Recent advances in collisional excitation of interstellar polyatomic molecules - S. Demes and F. Lique

Recent astronomical observations have been showed that the interstellar molecular clouds exhibit a very rich and complex chemistry [1]. To determine the chemical composition of these environments, a non-LTE analysis of the emission spectra is required. Such analysis however requires the prior collisional rate coefficients, since collisions, in competition with radiation, are responsible for the molecular excitation in interstellar clouds.

We report here our recent advances in rotational excitation studies of hydronium (H₃O⁺) and ammonia (NH₃) molecular species in collision with H₂ projectile. Both of these molecules are abundant in the interstellar medium (ISM), they have been detected in several interstellar molecular clouds [2,3].
State-to-state rotational de-excitation cross sections were computed using the exact close-coupling method [4]. The corresponding rate coefficients were derived up to 300 K kinetic temperatures for the H₃O⁺ and up to 800 K for the NH₃ target species. A detailed analysis of the cross sections and rate coefficients was performed for both molecules. The significant impact of the new rate coefficients for the modeling of the excitation of interstellar clouds is demonstrated in contrast to the previously published collisional data for H₃O⁺ and NH₃.


References
[1] E. Herbst, E. van Dishoeck, Annu. Rev. Astron. Astrophys. 47 427-480 (2009).
[2] F.F.S van der Tak, S. Aalto, and R. Meijerink, A&A 477 L5 (2008).
[3] A.C. Cheung, D.M. Rank, C.H. Townes et al., Phys. Rev. Lett. 21, 1701 (1968).
[4] S. Demes, F. Lique, F.F.S. van der Tak, A. Faure, C. Rist, P. Hily-Blant, MNRAS 509, 1252 (2022).