Excitation, reactivity and detection of anions in the interstellar and circumstellar media
Intervenant : François Lique
LOMC – UMR 6294, CNRS – Université du Havre, France
The molecular diversity of interstellar and circumstellar matters has been recently enriched with the detection of molecular anions. Usually by far less abundant than neutrals, their presence affects the density of free electrons which controls the rate of cloud collapse, and therefore of star formation. The detections of anions have simulated research on chemistry of negative ions. However, current models still fail to reproduce anion-to-neutral abundance ratios. Accurate knowledge of the physical and chemical processes involving these species in astrophysical media is then crucial.
First, accurate modelling of molecular anions’ emission spectra to accurately determine the anionic abundances in space requires the calculation of rate coefficients for the collisional excitation of the anions by the most abundant species. Without these rates, only approximate estimates of the molecular abundances are possible assuming local thermodynamic equilibrium, which is generally not a good approximation at typical interstellar and circumstellar densities. We present here calculations that were recently performed to compute collisional data for the C6H- and CN- molecules [1,2], two of the most abundant anions, and we discuss their impact on the astrophysical modelling.
Second, the understanding of the chemical evolution of the anionic species relies on reactive rate constants for their formation and destruction processes. Some experimental studies have determined the rate constants for some of the reactions. Unfortunately, these rates are usually obtained at room temperatures, covering only a small part of the temperature range and leading the astrophysical community with a real lack of reactive data. We present here the first combined theoretical and experimental study dealing with the reactivity of anionic species under very cold conditions. The reactivity of the CxN- anionic carbon chains with cyanopolyynes and formic acid was studied down to 36K . Surprisingly, A very weak temperature dependence of the rate constants was found leading to a lower anionic destruction rates at low temperatures than previously thought.
Third, we also show how first principles calculations can predict the existence of new molecular anions. We present the global potential energy surface and the bound states of the N2H- van der Waals complex . The electronic calculations were performed using state-of-the-art ab initio methods and the nuclear motions were solved using a quantum close-coupling scattering theory. A T-shaped equilibrium structure was found, with a well depth of 349 cm-1, where many bound states have been located. This anion could be formed after low energy collisions between N2 and H- through radiative association. The importance of this prediction in astrophysics and the possible use of N2H- as a tracer of N2 and H- in the interstellar medium is discussed.
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 Walker et al., MNRAS 466, 831–837 (2017)
 Joalland et al., J. Phys. Chem. Lett. 7, 2957 (2016)
 Lique et al., J. Chem. Phys. 136, 244302 (2012)