CH+ depletion by atomic hydrogen: accuracy of new rates in photo-dominated and self-shielded environments

TL;DR

This study provides new quantum-derived reaction rates for CH+ depletion by atomic hydrogen, demonstrating their impact on astrochemical models in PDR and MC environments, especially above 100 K, and offers insights into CH+ abundance discrepancies.

Contribution

It introduces accurate quantum reaction rates for CH+ depletion and integrates them into chemical models, improving understanding of CH+ evolution in space environments.

Findings

New reaction rates are accurate above 100 K.

Differences with low-temperature experiments are negligible at relevant astrochemical temperatures.

Slight changes in initial oxygen abundance affect CH+ abundance predictions.

Abstract

A detailed quantum analysis of a ionic reaction with a crucial role in the ISM is carried out to generate ab initio reactive cross sections with a quantum method. From them we obtain the corresponding CH+ depletion rates over a broad range of temperatures. The new rates are further linked to a complex chemical network that shows the evolution in time of the CH+ abundance in photodissociation region (PDR) and molecular cloud (MC) environments. The evolutionary abundances of CH+ are given by numerical solutions of a large set of coupled, first-order kinetics equations by employing the new chemical package KROME. The differences found between all existing calculations from low-T experiments are explained via a simple numerical model that links the low-T cross section reductions to collinear approaches where nonadiabatic crossings dominate. The analysis of evolutionary abundance of CH+ reveals that the important region for the depletion reaction of this study is that above 100 K, hence showing that, at least for this reaction, the differences with the existing low-temperature experiments are of essentially no importance within the astrochemical environments. 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+ abundance since the main chemical carbon ion depletion channel is reduced in efficiency. This simplified observation might provide an alternative starting point to understand the problem of astrochemical models in matching the observed CH+ abundances.