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Collisional excitation of interstellar molecules: towards reactive systems

Periodic Reporting for period 1 - COLLEXISM (Collisional excitation of interstellar molecules: towards reactive systems)

Reporting period: 2019-07-01 to 2020-11-30

Accurate determination of physical conditions of interstellar molecular clouds is a crucial step to better understand the life cycle of the interstellar matter and particularly the formation of stars and planets as well as the synthesis of organic molecules that may lead to emergence of life in the universe. A key parameter for the determination of these conditions from interstellar spectra is the calculation of accurate collisional rate coefficients of interstellar molecules with the most abundant species (H, He, H2 and e-).
Indeed, in order to make the most of the observational spectra, it is crucial to model the excitation conditions of the interstellar molecules that are primarily governed by collisional and radiative processes.
Whereas the knowledge of collisional processes has reached a certain level of maturity for collisions involving non-reactive molecules, very few reliable data exist for collisions involving reactive radicals and ions.

The computation of such data is a real challenge since inelastic and reactive processes compete during collisions.
The COLLEXISM project aims at overcoming this complex problem and to provide collisional data for these radicals and ions in order to derive as much information as possible from the molecular spectra collected by current telescopes.
As it is hardly possible to consider both collisional and reactive processes simultaneously, the COLLEXISM consortium will set up a new methodology based on quantum approach that allows obtaining accurate data. It will focus on molecular hydrides that are good candidates because of both their astrophysical importance and their quantum accessibility. The COLLEXISM consortium will carry out the determination of interaction potentials using quantum chemistry ab initio methods while the treatment of the dynamics of the nuclei will primarily be done using quantum time-independent reactive and non-reactive approaches. When exact quantum calculations will not be usable, innovative statistical quantum mechanical methods will also be explored.

The new data will then be used in radiative transfer models and the predictions will be finally compared to observations in order to derive the abundances of reactive radicals with unprecedented accuracy.
By providing, for the first time, collisional data for reactive systems, the present projects represents a major step forward in both the understanding of molecular energy transfer and the analysis of interstellar spectra.
We started working on two different aspects of the project: The study of the collisional excitation of reactive species using reduced dimensional approaches and the development of statistical approaches to treat collisional excitation. We have obtained new collisional data for the OD+, H3O+, OH, HeH+ and HS+ interstellar molecules. A new code to compute collisional data using statistical approaches is under development. The new data will be soon use in astrochemical modelling in order to derive more precisely the chemical content of interstellar molecular clouds.
All together, we have published 7 papers in refereed journal and one is in press. The results have also been presented in several national and international conferences as well as in different laboratory. Outreach conference were also given.
Despite the recent increase of interest for the grain surface chemistry in the ISM, there are still many basic gas-phase processes (like collisional excitation of radicals) that are not fully understood.
By providing, for the first time, collisional data for reactive systems, the present proposal represents a major step forward in both the understanding of molecular energy transfer and the analysis of interstellar spectra. In addition, we will not only provide new data for selected systems but we will propose a new methodology based on statistical method that will provide collisional rate coefficients for any (new) radicals that will be detected by present and future telescopes. Let us cite the James Webb Space Telescope (JWST), a space observatory to be launched in 2021 and the Square Kilometre Array (SKA) telescope, a large multi radio telescope under construction that will provide new observations of molecular clouds and star-forming clusters with unprecedented resolution and sensitivity from long-wavelength visible to the mid-infrared that should allow the discovery of new interstellar molecules.

Also, from the strong multi-disciplinary character of the proposal, we expect a broad impact of the results, both in the astrochemistry and physical chemistry communities.