by François Lique & Marie Gueguen & Cheikh Bop & Sándor Demes & Michał Żółtowski & Benjamin Desrousseaux & Paul Pirlot Jankowiak & Amélie Godard Palluet & Maxime Gontel & Alexandre Faure & Guillaume Raffy, on

Presentation of the workshop

Group pictures of all participants to the 6th workshop on physical-chemical processes of astrophysical interest:towards a state-to-state chemistry.

From the 14th to the 18th of June 2021, the workshop on "Physics-chemical processes of astrophysical interest: towards state-to-state chemistry" was taking place in Saint-Florent (Corsica, France). It was organized by F. Lique, A. Faure and O. Roncero.

Astronomers, experimentalists and theoreticians were gathered to discuss the state-to-state chemistry and its effect on astrophysical modeling.

The whole team attended the workshop and B. Desrousseaux, M. Gueguen, M. Żółtowski, S. Demes, C. Bop, A. Faure and G. Raffy presented their work during this workshop.
During the young researcher session, P. Pirlot Jankowiak, M. Gontel and A. Godard Palluet also gave talks.

Centre d'Études Sous Marines (CESM) - Saint Florent, Corsica

The CESM is an associative sailing and diving club created in 1949.
This non-profit organization is offering sailing and diving activities to vacationers for over 60 years.

Since 2011, the center lends its premises to workshops about physical and chemical processes in the interstellar medium organised by François Lique and Carole Le Guen.

View from the Centre d'Etude Sous-Marine (CESM), St Florent.

Beyond Verification and Validation: testing the reliability of complex simulations in astrophysics - M. Gueguen

Oral communication of M. Gueguen.

A debate has recently taken place in the philosophical literature about the specific challenges that arise for assessing the reliability of complex simulations. Lenhard and Winsberg (2010), in particular, have argued that for sufficiently complex simulations, the numerical scheme and the model assumptions become entangled in such a way that, when the model disagrees with the data, the sources of the failure become impossible to locate a challenge they refer to as 'fuzzy modularity'.

Traditional procedures of verification and validation supposed to ensure that the computer simulation is free of numerical errors and based on correct modeling assumptions can no longer be separated and undermines the overall assessment of the simulation's reliability. I argue that the spectrum of methodologies available to test the reliability of simulations is broader than what the language of verification and validation can capture. Other procedures, that have not been given a precise formulation yet but have nonetheless been used by astrophysicists, succeed in escaping the challenges that a lack of modularity may generate.

My aim here is first to flesh out more precisely these methodologies and to explain how this methodology permits to minimize the holistic challenges associated with fuzzy modularity.

Collisional excitation of H3O+ by H2 and its astrophysical applications - S. Demes, F. Lique, A. Faure, F. van dr Talk, C. Rist and P. Hily-Blant

Recent astronomical observations have been showed that molecular clouds in the interstellar medium (ISM) exhibit a very rich and complex chemistry [1]. However, in order to interpret these observations, complementary research is required. So for example, to correctly describe the physico- chemical conditions in interstellar environments, where local thermodynamic equilibrium conditions are usually not fulfilled, a complex theoretical analysis is required, which involves the computation of state-to-state collisional rate coefficients for the rotational transitions of molecular species.

Hydronium (H3O+) has been detected in both dense and diffuse molecular clouds, and plays a crucial role in oxygen and water chemistry [2]. Since there are only limited works devoted to study its collisional excitation by interstellar colliders [3], accurate rate coefficients are obviously needed.

The rotational excitation of the H3O+ cation in collision with H2 molecule (the most dominant collider in the ISM) is studied for the first time [4]. State-to-state rotational de-excitation cross sections were computed using the close-coupling method, based on a highly accurate 5D potential energy surface [5]. The thermal rate coefficients were derived up to 300 K kinetic temperatures by integrating the cross sections over a Maxwell–Boltzmann distribution of velocities. The calculated rate coefficients are used then in radiative transfer modeling of hydronium excitation in interstellar molecular clouds, giving new insights into H3O+ observations and more details about water chemistry in Space.

[1] Gas-Phase Chemistry in Space, ed. F. Lique and A. Faure (IOP Publishing, Bristol, UK, 2019).
[2] F. F. S. Van Der Tak, S. Aalto, R. Meijerink, Astron. Astrophys., 477 L5 (2008).
[3] A. R. Offer and M. C. van Hemert, Chem. Phys., 163 83 (1992).
[4] S. Demes, F. Lique, F.F.S. van der Tak et al., Mon. Notices Royal Astron. Soc., 509 1252 (2022).
[5] S. Demes, F. Lique, A. Faure, C. Rist, J. Chem. Phys., 153, 094301 (2020).

Oral communication of S. Demes.

New results on the excitation of H2O and its isotopologues - M. Żółtowski, F. Lique, A. Karska and P. S. Żuchowski

Systematic comparison of rate coefficients from [1] and [2] for ortho- and para-H2O at different temperature.

Water is a key molecule for interstellar chemistry. Observations with Herschel telescope show significant population of very high rotational transitions (j ~ 8) in young stellar objects, indicating significant amounts of water in hot (T ~ 1500 K) and dense (n ~ 106 cm-3) gas.

Non-local thermodynamic equilibrium (LTE) modeling of these observations requires the knowledge of the collisional and radiative properties of highly excited water at high temperatures. The aim of this work is to calculate a new set of excitation rate coefficients for both para- and ortho-H2O induced by collisions with H2 for energy levels up to j = 17.

Quantum scattering calculations were performed using a reduced dimensional approach and the coupled states approximation. Rate coefficients were obtained for 97 pure rotational energy levels of both para- and ortho-H2O and for temperatures up to 2000 K. The isotopic substitution of the water molecule was also investigated.

[1] F. Daniel, M.-L. Dubernet, A. Grosjean, A&A 536, A76 (2011)
[2] M. Żółtowski, F. Lique, A. Karska, P. S. Żuchowski, Mon. Not. Roy. Astr. Soc., 502, 5356-5361 (2021)

State-to-state astrochemistry in the primordial universe - M. Gontel, A. Faure, P. Hily-Blant and F. Lique

Hydrogen plays a crucial role in the chemistry and the thermodynamics of the primordial universe, prior to the formation of the first stars. H2 and HD, two molecules whose cooling effect is essential in the formation of the first stars and structures, are formed mainly through ionic reactions involving charged species like H2+ , H- or D+. In addition, excitation of H2 by H and H+ plays an important role since it allows ortho-para-H2 conversion, strongly influencing the cooling essential for the star formation.

In this talk an up-to-date and improved chemical network for primordial chemistry based on previous commonly used networks is presented. It includes the most recent experimental and theoretical results on the rate coefficients of the main reactions and is valid for most of the reactions between 10 K and 10000 K. We will also present new state-to-state data and ongoing work on the collisional excitation of H2 by H, H+ and He. In particular, improve- ments at low temperatures of recently calculated rate coefficients for H2-H will be presented. Our chemical network and collisional data should be of valuable uses, most notably for future simulations of primordial gas evolution.

Oral communication of M. Gontel during the young researcher session.

Collisional excitation of NH by H2 in the interstellar medium - P. Pirlot Jankowiak, Y. N. Kalugina, R. Ramachandran, G. Raffy, P. J. Dagdigian and F. Lique

Oral communication of P. Pirlot during the young researcher session.

Despite the increasing number of observations of several nitrogen hydrides molecules (NH, NH2, NH3 ...) in molecular clouds, the nitrogen abundance in the interstellar medium (ISM) is still subject to debate.

Among nitrogen hydrides, the NH radical is of key importance since it is produced from at least two main paths, the dissociative recombination of N2H+ and NH4+. It is then a potential accurate probe of nitrogen chemical network [1] and accurately modeling the NH abundance in molecular clouds is of great interest. Density conditions in the ISM does not allow maintaining collisional processes large enough to sustain the local thermodynamic equilibrium conditions. Hence, the modeling of the observational spectra requires to consider the competition between radiative and collisional processes. In the ISM, H2 is the dominant collider but to the best of our knowledge, there are lack of collisional data for the NH-H2 system.

Calculations of fine structure resolved excitation cross sections and rate coefficients for the NH-He collisional complex will be presented. The collisional data were computed from a new four-dimensional potential energy surface (PES) [2]. Binding energy of the complex was calculated to check the validity of the PES through experimental data as benchmark. Cross sections calculations are performed using the close-coupling approach for NH and with both ortho and para H2 partners. It appears a very different general behavior between the two partners. Calculations implying NH with ortho-H2 show a propensity rule in favor of ΔN = 1 transitions whereas a propensity rule in favor of ΔN = 2 is seen for collisions with para-H2. F-conserving transitions were found to be larger than F-changing transitions. We compare the new NH-para-H2 rate coefficients with previous NH-He data. The new data are expected to improve abundances calculations for astrophysical applications.

Collisional excitation of CO2 by He: New potential energy surface and scattering calculations - A. Godard Palluet, F. Thibault and F. Lique

The CO2-He collisional system is suited for benchmarking new experimental and theoretical methods. Thus, it was well studied during the last decades.

In this talk, I will present our new study of the CO2-He collisional system. A new potential energy surface was calculated at the coupled cluster level of theory with a complete basis set extrapolation. The potential energy surface accuracy was tested through the comparison of bound states and pressure broadening coefficients with experimental data.

Finally, revised collisional rate coeffcients were provided in the 10 - 300 K temperature range. Such data can also be used for the interpretation of astrophysical observations in media where both CO2 and He species are abundant. We expect that these new collisional data will stimulate new experimental studies at low or room temperature.

Oral communication of A. Godard Palluet during the young researcher session.

New abundance determination of the HC3N isomers in cold clouds - C. Bop

Oral communication of C. Bop.

The isomers of HC3N, namely HC2NC and HNC3, are widely observed in the interstellar medium and in circumstellar envelopes. Their abundance has been determined under the assumption of local thermodynamic equilibrium (LTE) conditions or non-LTE radiative transfer models, but in considering the collisional excitation of HC3N as the same for all isomers.

Chemical models for the prototypical cold cores, TMC-1 and L1544, reproduced the abundance of HC3N fairly well, but they tend to overestimate the abundances of HC2NC and HNC3 with respect to the observations. It is therefore worth revisiting the interpretation of the observational spectra of these isomers using a rigorous non-LTE modelling. The abundance of HC2NC and HNC3 were then determined using non-LTE radiative transfer calculations based on the proper rate coefficients for the first time in this work. Modeling the brightness temperature of HC2NC and HNC3 when using their proper collision rate coefficients shows that models based on LTE or non-LTE with approximate collision data may lead to deviations of up to a factor of∼ 1.5.

Reinterpreting the observational spectra led us to significant differences relative to the observed abundances previously determined. Our findings suggest quite similar abundance ratios for the TMC-1 and L1544 cold cores as well as the L483 protostar. This work will encourage further modelling with more robust non-LTE radiative transfer calculations and future studies to revisit the chemistry of HC3N and its isomers in cold molecular clouds.

CF+, a new tracer of PDR’s physical conditions - B. Desrousseaux, F. Lique, J. Goicoechea, E. Quintas-Sánchez, and R. Dawes

The detection of CF+ in interstellar clouds potentially allows astronomers to infer the elemental fluorine abundance and the ionization fraction in ultraviolet-illuminated molecular gas. Because local thermodynamic equilibrium (LTE) conditions are hardly fulfilled in the interstellar medium (ISM), the accurate determination of the CF+ abundance requires one to model its non-LTE excitation via both radiative and collisional processes. CF+ being mostly detected in warm molecular gas, where the H2 ortho-to-para ratio is found to be large, obtaining rate coefficients for collisional excitation with both spin isomers of H2 can be crucial.

In this work, we carried out close-coupling calculations of the rotational (de)excitation cross sections for collisions between CF+ and both para- and ortho-H2 for transitions between the 22 first rotational levels of CF+. Collisional energies up to 1500 cm−1 were explored in order to determine rate coefficients for temperatures up to 150 K. As already observed for a wide variety of ion–molecule collisions, para- and ortho-H2 rate coefficients are found to be similar in magnitude.

Then, we present non-LTE excitation models that reveal population inversion in physical conditions typical of photodissociation regions (PDRs). We successfully applied these models to fit the CF+ emission lines previously observed toward the Orion Bar and Horsehead PDRs, constraining gas density and temperature at the surface of the Horsehead and Orion Bar PDRs. The radiative transfer models achieved with these new rate coefficients allow the use of CF+ as a powerful probe to study molecular clouds exposed to strong stellar radiation fields.

Oral communication of B. Desrousseaux.

Hibridon 5.0 - G. Raffy, B. Desrousseaux and F. Lique

Oral communication of G. Raffy.

Hibridon© is a program package to solve the close-coupled equations which occur in the quantum treatment of inelastic atomic and molecular collisions. Gas-phase scattering, photodissociation, collisions of atoms and/or molecules with flat surfaces, and bound states of weakly-bound complexes can be treated.

Currently, Hibridon© is officially distributed in its 4.3.7 version, with last modifications from September 2006. We are currently working on the 5th version of the software, integrating some changes to the code:
Use git to manage the source code (stored on github)
 - Gather all user’s custom modifications of the Hibridon© code and merge them to obtain an up-to-date starting point
 - Add Automatic testing of the Hibridon© installation and continuous integration (CI)
 - Replace the previous custom build system with cmake to make Hibridon© easy to install on various platforms
 - Replace the previous custom fortran preprocessing system with the more standard fpp
 - Add dynamic memory allocation

New scattering code: SARAS - B. Desrousseaux and F. Lique

As for now, full quantum time-independent close-coupling calculations is the method of choice to obtain accurate collisional rate coefficients at typically low interstellar temperatures (< 100 K). However, in the case of reactive systems, i.e. open-shell molecules and ions that can undergo a reaction with the most dominant interstellar species H or H2, this method is impractical due to its memory and CPU requirements. As a result, reliable collisional data is missing for many detected reactive molecules of key importance in astrochemistry (NH, OH+, CH+, HCl+, H2O+, …), preventing a proper determination of their abundance.

We intend to develop a new approach [2-5] based on the statistical adiabatic channel model (SACM) of Quack and Troe [6, 7] to compute collisional rate coefficients in the case of reactive molecules. This efficient approach would allow the determination of the rate coefficients with an accuracy meeting the needs of astrophysical applications while overcoming the memory and CPU limitations of the close-coupling method.

In order to efficiently apply this method, I am currently developping a new software: SARAS. The source code benefits from the object oriented features and modularity of the Fortran 2008 standard. It is made massively parallel using the Message Passing Interface (MPI) and thus fully profits from nowadays supercomputer architectures.

[1] Roueff, E. & Lique, F. Molecular Excitation in the Interstellar Medium: Recent Advances in Collisional, Radiative, and Chemical Processes. Chemical Reviews 113, 8906–8938 (2013).
[2] Loreau, J., Lique, F. & Faure, A. An efficient statistical method to compute molecular collisional rate coefficients. ApJ 853, L5 (2018).
[3] Loreau, J., Faure, A. & Lique, F. Scattering of CO with H2O: Statistical and classical alternatives to close-coupling calculations. J. Chem. Phys. 8 (2018).
[4] Desrousseaux, B., Konings, M., Loreau, J., Lique, F. HD-H+ collisions: statistical and quantum state-to-state studies. Physical Chemistry Chemical Physics 23-35 (2021).
[5] Konings, M., Desrousseaux, B., Lique, F., Loreau, J. Benchmarking an Improved Statistical Adiabatic Channel Model for Competing Inelastic and Reactive Processes. The Journal of Chemical Physics 155-10 (2021).
[6] Quack, M. & Troe, J. Complex Formation in Reactive and Inelastic Scattering: Statistical Adiabatic Channel Model of Unimolecular Processes III. Berichte der Bunsengesellschaft für physikalische Chemie 79, 170–183 (1975).
[7] Quack, M. & Troe, J. Specific Rate Constants of Unimolecular Processes II. Adiabatic Channel Model. Berichte der Bunsengesellschaft für physikalische Chemie 78, 240–252 (1974).

Oral communication of B. Desrousseaux.