2020A&A...638A..72L

Context. SH⁺ is a surprisingly widespread molecular ion in diffuse interstellar clouds. There, it plays an important role by triggering the sulfur chemistry. In addition, SH⁺ emission lines have been detected at the UV-illuminated edges of dense molecular clouds, so-called photo-dissociation regions (PDRs), and toward high-mass protostars. An accurate determination of the SH⁺ abundance and of the physical conditions prevailing in these energetic environments relies on knowing the rate coefficients of inelastic collisions between SH⁺ molecules and hydrogen atoms, hydrogen molecules, and electrons.
Aims. We derive SH⁺-H fine and hyperfine-resolved rate coefficients from recent quantum calculations for the SH⁺-H collisions, including inelastic, exchange, and reactive processes.
Methods. The method we used is based on the infinite-order sudden approach.
Results. State-to-state rate coefficients between the first 31 fine levels and 61 hyperfine levels of SH⁺ were obtained for temperatures ranging from 10 to 1000K. Fine-structure resolved rate coefficients present a strong propensity rule in favor of Δj=ΔN transitions. The Δj=ΔF propensity rule is observed for the hyperfine transitions.
Conclusions. The new rate coefficients will help significantly in the interpretation of SH⁺ spectra from PDRs and UV-irradiated shocks where the abundance of hydrogen atoms with respect to hydrogen molecules can be significant.