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Excitation and chemistry of interstellar hydrides – HYDRIDES


Excitation and chemistry of interstellar hydrides


In the period 2010-2013, the European Space Agency’s Herschel infrared observatory has mapped interstellar hydrides3 with unprecedented spectral resolution and sensitivity. Comprehensive analysis of such spectra requires the knowledge of collisional rate coefficients for hydrides excited by H, H2 and electrons. Before 2013, accurate collisional data for hydrides were available for only two species: HF and H2O. For a few other hydrides, calculations employed the helium atom as a substitute for H2, for simplicity. Significant collisional differences however exist between He, H and H2. In addition, all previous studies were performed within the rigid-rotor approximation. This approximation is questionable in the case of ions or radicals because reactive channels with H or H2 can play a role, even at low temperature. The three main objectives of the project HYDRIDES (2013-2017) were i) to tackle the collisional excitation of radical and ionic hydrides with the best available theoretical tools ii) to compare theory to experiment at the state-to-state level and iii) to assess the impact of the new collisional rate coefficients on radiative transfer models and to compare predictions with observations.

Our project was composed of three main tasks: the first task concerned the computation of collisional cross sections and rate coefficients for small hydrides detected in the interstellar medium. The so-called close-coupling quantum scattering theory was used massively and combined with the best available potential energy surfaces. The second task was devoted to laboratory measurements and comparisons with theoretical data, at low-temperature and state-resolved. The complementary techniques of double resonance (in cold flows) and crossed molecular beams were implemented. The third and last task focused on the impact of the collisional data on realistic radiative transfer models. To this aim, we have modeled observational spectra using standard (escape probability) as well as sophisticated (non-local iterative methods) approaches. In parallel with these tasks, we have developed a gas-phase astrochemical network distinguishing the nuclear-spin states of interstellar hydrides.

1. Accurate collisional data (with H, H2 or electrons) are now available for a dozen of hydrides (not including isotopologues), including highly reactive species such as CH+.
2. Comparisons between state-to-state theory and experiment for the benchmark systems CO+H2 and H2O+H2 have validated in detail the high precision of quantum calculations in the cold interstellar regime (< -260 degrees Celsius), near absolute zero.
3. We have quantitatively reproduced, for the first time, the emission spectrum of CH+ and OH+ as measured by Herschel satellite in photodissociation regions.
4. We have explained the “anomalous” (out of equilibrium) ortho-to-para ratio of nitrogen hydrides NH2 and NH3 in the cold interstellar medium. This development, not initially planned, has paved the way to the nuclear-spin specific astrochemistry.

The different results of the HYDRIDES project place our laboratories at the forefront of molecular excitation studies for astrophysics. The next natural challenge is the extension of our calculations to tetratomic, pentatomic and even larger reactive systems such as CH+ + H2 and H2O+ + H2. The CH3+ potential energy surface has been recently determined in Bordeaux but scattering calculations are currently only possible with approximate methods (statistical or classical) due to the computational cost of close- coupling calculations. These alternative methods now need to be fully tested. Finally, in addition to their role in molecular energy transfer, such reactive collisions (involving hydrogen atom exchanges) are crucial to model the nuclear-spin chemistry of hydrides and to predict reliable ortho-to-para ratios.

All publications, including PhD thesis, are online at:

Hydrides play a central role in molecular astrophysics as significant reservoirs of heavy elements. Excitation studies of interstellar hydrides deserve a particular attention because the reactive processes, negligible at low temperature for most of the molecules, can compete with or even dominate the energy transfer processes. The objective of the present cross-disciplinary project is to address this challenging problem both theoretically and experimentally. The targeted species are light diatomic carbon, nitrogen and oxygen hydrides, which are the building blocks of the whole interstellar chemistry. In addition to the production of fundamental molecular data, the analysis of available astronomical spectra will be carried out through coupled radiative transfer and chemical models. Such studies have been so far hampered by the lack of accurate state-to-state collisional cross sections and rate coefficients. A strong impact of the present proposal is thus expected both in molecular sciences and in astrochemistry.

Project coordinator

Monsieur Alexandre FAURE (Insitut de Planétologie et d'Astrophysique de Grenoble) –

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.


IPR Institut de Physique de Rennes
IPAG Insitut de Planétologie et d'Astrophysique de Grenoble
ISM Institut des Sciences Moléculaires
LOMC Laboratoire Ondes et Milieux Cmplexes

Help of the ANR 639,757 euros
Beginning and duration of the scientific project: December 2012 - 48 Months

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