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.
Highlights
Cross-sections for electron collisions with molecular ions can be very large (>1000 Å2)
If the ion in question contains a permanent dipole moment, the electron-impact rotational excitation rate coefficients far exceed those of H and H2 meaning that in comparatively electron-rich regions, electron collisions can become the dominant excitation process
We considerexcitation of the hydoxyl and sulphanyl ions: OH+ and SH+, respectively. Both the species have electronic ground states of 3 − symmetry which adds an extra complication as the rotational levels display fine structure due to the electron spin of the two unpaired electrons and a hyperfine structure due to the nuclear spin of the hydrogen atom. Both OH+ and SH+ were only detected in the interstellar medium (ISM) within the last decade; OH+ being first observed by Wyrowski et al (2010) and SH+ by Benz et al (2010) and Menten et al (2011)
Summary
Cross-sections for electron collisions with molecular ions can be very large (>1000 Å2). We consider (de)excitation of the hydoxyl and sulphanyl ions: OH+ and SH+, respectively Both the species have electronic ground states of 3 − symmetry which adds an extra complication as the rotational levels display fine structure due to the electron spin of the two unpaired electrons and a hyperfine structure due to the nuclear spin of the hydrogen atom. We present electron-impact electronic excitation cross-sections for the two ions considered While these are unlikely to be important for models of ISM, OH+ can be found in planetary ionospheres (Fox et al 2015), and cometary coma (Nordholt et al 2003; Haider & Bhardwaj 2005; Rubin et al 2009), as well as around Enceladus (Gupta et al 2010).
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