Hyperfine excitation of SH+ by H

Abstract

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 1000 K. 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.