Collisional excitation of radical species in the interstellar medium

International audienc

Hyperfine excitation of 13CCH and C13CH by collisions with para- H2

International audienceABSTRACT The computation of hyperfine resolved cross sections and rate coefficients for open-shell molecules in collision with H2 is a true methodological and numerical challenge. Such collisional data are however required to interpret astrophysical observations. We report the first hyperfine resolved rate coefficients for (de-)excitation of 13CCH and C13CH isotopologues induced by collisions with para-H2. These calculations have been performed using a recently published C2H–H2 potential energy surface. Hyperfine resolved cross sections and rate coefficients between the first 98 energy levels of the two isotopologues were determined using a recoupling technique for temperatures ranging from 5 to 100 K. Significant isotopic substitution effects were found, showing the necessity of computing isotopologue specific collisional data. These rate coefficents have then been used in a simple radiative transfer modelling for typical molecular cloud conditions

The excitation of ethynyl in the interstellar medium: A key to understand isotopic fractionation

International audienc

Hyperfine excitation of C2H and its isotopologues in the interstellar medium

International audienc

The excitation of ethynyl in the interstellar medium: A key to understand isotopic fractionation

International audienc

Hyperfine excitation of NH and ND by molecular hydrogen: Rate coefficients and astrophysical modeling

12 pags., 13 figs., 2 tabs.The NH and ND radicals are of key importance in the comprehension of nitrogen chemistry and the enhancement of deuterated molecules in the interstellar medium. Observations by space telescopes yield spectra that can resolve the fine and hyperfine structure of these radicals, a consequence of the electronic and magnetic interactions of nitrogen, hydrogen, and deuterium nuclei. Accurate rate coefficients, induced by collisions with H, are required to interpret spectra of these radicals. We report the first rate coefficients for fine and hyperfine transitions of NH and ND in collision with both ortho- and para-H. Based on a recent four-dimensional potential energy surface, fine-structure resolved cross sections and rate coefficients are computed with the time-independent close-coupling method over a temperature range of 5- 300 K. Our calculations include the first 25 energy levels of NH and ND. Hyperfine resolved cross sections and rate coefficients are determined using the infinite-order sudden (IOS) approximation between 5 and 200 K for NH and 100 K for ND. We consider the first 71 and 105 energy levels of NH and ND, respectively. General propensity rules are discussed. We found a significant isotopic substitution effect in the rate coefficients. In addition, the rate coefficients for collisions with H are larger than those with He by a factor of up to 5, leading to lower critical densities for collisional excitation with H than He. The impact of the new set of collisional data has been investigated in simple radiative transfer models of the NH emission seen toward the Orion Bar and the ejecta of the η Carinae binary star. We observed significant differences by a factor of 5 between the presently determined column densities for NH compared to those from the literature using He as a collider.We acknowledge financial support from the European Research Council (Consolidator Grant COLLEXISM, Grant Agreement No. 811363). We wish to acknowledge the support from the CEA/GENCI (Grand Equipement National de Calcul Intensif) for awarding us access to the TGCC (Très Grand Centre de Calcul) Joliot Curie/IRENE supercomputer within the A0110413001 project. J.R.G. thanks the Spanish MICINN for funding support under grant PID2019-106110GB-I00

Theoretical study of collisions of NH by H2 in the interstellar medium

International audienc

Étude théorique des collisions entre NH et H2 dans le milieu interstellaire

National audienc

Theoretical study of collisions of NH by H2 in the interstellar medium

International audienc

Theoretical study of collisional excitation of NH by H2 in the interstellar medium

International audienc