$. △ Less

Submitted 4 September, 2023; originally announced September 2023.

An accurate set of H$_3$O$^+ -$ H$_2$ collisional rate coefficients for non-LTE modelling of warm interstellar clouds

Authors: Sándor Demes, François Lique, Alexandre Faure, Floris F. S. van der Tak

Abstract: Hydronium (H$_3$O$^+$) was first detected in 1986 in interstellar molecular clouds. It was reported in many galactic diffuse and dense regions, as well as in extragalactic sources. H$_3$O$^+$ plays a major role both in interstellar oxygen and water chemistry. However, despite the large number of H$_3$O$^+$ observations, its collisional excitation was investigated only partially. In the present wor… ▽ More Hydronium (H$_3$O$^+$) was first detected in 1986 in interstellar molecular clouds. It was reported in many galactic diffuse and dense regions, as well as in extragalactic sources. H$_3$O$^+$ plays a major role both in interstellar oxygen and water chemistry. However, despite the large number of H$_3$O$^+$ observations, its collisional excitation was investigated only partially. In the present work we study the state-to-state rotational de-excitation of $ortho$- and $para$-H$_3$O$^+$ in collisions both with $ortho$- and $para$-H$_2$. The cross sections are calculated within the close-coupling formalism using a highly accurate potential energy surface developed for this system. The rate coefficients are computed up to $300$ K kinetic temperature. Transitions between the lowest 21 rotation-inversion states were studied for $para$-H$_3$O$^+$, and the lowest 11 states for $ortho$-H$_3$O$^+$, i.e. all levels with rotational energies below 430 K ($\sim 300$ cm$^{-1}$) are considered (up to $j\leq5$). In order to estimate the impact of the new rate coefficients on the astrophysical models for H$_3$O$^+$, radiative transfer calculations were also carried out. We have examined how the new collisional data affect the line intensities with respect to older data previously used for the interpretation of observations. By analysing all detected transitions we find that our new, accurate rate coefficients have a significant impact (typically within a factor of 2) on radiation temperatures, allowing more accurate estimation of column densities and relative abundances of hydronium, especially in warm molecular clouds, paving the path towards better interpretation of interstellar water and oxygen chemistry. △ Less

Submitted 4 November, 2022; originally announced November 2022.

Comments: 13 pages, 7 figures

MSC Class: 85-10; 85-08

The effect of water and electron collisions in the rotational excitation of HF in comets

Authors: J. Loreau, A. Faure, F. Lique

Abstract: We present the first set of rate coefficients for the rotational excitation of the 7 lowest levels of hydrogen fluoride (HF) induced by collision with water molecules, the dominant collider in cometary comas, in the 5-150 K temperature range. The calculations are performed with a quantum statistical approach from an accurate rigid rotor ab initio interaction potential. Rate coefficients for excita… ▽ More We present the first set of rate coefficients for the rotational excitation of the 7 lowest levels of hydrogen fluoride (HF) induced by collision with water molecules, the dominant collider in cometary comas, in the 5-150 K temperature range. The calculations are performed with a quantum statistical approach from an accurate rigid rotor ab initio interaction potential. Rate coefficients for excitation of HF by electron-impact are also computed, within the Born approximation, in the 10-10,000 K temperature range. These rate coefficients are then used in a simplified non-local thermodynamic equilibrium (non-LTE) model of a cometary coma that also includes solar radiative pumping and radiative decay. We investigate the range of H2O densities that lead to non-LTE populations of the rotational levels of HF. We show that to describe the excitation of HF in comets, considering collisions with both water molecules and electrons is needed as a result of the large dipole of HF. △ Less

Submitted 21 October, 2022; originally announced October 2022.

Comments: 8 pages

Journal ref: MNRAS 516 5964 (2022)

Absolute measurements of state-to-state rotational energy transfer between CO and H2 at interstellar temperatures

Authors: H. Labiad, M. Fournier, L. A. Mertens, A. Faure, D. Carty, T. Stoecklin, P. Jankowski, K. Szalewicz, S. D. Le Picard, I. R. Sims

Abstract: Experimental measurements and theoretical calculations of state-to-state rate coefficients for rotational energy transfer of CO in collision with H$_2$ are reported at the very low temperatures prevailing in dense interstellar clouds (5 - 20 K). Detailed agreement between quantum state-selected experiments performed in cold supersonic flows using time-resolved infrared - vacuum-ultraviolet double… ▽ More Experimental measurements and theoretical calculations of state-to-state rate coefficients for rotational energy transfer of CO in collision with H$_2$ are reported at the very low temperatures prevailing in dense interstellar clouds (5 - 20 K). Detailed agreement between quantum state-selected experiments performed in cold supersonic flows using time-resolved infrared - vacuum-ultraviolet double resonance spectroscopy and close-coupling quantum scattering calculations confirms the validity of the calculations for collisions between the two most abundant molecules in the interstellar medium. △ Less

Submitted 1 February, 2022; originally announced February 2022.

Comments: 6 pages, 4 figures, accepted for publication in Phys. Rev. A. Letter (21/12/2021)

The ortho-to-para ratio of water in interstellar clouds

Authors: A. Faure, P. Hily-Blant, C. Rist, G. Pineau des Forêts, A. Matthews, D. R. Flower

Abstract: The nuclear-spin chemistry of interstellar water is investigated using the University of Grenoble Alpes Astrochemical Network (UGAN). This network includes reactions involving the different nuclear-spin states of the hydrides of carbon, nitrogen, oxygen and sulphur, as well as their deuterated forms. Nuclear-spin selection rules are implemented within the scrambling hypothesis for reactions involv… ▽ More The nuclear-spin chemistry of interstellar water is investigated using the University of Grenoble Alpes Astrochemical Network (UGAN). This network includes reactions involving the different nuclear-spin states of the hydrides of carbon, nitrogen, oxygen and sulphur, as well as their deuterated forms. Nuclear-spin selection rules are implemented within the scrambling hypothesis for reactions involving up to seven protons. The abundances and ortho-to-para ratios (OPRs) of gas-phase water and water ions (H$_2$O$^+$ and H$_3$O$^+$) are computed under the steady-state conditions representative of a dark molecular cloud and during the early phase of gravitational collapse of a prestellar core. The model incorporates the freezing of the molecules on to grains, simple grain surface chemistry and cosmic-ray induced and direct desorption of ices. The predicted OPRs are found to deviate significantly from both thermal and statistical values and to be independent of temperature below $\sim $30~K. The OPR of H$_2$O is shown to lie between 1.5 and 2.6, depending on the spin-state of H$_2$, in good agreement with values derived in translucent clouds with relatively high extinction. In the prestellar core collapse calculations, the OPR of H$_2$O is shown to reach the statistical value of 3 in regions with severe depletion ($n_{\rm H}> 10^7$~cm$^{-3}$). We conclude that a low water OPR ($\lesssim 2.5$) is consistent with gas-phase ion-neutral chemistry and reflects a gas with OPR(H$_2)\lesssim 1$. Available OPR measurements in protoplanetary disks and comets are finally discussed. △ Less

Submitted 6 January, 2022; originally announced January 2022.

Comments: 14 pages, 7 figures, accepted for publication in MNRAS on 29 May 2019

Journal ref: Monthly Notices of the Royal Astronomical Society, Volume 487, Issue 3, p.3392-3403 (2019)

Collisional excitation and non-LTE modelling of interstellar chiral propylene oxide

Authors: K. Dzenis, A. Faure, B. A. McGuire, A. J. Remijan, P. J. Dagdigian, C. Rist, R. Dawes, E. Quintas-Sanchez, F. Lique, M. Hochlaf

Abstract: The first set of theoretical cross sections for propylene oxide (CH3CHCH2O) colliding with cold He atoms has been obtained at the full quantum level using a high-accuracy potential energy surface. By scaling the collision reduced mass, rotational rate coefficients for collisions with para-H2 are deduced in the temperature range 5-30 K. These collisional coefficients are combined with radiative dat… ▽ More The first set of theoretical cross sections for propylene oxide (CH3CHCH2O) colliding with cold He atoms has been obtained at the full quantum level using a high-accuracy potential energy surface. By scaling the collision reduced mass, rotational rate coefficients for collisions with para-H2 are deduced in the temperature range 5-30 K. These collisional coefficients are combined with radiative data in a non-LTE radiative transfer model in order to reproduce observations of propylene oxide made towards the Sagittarius B2(N) molecular cloud with the Green Bank and Parkes radio telescopes. The three detected absorption lines are found to probe the cold (~ 10 K) and translucent (nH ~ 2000 cm-3) gas in the outer edges of the extended Sgr B2(N) envelope. The derived column density for propylene oxide is Ntot ~ 3e12 cm-2, corresponding to a fractional abundance relative to total hydrogen of ~ 2.5e-11. The present results are expected to help our understanding of the chemistry of propylene oxide, including a potential enantiomeric excess, in the cold interstellar medium. △ Less

Submitted 5 February, 2022; v1 submitted 16 December, 2021; originally announced December 2021.

Comments: 9 pages, 5 figures, accepted for publication in ApJ (2021/12/14)

Sulfur gas-phase abundance in dense cores

Authors: P. Hily-Blant, G. Pineau des Forêts, A. Faure, F. Lique

Abstract: The abundance of volatile sulfur in dense clouds is long-standing problem in studies of the physics and chemistry of star-forming regions. Sulfur is an important species because its low ionization potential may possibly make it an important charge carrier. The observed sulfur-bearing species in the gas-phase of dense clouds represent only a minor fraction of the cosmic sulfur abundance, which has… ▽ More The abundance of volatile sulfur in dense clouds is long-standing problem in studies of the physics and chemistry of star-forming regions. Sulfur is an important species because its low ionization potential may possibly make it an important charge carrier. The observed sulfur-bearing species in the gas-phase of dense clouds represent only a minor fraction of the cosmic sulfur abundance, which has been interpreted as a signature of sulfur depletion into ices at the surface of dust grains. However, atomic sulfur, which could be the main gas-phase carrier, cannot be observed directly in cold cores. We present measurements of the nitrogen sulfide (NS) radical toward four dense cores performed with the IRAM-30m telescope. Analytical chemical considerations and chemical models over a wide parameter space show that the NS:N2H+ abundance ratio provides a direct constraint on the abundance of gas-phase atomic sulfur. Toward early-type cores, we find that $n(\rm S)/n_{\rm H}$ is close, or even equal, to the cosmic abundance of sulfur, 14$\times 10^{-6}$, demonstrating that sulfur is not depleted and is atomic, which is in agreement with chemical models. More chemically evolved cores show sulfur depletion by factors up to 100 in their densest parts. In L1544, atomic sulfur depletion is shown to increase with increasing density. Future observations are needed to discover the solid-phase carrier of sulfur. The initial steps of the collapse of pre-stellar cores in the high sulfur abundance regime also need to be explored from their chemical and dynamical perspectives. △ Less

Submitted 5 December, 2021; v1 submitted 2 December, 2021; originally announced December 2021.

Comments: Accepted for publication in A&A

Journal ref: A&A 658, A168 (2022)

Rotational excitation of H$_3$O$^+$ cations by para-H$_2$: improved collisional data at low temperatures

Authors: S. Demes, François Lique, Alexandre Faure, Floris van der Tak, Claire Rist, Pierre Hily-Blant

Abstract: The hydronium cation plays a crucial role in interstellar oxygen and water chemistry. While its spectroscopy was extensively investigated earlier, the collisional excitation of H$_3$O$^+$ is not well studied yet. In this work we present state-to-state collisional data for rotational de-excitation of both ortho- and para-H$_3$O$^+$ due to para-H$_2$ impact. The cross sections are calculated within… ▽ More The hydronium cation plays a crucial role in interstellar oxygen and water chemistry. While its spectroscopy was extensively investigated earlier, the collisional excitation of H$_3$O$^+$ is not well studied yet. In this work we present state-to-state collisional data for rotational de-excitation of both ortho- and para-H$_3$O$^+$ due to para-H$_2$ impact. The cross sections are calculated within the close-coupling formalism using our recent, highly accurate rigid-rotor potential energy surface for this collision system. The corresponding thermal rate coefficients are calculated up to 100 K. For para-H$_3$O$^+$ the lowest 20 rotation-inversion states were considered in the calculations, while for ortho-H$_3$O$^+$ the lowest 11 states are involved (up to $j\leq5$), so all levels with rotational energy below 420 K (292 cm$^{-1}$) are studied. In order to analyse the impact of the new collisional rates on the excitation of H$_3$O$^+$ in astrophysical environments radiative transfer calculations are also provided. The most relevant emission lines from an astrophysical point of view are studied, taking into account the transitions at 307, 365, 389 and 396 GHz. We show that our new collisional data have a non-negligible impact (from a few percents up to about a factor of 3) on the brightness and excitation temperatures of H$_3$O$^+$, justifying the revision of the physical conditions in the appropriate astrophysical observations. The calculated rate coefficients allow one to recalculate the column density of hydronium in interstellar clouds, which can lead to a better understanding of interstellar water and oxygen chemistry. △ Less

Submitted 2 November, 2021; originally announced November 2021.

Journal ref: Monthly Notices of the Royal Astronomical Society, Oxford University Press (OUP): Policy P - Oxford Open Option A, 2021

Observations and analysis of CH$^+$ vibrational emissions from the young, carbon-rich planetary nebula NGC 7027: a textbook example of chemical pumping

Authors: David A. Neufeld, Benjamin Godard, P. Bryan Changala, Alexandre Faure, T. R. Geballe, Rolf Güsten, Karl M. Menten, Helmut Wiesemeyer

Abstract: We discuss the detection of 14 rovibrational lines of CH$^+$, obtained with the iSHELL spectrograph on NASA's Infrared Telescope Facility (IRTF) on Maunakea. Our observations in the 3.49 - 4.13 $μ$m spectral region, obtained with a 0.375" slit width that provided a spectral resolving power $λ/Δλ\sim 80,000$, have resulted in the unequivocal detection of the $R(0) - R(3)$ and $P(1)-P(10)$ transitio… ▽ More We discuss the detection of 14 rovibrational lines of CH$^+$, obtained with the iSHELL spectrograph on NASA's Infrared Telescope Facility (IRTF) on Maunakea. Our observations in the 3.49 - 4.13 $μ$m spectral region, obtained with a 0.375" slit width that provided a spectral resolving power $λ/Δλ\sim 80,000$, have resulted in the unequivocal detection of the $R(0) - R(3)$ and $P(1)-P(10)$ transitions within the $v=1-0$ band of CH$^+$. The $R$-branch transitions are anomalously weak relative to the $P$-branch transitions, a behavior that is explained accurately by rovibronic calculations of the transition dipole moment reported in a companion paper (Changala et al. 2021). Nine infrared transitions of H$_2$ were also detected in these observations, comprising the $S(8)$, $S(9)$, $S(13)$ and $S(15)$ pure rotational lines; the $v=1-0$ $O(4) - O(7)$ lines, and the $v=2-1$ $O(5)$ line. We present a photodissociation region model, constrained by the CH$^+$ and H$_2$ line fluxes that we measured, that includes a detailed treatment of the excitation of CH$^+$ by inelastic collisions, optical pumping, and chemical ("formation") pumping. The latter process is found to dominate the excitation of the observed rovibrational lines of CH$^+$, and the model is remarkably successful in explaining both the absolute and relative strengths of the CH$^+$ and H$_2$ lines. △ Less

Submitted 28 May, 2021; originally announced May 2021.

Comments: 38 pages, 15 figures, accepted for publication in the Astrophysical Journal

Deuterium fractionation of nitrogen hydrides: detections of NHD and ND$_2$

Authors: A. Bacmann, A. Faure, P. Hily-Blant, K. Kobayashi, H. Ozeki, S. Yamamoto, L. Pagani, F. Lique

Abstract: Although ammonia is an abundant molecule commonly observed towards the dense interstellar medium, it has not yet been established whether its main formation route is from gas-phase ion-molecule reactions or grain-surface hydrogen additions on adsorbed nitrogen atoms. Deuterium fractionation can be used as a tool to constrain formation mechanisms. High abundances of deuterated molecules are routine… ▽ More Although ammonia is an abundant molecule commonly observed towards the dense interstellar medium, it has not yet been established whether its main formation route is from gas-phase ion-molecule reactions or grain-surface hydrogen additions on adsorbed nitrogen atoms. Deuterium fractionation can be used as a tool to constrain formation mechanisms. High abundances of deuterated molecules are routinely observed in the dense interstellar medium, with the ratio between deuterated molecules and the main isotopologue enhanced by several orders of magnitude with respect to the elemental D/H ratio. In the case of ammonia, the detection of its triply deuterated isotopologue hints at high abundances of the deuterated intermediate nitrogen radicals, ND, NHD and ND$_2$. So far however, only ND has been detected in the interstellar medium. In this paper, to constrain the formation of ammonia, we aim at determining the NHD/NH$_2$ and ND$_2$/NHD abundance ratios, and compare them with the predictions of both pure gas-phase and grain-surface chemical models. We searched for the fundamental rotational transitions of NHD and ND$_2$ towards the class 0 protostar IRAS16293-2422, towards which NH, NH$_2$ and ND had been previously detected. Both NHD and ND$_2$ are detected in absorption towards the source. The relative abundance ratios NH$_2$ : NHD : ND$_2$ are close to 8 : 4 : 1. These ratios can be reproduced by our gas-phase chemical model within a factor of two-three. Statistical ratios as expected from grain-surface chemistry are also consistent with our data. Further investigations of the ortho-to-para ratio in ND$_2$ , both theoretical and observational, could bring new constraints to better understand nitrogen hydride chemistry. △ Less

Submitted 25 September, 2020; originally announced September 2020.

Comments: 10 pages, 5 figures, accepted for publication in MNRAS

Depletion and fractionation of nitrogen in collapsing cores

Authors: P. Hily-Blant, G. Pineau des Forêts, A. Faure, D. R. Flower

Abstract: Measurements of the nitrogen isotopic ratio in Solar System comets show a constant value, ~140, which is three times lower than the protosolar ratio, a highly significant difference that remains unexplained. Observations of static starless cores at early stages of collapse confirm the theoretical expectation that nitrogen fractionation in interstellar conditions is marginal for most species. Yet,… ▽ More Measurements of the nitrogen isotopic ratio in Solar System comets show a constant value, ~140, which is three times lower than the protosolar ratio, a highly significant difference that remains unexplained. Observations of static starless cores at early stages of collapse confirm the theoretical expectation that nitrogen fractionation in interstellar conditions is marginal for most species. Yet, observed isotopic ratios in N2H+ are at variance with model predictions. These gaps in our understanding of how the isotopic reservoirs of nitrogen evolve, from interstellar clouds to comets, and, more generally, to protosolar nebulae, may have their origin in missing processes or misconceptions in the chemistry of interstellar nitrogen. So far, theoretical studies of nitrogen fractionation in starless cores have addressed the quasi-static phase of their evolution such that the effect of dynamical collapse on the isotopic ratio is not known. In this paper, we investigate the fractionation of 14N and 15N during the gravitational collapse of a pre-stellar core through gas-phase and grain adsorption and desorption reactions. The initial chemical conditions, which are obtained in steady state after typically a few Myr, show low degrees of fractionation in the gas phase, in agreement with earlier studies. However, during collapse, the differential rate of adsorption of 14N- and 15N-containing species onto grains results in enhanced 15N:14N ratios, in better agreement with the observations. Furthermore, we find differences in the behavior, with increasing density, of the isotopic ratio in different species. We find that the collapse must take place on approximately one free-fall timescale, based on the CO abundance profile in L183 [see the end in the PDF file] △ Less

Submitted 14 September, 2020; originally announced September 2020.

Comments: Accepted for publication in A&A

Journal ref: A&A 643, A76 (2020)

The Leiden Atomic and Molecular Database (LAMDA): Current Status, Recent Updates, and Future Plans

Authors: Floris van der Tak, François Lique, Alex Faure, John Black, Ewine van Dishoeck

Abstract: The Leiden Atomic and Molecular Database (LAMDA) collects spectroscopic information and collisional rate coefficients for molecules, atoms, and ions of astrophysical and astrochemical interest. We describe the developments of the database since its inception in 2005, and outline our plans for the near future. Such a database is constrained both by the nature of its uses and by the availability of… ▽ More The Leiden Atomic and Molecular Database (LAMDA) collects spectroscopic information and collisional rate coefficients for molecules, atoms, and ions of astrophysical and astrochemical interest. We describe the developments of the database since its inception in 2005, and outline our plans for the near future. Such a database is constrained both by the nature of its uses and by the availability of accurate data: we suggest ways to improve the synergies among users and suppliers of data. We summarize some recent developments in computation of collisional cross sections and rate coefficients. We consider atomic and molecular data that are needed to support astrophysics and astrochemistry with upcoming instruments that operate in the mid- and far-infrared parts of the spectrum. △ Less

Submitted 23 April, 2020; originally announced April 2020.

Comments: Accepted by Atoms for the special issue on Atomic and molecular databases; 26 pages, 6 tables

Ammonium salts are a reservoir of nitrogen on a cometary nucleus and possibly on some asteroids

Authors: O. Poch, I. Istiqomah, E. Quirico, P. Beck, B. Schmitt, P. Theulé, A. Faure, P. Hily-Blant, L. Bonal, A. Raponi, M. Ciarniello, B. Rousseau, S. Potin, O. Brissaud, L. Flandinet, G. Filacchione, A. Pommerol, N. Thomas, D. Kappel, V. Mennella, L. Moroz, V. Vinogradoff, G. Arnold, S. Erard, D. Bockelée-Morvan , et al. (7 additional authors not shown)

Abstract: The measured nitrogen-to-carbon ratio in comets is lower than for the Sun, a discrepancy which could be alleviated if there is an unknown reservoir of nitrogen in comets. The nucleus of comet 67P/Churyumov-Gerasimenko exhibits an unidentified broad spectral reflectance feature around 3.2 micrometers, which is ubiquitous across its surface. On the basis of laboratory experiments, we attribute this… ▽ More The measured nitrogen-to-carbon ratio in comets is lower than for the Sun, a discrepancy which could be alleviated if there is an unknown reservoir of nitrogen in comets. The nucleus of comet 67P/Churyumov-Gerasimenko exhibits an unidentified broad spectral reflectance feature around 3.2 micrometers, which is ubiquitous across its surface. On the basis of laboratory experiments, we attribute this absorption band to ammonium salts mixed with dust on the surface. The depth of the band indicates that semivolatile ammonium salts are a substantial reservoir of nitrogen in the comet, potentially dominating over refractory organic matter and more volatile species. Similar absorption features appear in the spectra of some asteroids, implying a compositional link between asteroids, comets, and the parent interstellar cloud. △ Less

Submitted 12 March, 2020; originally announced March 2020.

Comments: Main manuscript and Supplementary material document, Accepted for publication in Science on February 14, 2020

Journal ref: Science 367, (2020) eaaw7462

The effect of CO-H2O collisions in the rotational excitation of cometary CO

Authors: A. Faure, F. Lique, J. Loreau

Abstract: We present the first accurate rate coefficients for the rotational excitation of CO by H2O in the kinetic temperature range 5-100 K. The statistical adiabatic channel method (SACM) is combined with a high-level rigid-rotor CO-H2O intermolecular potential surface. Transitions among the first 11 rotational levels of CO and the first 8 rotational levels of both para-H2O and ortho-H2O are considered.… ▽ More We present the first accurate rate coefficients for the rotational excitation of CO by H2O in the kinetic temperature range 5-100 K. The statistical adiabatic channel method (SACM) is combined with a high-level rigid-rotor CO-H2O intermolecular potential surface. Transitions among the first 11 rotational levels of CO and the first 8 rotational levels of both para-H2O and ortho-H2O are considered. Our rate coefficients are compared to previous data from the literature and they are also incorporated in a simple non-LTE model of cometary coma including collision-induced transitions, solar radiative pumping and radiative decay. We find that the uncertainties in the collision data have significant influence on the CO population distribution for H2O densities in the range 10^3-10^8 cm^-3. We also show that the rotational distribution of H2O plays an important role in CO excitation (owing to correlated energy transfer in both CO and H2O), while the impact of the ortho-to-para ratio of H2O is found to be negligible. △ Less

Submitted 12 February, 2020; v1 submitted 24 January, 2020; originally announced January 2020.

Comments: 8 pages, 7 figures, accepted for publication in MNRAS (January 20 2020)

Cyanoacetylene in the outflow/hot molecular core G331.512-0.103

Authors: N. U. Duronea, L. Bronfman, E. Mendoza, M. Merello, R. Finger, N. Reyes, C. Hervías-Caimapo, A. Faure, C. E. Cappa, E. M. Arnal, J. R. D. Lépine, I. Kleiner, L-Ä Nyman

Abstract: Using APEX-1 and APEX-2 observations, we have detected and studied the rotational lines of the HC$_3$N molecule (cyanoacetylene) in the powerful outflow/hot molecular core G331.512-0.103. We identified thirty-one rotational lines at $J$ levels between 24 and 39; seventeen of them in the ground vibrational state $v$=0 (9 lines corresponding to the main C isotopologue and 8 lines corresponding to th… ▽ More Using APEX-1 and APEX-2 observations, we have detected and studied the rotational lines of the HC$_3$N molecule (cyanoacetylene) in the powerful outflow/hot molecular core G331.512-0.103. We identified thirty-one rotational lines at $J$ levels between 24 and 39; seventeen of them in the ground vibrational state $v$=0 (9 lines corresponding to the main C isotopologue and 8 lines corresponding to the $^{13}$C isotopologues), and fourteen in the lowest vibrationally excited state $v_7$=1. Using LTE-based population diagrams for the beam-diluted $v$=0 transitions, we determined $T_{\rm exc}$=85$\pm$4 K and $N$(HC$_3$N)=(6.9$\pm$0.8)$\times$10$^{14}$ cm$^{-2}$, while for the beam-diluted $v_7$=1 transitions we obtained $T_{\rm exc}$=89$\pm$10 K and $N$(HC$_3$N)=2$\pm$1$\times$10$^{15}$ cm$^{-2}$. Non-LTE calculations using H$_2$ collision rates indicate that the HC$_3$N emission is in good agreement with LTE-based results. From the non-LTE method we estimated $T_{\rm kin}$ $\simeq$90~K, $n$(H$_2$)$\simeq$2$\times$10$^7$~cm$^{-3}$ for a central core of 6 arcsec in size. A vibrational temperature in the range from 130~K to 145~K was also determined, values which are very likely lower limits. Our results suggest that rotational transitions are thermalized, while IR radiative pumping processes are probably more efficient than collisions in exciting the molecule to the vibrationally excited state $v_7$=1. Abundance ratios derived under LTE conditions for the $^{13}$C isotopologues suggest that the main formation pathway of HC$_3$N is ${\rm C}_2{\rm H}_2 + {\rm CN} \rightarrow {\rm HC}_3{\rm N} + {\rm H}$. △ Less

Submitted 22 July, 2019; originally announced July 2019.

Comments: 15 pages, 8 figures, 2 tables. Accepted for publication in MNRAS

Deuterated methanol toward NGC 7538-IRS1

Authors: Juan Ospina-Zamudio, Cécile Favre, Marina Kounkel, Li-Hong Xu, Justin Neill, Bertrand Lefloch, Alexandre Faure, Edwin Bergin, Davide Fedele, Lee Hartmann

Abstract: We investigate the deuteration of methanol towards the high-mass star forming region NGC 7538-IRS1. We have carried out a multi-transition study of CH$_3$OH, $^{13}$CH$_3$OH and of the deuterated fllavors, CH$_2$DOH and CH$_3$OD, between 1.0--1.4 mm with the IRAM-30~m antenna. In total, 34 $^{13}$CH$_3$OH, 13 CH$_2$DOH lines and 20 CH$_3$OD lines spanning a wide range of upper-state energies (E… ▽ More We investigate the deuteration of methanol towards the high-mass star forming region NGC 7538-IRS1. We have carried out a multi-transition study of CH$_3$OH, $^{13}$CH$_3$OH and of the deuterated fllavors, CH$_2$DOH and CH$_3$OD, between 1.0--1.4 mm with the IRAM-30~m antenna. In total, 34 $^{13}$CH$_3$OH, 13 CH$_2$DOH lines and 20 CH$_3$OD lines spanning a wide range of upper-state energies (E$_{up}$) were detected. From the detected transitions, we estimate that the measured D/H does not exceed 1$\%$, with a measured CH$_2$DOH/CH$_3$OH and CH$_3$OD/CH$_3$OH of about (32$\pm$8)$\times$10$^{-4}$ and (10$\pm$4)$\times$10$^{-4}$, respectively. This finding is consistent with the hypothesis of a short-time scale formation during the pre-stellar phase. We find a relative abundance ratio CH$_2$DOH/CH$_3$OD of 3.2 $\pm$ 1.5. This result is consistent with a statistical deuteration. We cannot exclude H/D exchanges between water and methanol if water deuteration is of the order 0.1$\%$, as suggested by recent Herschel observations. △ Less

Submitted 23 May, 2019; originally announced May 2019.

Comments: Accepted for publication in Astronomy & Astrophysics

Journal ref: A&A 627, A80 (2019)

Efficient methanol production on the dark side of a prestellar core

Authors: Jorma Harju, Jaime E. Pineda, Anton I. Vasyunin, Paola Caselli, Stella S. R. Offner, Alyssa A. Goodman, Mika Juvela, Olli Sipilae, Alexandre Faure, Romane Le Gal, Pierre Hily-Blant, Joao Alves, Luca Bizzocchi, Andreas Burkert, Hope Chen, Rachel K. Friesen, Rolf Guesten, Philip C. Myers, Anna Punanova, Claire Rist, Erik Rosolowsky, Stephan Schlemmer, Yancy Shirley, Silvia Spezzano, Charlotte Vastel , et al. (1 additional authors not shown)

Abstract: We present ALMA maps of the starless molecular cloud core Ophiuchus/H-MM1 in the lines of deuterated ammonia (ortho-NH2D), methanol (CH3OH), and sulphur monoxide (SO). The dense core is seen in NH2D emission, whereas the CH3OH and SO distributions form a halo surrounding the core. Because methanol is formed on grain surfaces, its emission highlights regions where desorption from grains is particul… ▽ More We present ALMA maps of the starless molecular cloud core Ophiuchus/H-MM1 in the lines of deuterated ammonia (ortho-NH2D), methanol (CH3OH), and sulphur monoxide (SO). The dense core is seen in NH2D emission, whereas the CH3OH and SO distributions form a halo surrounding the core. Because methanol is formed on grain surfaces, its emission highlights regions where desorption from grains is particularly efficient. Methanol and sulphur monoxide are most abundant in a narrow zone that follows the eastern side of the core. This side is sheltered from the stronger external radiation field coming from the west. We show that photodissociation on the illuminated side can give rise to an asymmetric methanol distribution, but that the stark contrast observed in H-MM1 is hard to explain without assuming enhanced desorption on the shaded side. The region of the brightest emission has a wavy structure that rolls up at one end. This is the signature of Kelvin-Helmholtz instability occurring in sheared flows. We suggest that in this zone, methanol and sulphur are released as a result of grain-grain collisions induced by shear vorticity. △ Less

Submitted 4 May, 2020; v1 submitted 27 March, 2019; originally announced March 2019.

Comments: Accepted for publication in the Astrophysical Journal

The Dense Gas Fraction in Galactic Center Clouds

Authors: E. A. C. Mills, A. Ginsburg, K. Immer, J. M Barnes, L. Wiesenfeld, A. Faure, M. R. Morris, M. A. Requena-Torres

Abstract: We present an analysis of gas densities in the central R=300 parsecs of the Milky Way, focusing on three clouds: GCM-0.02-0.07 (the 50 km/s cloud), GCM-0.13-0.08 (the 20 km/s cloud), and GCM0.25+0.01 (the "Brick"). Densities are determined using observations of the J=(3-2), (4-3), (5-4), (10-9), (18-17), (19-18), (21-20), and (24-23) transitions of the molecule HC3N. We find evidence of at least t… ▽ More We present an analysis of gas densities in the central R=300 parsecs of the Milky Way, focusing on three clouds: GCM-0.02-0.07 (the 50 km/s cloud), GCM-0.13-0.08 (the 20 km/s cloud), and GCM0.25+0.01 (the "Brick"). Densities are determined using observations of the J=(3-2), (4-3), (5-4), (10-9), (18-17), (19-18), (21-20), and (24-23) transitions of the molecule HC3N. We find evidence of at least two excitation regimes for HC3N and constrain the low-excitation component to have a density less than 10^4 cm^-3 and the high-excitation component to have a density between 10^5 and 10^6 cm^-3. This is much less than densities of 10^7 cm^-3 that are found in Sgr B2, the most actively star-forming cloud in the Galactic center. This is consistent with the requirement of a higher density threshold for star formation in the Galactic center than is typical in the Galactic disk. We are also able to constrain the column density of each component in order to determine the mass fraction of 'dense' (n>10^5 cm^-3) gas for these clouds. We find that this is ~15% for all three clouds. Applying the results of our models to ratios of the (10-9) and (3-2) line across the entire central R=300 pc, we find that the fraction of gas with n>10^4 cm^-3 increases inward of a radius of ~140 pc, consistent with the predictions of recent models for the gas dynamics in this region. Our observations show that HC3N is an excellent molecule for probing the density structure of clouds in the Galactic center. △ Less

Submitted 29 September, 2018; originally announced October 2018.

Comments: 20 pages, 8 figures, 3 tables. ApJ accepted

The nitrogen isotopic ratio of HC3N towards the L1544 prestellar core

Authors: P. Hily-Blant, A. Faure, C. Vastel, V. Magalhaes, B. Lefloch, R. Bachiller

Abstract: The origin of the heavily fractionated reservoir of nitrogen in comets remains an issue in the theory of their formation and hence of the solar system. Whether the fractionated reservoir traced by comets is inherited from the interstellar cloud or is the product of processes taking place in the protostar, or in the protoplanetary disk, remains unclear. So far, observations of nitrogen isotopic rat… ▽ More The origin of the heavily fractionated reservoir of nitrogen in comets remains an issue in the theory of their formation and hence of the solar system. Whether the fractionated reservoir traced by comets is inherited from the interstellar cloud or is the product of processes taking place in the protostar, or in the protoplanetary disk, remains unclear. So far, observations of nitrogen isotopic ratios in protostars or prestellar cores have not securely identified such a fractionated reservoir owing to the intrinsic difficulty of direct isotopic ratios measurements. In this article, we report the detection of 5 rotational lines of \ce{HC3N}, {including the weaker components of the hyperfine multiplets}, and two rotational lines of its $^{15}$N isotopologue, towards the L1544 prestellar core. Based on a MCMC/non-LTE multi-line analysis at the hyperfine level, we derive the column densities of \ce{HC3N} ($8.0\pm0.4\tdix{13}$\cc) and \ce{HC3^{15}N} ($2.0\pm0.4\tdix{11}$\cc) and derive an isotopic ratio of 400$\pm$20(1$σ$). This value suggests that \ce{HC3N} is slightly depleted in $^{15}$N in L1544 with respect to the elemental $^{14}$N/$^{15}$N ratio {of $\approx$330} in the present-day local interstellar medium. Our study also stresses the need for radiative calculations at the hyperfine level. Finally, the comparison of the derived ratio with those obtained in CN and HCN in the same core seems to favor CN+C$_2$H$_2$ as the dominant formation route to HC$_3$N. However, uncertainties in the isotopic ratios preclude definitive conclusions. △ Less

Submitted 12 June, 2018; originally announced June 2018.

Comments: Accepted for publication in MNRAS

Collisional Excitation and Weak Maser Action of Interstellar Methanimine

Authors: Alexandre Faure, François Lique, Anthony J. Remijan

Abstract: The inelastic cross sections and rate coefficients for the rotational excitation of methanimine (CH$_2$NH) by cold H$_2$ have been determined quantum mechanically based on a new highly correlated five-dimensional potential energy surface. This surface was fitted to more than 60 000 ab initio points with a root mean square error of $\sim$ 1-2 cm$^{-1}$ in the region of the potential well whose dept… ▽ More The inelastic cross sections and rate coefficients for the rotational excitation of methanimine (CH$_2$NH) by cold H$_2$ have been determined quantum mechanically based on a new highly correlated five-dimensional potential energy surface. This surface was fitted to more than 60 000 ab initio points with a root mean square error of $\sim$ 1-2 cm$^{-1}$ in the region of the potential well whose depth is -374.0 cm$^{-1}$. The rotationally inelastic rate coefficients have been combined with spectroscopic data to generate the non-equilibrium spectrum of CH$_2$NH towards the interstellar molecular cloud Sgr~B2(N). The transition $1_{10}\to 1_{11}$ at 5.29 GHz is found to be inverted and the predicted weak maser emission spectrum is in excellent agreement with new observations performed with the 100-meter Green Bank Telescope. △ Less

Submitted 5 June, 2018; originally announced June 2018.

Comments: 16 pages, 4 figures

Journal ref: The Journal of Physical Chemistry Letters 2018 9 3199-3204

Modelling the molecular composition and nuclear-spin chemistry of collapsing prestellar sources

Authors: P. Hily-Blant, A. Faure, C. Rist, G. Pineau des Foreêts, D. R. Flower

Abstract: We study the gravitational collapse of prestellar sources and the associated evolution of their chemical composition. We use the University of Grenoble Alpes Astrochemical Network (UGAN), which includes reactions involving the different nuclear--spin states of H2, H3+, and of the hydrides of carbon, nitrogen, oxygen, and sulfur, for reactions involving up to seven protons. In addition, species-to-… ▽ More We study the gravitational collapse of prestellar sources and the associated evolution of their chemical composition. We use the University of Grenoble Alpes Astrochemical Network (UGAN), which includes reactions involving the different nuclear--spin states of H2, H3+, and of the hydrides of carbon, nitrogen, oxygen, and sulfur, for reactions involving up to seven protons. In addition, species-to-species rate coefficients are provided for the ortho/para interconversion of the H3+ + H2 system and isotopic variants. The composition of the medium is followed from an initial steady state through the early phase of isothermal gravitational collapse. Both the freeze--out of the molecules on to grains and the coagulation of the grains were incorporated in the model. The predicted abundances and column densities of the spin isomers of ammonia and its deuterated forms are compared with those measured recently towards the prestellar cores H-MM1, L16293E, and Barnard B1. We find that gas--phase processes alone account satisfactorily for the observations, without recourse to grain-surface reactions. In particular, our model reproduces both the isotopologue abundance ratios and the ortho:para ratios of NH2D and NHD2 within observational uncertainties. More accurate observations are necessary to distinguish between full scrambling processes---as assumed in our gas-phase network---and direct nucleus- or atom-exchange reactions. △ Less

Submitted 4 April, 2018; originally announced April 2018.

Comments: Main paper: 18 pages; supplementary material (University of Grenoble Alpes chemical Network): 62 pages

Abundance of HCN and its C and N isotopologues in L1498

Authors: Victor S. Magalhaes, Pierre Hily-Blant, Alexandre Faure, Mario Hernandez-Vera, Francois Lique

Abstract: The isotopic ratio of nitrogen in nearby protoplanetary disks, recently measured in CN and HCN, indicates that a fractionated reservoir of nitrogen is available at the stage of comet formation. This reservoir presents a 3:1 enrichment in 15N relative to the elemental ratio of 330, identical to that between comets and the sun, suggesting that similar processes are responsible for the fractionation… ▽ More The isotopic ratio of nitrogen in nearby protoplanetary disks, recently measured in CN and HCN, indicates that a fractionated reservoir of nitrogen is available at the stage of comet formation. This reservoir presents a 3:1 enrichment in 15N relative to the elemental ratio of 330, identical to that between comets and the sun, suggesting that similar processes are responsible for the fractionation in the protosolar nebula (PSN) and in PSN analogs. However, how the fractionation of nitrogen takes place is an open question. Previously obtained HCN/HC15N abundance ratios suggest that HCN may be enriched in 15N in prestellar cores, although doubts remain on these measurements, which rely on the double-isotopologue method. Here we present direct measurements of the HCN/H13CN and HCN/HC15N abundance ratios in the L1498 prestellar core based on spatially resolved spectra of HCN(1-0), (3-2), H13CN(1-0), and HC15N(1-0). We use state-of-the-art radiative transfer calculations using ALICO, a 1D radiative transfer code capable of treating hyperfine (hf) overlaps. From an analysis of dust emission maps of L1498, we derive a new physical structure. We also use new, high-accuracy HCN-H2 hf collisional rates, which enable us to quantitatively reproduce all the features of the HCN(1-0) and HCN(3-2) spectra, especially the anomalous hf line ratios. Special attention is devoted to derive meaningful uncertainties on the abundance ratios. The obtained values, HCN/H13CN=45(3) and HCN/HC15N=338(28), indicate that carbon is fractionated in HCN, but nitrogen is not. For the HCN/HC15N abundance ratio, our study validates to some extent analyses based on the single excitation temperature assumption. Comparisons with other measurements suggest significant core-to-core variability. Furthermore, the 13C enrichment we found in HCN could explain the superfractionation of nitrogen measured in solar system chondrites. △ Less

Submitted 29 March, 2018; originally announced March 2018.

Comments: 16 pages, 12 figures

Journal ref: A&A 615, A52 (2018)