Electron-impact excitation of diatomic hydride cations I: HeH$^+$, CH$^+$, ArH$^+$

TL;DR

This study uses advanced R-matrix calculations to compute electron-impact excitation rates for three diatomic cations, revealing significant low-temperature rate changes and providing a model consistent with astrophysical observations.

Contribution

The paper introduces improved threshold effect treatments in excitation rate calculations for diatomic cations, enhancing accuracy over previous studies.

Findings

Low-temperature excitation rates are significantly affected by threshold effects.

CH$^+$ excitation is dominated by $ riangle J=1$ transitions.

The ArH$^+$ excitation model aligns with Crab Nebula observations at specific electron densities.

Abstract

{\bf R}-matrix calculations combined with the adiabatic nuclei approximation are used to compute electron-impact rotiational excitation rates for three closed-shell diatomic cations, HeH$^+$, CH$^+$, ArH$^+$. Comparisons with previous studies show that an improved treatment of threshold effects leads to significant changes in the low temperature rates, furthermore the new calculations suggest that excitation of CH$^+$ is dominated by $\Delta J =1$ transitions as is expected for cations with a large dipole moment. A model for ArH$^+$ excitation in the Crab Nebula is presented which gives results consistent with the observations for electron densities in the range $2-3\times 10^3$~cm$^{-3}$.