Breakdown curves of CH₂⁽⁺⁾, CH₃⁽⁺⁾, and CH₄⁽⁺⁾ molecules

II. Application to chemical reactions and astrophysical implications

¹ Institut de Physique Nucléaire d’Orsay (IPNO), CNRS, Univ. Paris Sud, Université Paris-Saclay, 91406 Orsay, France
e-mail: chabot@ipno.in2p3.fr
² Institut des Sciences Moléculaires d’Orsay (ISMO), CNRS, Univ. Paris Sud, Université Paris-Saclay, 91405 Orsay, France
³ LERMA, Observatoire de Paris, PSL research university, CNRS, Sorbonne Université, 92186 Meudon, France
⁴ Laboratoire d’astrophysique de Bordeaux, Université de Bordeaux, CNRS, B18N, allée G. Saint-Hillaire, Pessac, France

Received: 11 March 2020
Accepted: 15 June 2020

Abstract

Aims. The aim of this work is to provide semi-empirical branching ratios (BRs) for the kinetic databases used in astrochemistry, such as the KInetic Database for Astrochemistry (KIDA). Our work focuses on the CH_y⁽⁺⁾ species (y = 2–4) excited by cosmic rays (CR), electrons, and photons (UV), or the intermediate excited complexes CH_y⁽⁺⁾ resulting from chemical reactions. It also intends to test the sensitivity of benchmark calculations to those new physical inputs in cold quiescent clouds and in photo-dissociation regions (PDRs).

Methods. We constructed semi-empirical breakdown curves (BDCs) based on the collision of CH_y⁺ (y = 2–4) projectiles of constant velocity (250 keV uma⁻¹) with He atoms as explained in a previous paper, where BRs for UV, CR, and electronic processing were also derived. The same BDCs were applied to predict BRs for chemical reactions (bi-molecular neutral and ionic reactions, charge exchange). The effect of the new BRs on the chemical composition of cold dark clouds was tested using the time-dependent Nautilus gas-grain model. The same effect on the chemical composition of PDRs was tested using the Meudon PDR code.

Results. Branching ratio predictions of the model are found to be in good agreement with available BR measurements for charge exchange reactions and the reaction between C and H₃⁺. The chemistry for both cold clouds and PDRs is found to be not strongly affected by this update of BRs.