Electron-impact excitation of diatomic hydride cations II: OH$^+$ and SH$^+$

TL;DR

This study calculates electron-impact excitation rates for OH$^+$ and SH$^+$ ions using advanced quantum methods, providing data crucial for astrophysical modeling and matching observations in the Orion Bar PDR.

Contribution

It offers new electron-impact rotational and hyperfine rate coefficients for OH$^+$ and SH$^+$, including a model that reproduces astrophysical observations.

Findings

Propensity rule $ riangle F= riangle j= riangle N= m extpm 1$ observed.

Model reproduces Herschel observations with specific electron fraction and column density.

Provides electronic excitation cross sections and rate coefficients for astrophysical applications.

Abstract

R-matrix calculations combined with the adiabatic-nuclei-rotation and Coulomb-Born approximations are used to compute electron-impact rotational rate coefficients for two open-shell diatomic cations of astrophysical interest: the hydoxyl and sulphanyl ions, OH$^+$ and SH$^+$. Hyperfine resolved rate coefficients are deduced using the infinite-order-sudden approximation. The propensity rule $\Delta F=\Delta j=\Delta N=\pm 1$ is observed, as is expected for cations with a large dipole moment. A model for OH$^+$ excitation in the Orion Bar photon-dominated region (PDR) is presented which nicely reproduces Herschel observations for an electron fraction $x_e=10^{-4}$ and an OH$^+$ column density of $3\times 10^{13}$~cm$^{-2}$. Electron impact electronic excitation cross sections and rate coefficients for the ions are also presented.