Abstract
The astrophysical media are extremely (if not impossible) difficult to probe. Their chemical characterization can only be done through the analysis of the emission spectra registered by telescopes. Since the lines composing the spectra represent the emission energy between quantified and variously populated energy levels of a specific chemical species, it is often said that the emission spectra captured by telescopes contains the chemical signature of the media. Transition within these levels can occur by absorption/emission of a photon or through a collisional energy transfer. In order to model the spectra, the efficiency of these processes needs to be evaluated. In astrochemistry, one of the challenging goal is thus to compute accurate collisional data such as the excitation rate coefficients induced by the dominant astrophysical species (He, H, H 2 , e − ), from which astrophysicists can then deduce energy levels population and thus, derive the abundance of species and model the spectra. The problem, however, is that an exact quantum calculations of these rates is not reachable in terms of computational time and memory. Therefore, the obtention of realistic rate coefficients for unstudied molecular system requires the setting of a correct methodology, which will include multiple approximations from which several uncertainties of different nature will arise. The standard methodology for the obtention of rate coefficients proceed in two steps. First, a quantum chemistry method is used to span the interaction potential between the colliders and obtain the so-called potential energy surface (PES) of the system. Then, the dynamics of the nuclei are studied on the computed surface. Through the procedure, some validation can be done, in order to ensure the validity of the rates: the PES can be validated through the computation of spectroscopic data based on this surface that can be compared to experimental measurements. The collisional data from which rate coefficients arise can be also validated through the comparison between computed and measured pressure broadening coefficients. Such validation, however, implies that spectroscopic data are available for the molecular system under consideration, which is not always the case. This talk will start by a presentation of the methodology developed to obtain highly accurate rate coefficients. Then, its application to the CO 2 -He and C 2 S-He collisional system will be presented. The CO 2 -He system was well studied during the last decades, from both a theoretical and an experimental point of view, and can as such be used as a benchmark system for the validation of our methodology. Afterwards, I will show how I extrapolated this methodology to the C 2 S-He system, for which no rate coefficients exist. Indeed, the C 2 S molecule and its 13 CCS and C 13 CS isotopologues have been detected in interstellar clouds?. But the spectra lines of 13 C isotopologues have significant differences in their intensities when they should be similar given the current astrochemical models used by astronomers. Can we resolve this anomaly through the computation of accurate rate cofficients? The intensity difference could be explained away if the rate coefficients for both isotopologues are different by the same factor that the spectroscopic lines. If the rate coefficients do not explain this difference, does this mean that the chemical model used to determine the respective abundancies of these isotopologues somewhat incorrect?
