CH+ Destruction by Reaction with H: Computing Quantum Rates To Model Different Molecular Regions in the Interstellar Medium

Abstract

A detailed analysis of an ionic reaction that plays a crucial role in the carbon chemistry of the interstellar medium (ISM) is carried out by computing ab initio reactive cross sections with a quantum method and by further obtaining the corresponding CH+ destruction rates over a range of temperatures that shows good overall agreement with existing experiments. The differences found between all existing calculations and the very-low-T experiments are discussed and explored via a simple numerical model that links these cross section reductions to collinear approaches where nonadiabatic crossing is expected to dominate. The new rates are further linked to a complex chemical network that models the evolution of the CH+ abundance in the photodissociation region (PDR) and molecular cloud (MC) environments of the ISM. The abundances of CH+ are given by numerical solutions of a large set of coupled, first-order kinetics equations that employs our new chemical package KROME. The analysis that we carry out reveals that the important region for CH+ destruction is that above 100 K, hence showing that, at least for this reaction, the differences with the existing laboratory low-T experiments are of essentially no importance within the astrochemical environments discussed here because, at those temperatures, other chemical processes involving the title molecule are taking over. A detailed analysis of the chemical network involving CH+ also shows that a slight decrease in the initial oxygen abundance might lead to higher CH+ abundances because the main chemical carbon ion destruction channel is reduced in efficiency. This might provide an alternative chemical route to understand the reason why general astrochemical models fail when the observed CH+ abundances are matched with the outcomes of their calculations.