Theoretical studies of interstellar molecular shocks – IV. The sulphur chemistry in diffuse clouds

• Published in: Monthly notices of the Royal Astronomical Society Vol. 223; no. 4; pp. 743 - 756
• Main Authors: Pineau des Forêts, G., Roueff, E., Flower, D. R.
• Format: Journal Article
• Language: English
• Published: Oxford, UK Oxford University Press 01.12.1986; Blackwell Science
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology; Interstellar medium (ism) and nebulae in milky way; Molecular clouds, h2 clouds, dense clouds, and dark clouds; Stellar systems. Galactic and extragalactic objects and systems. The universe; MHD model; Chemical reaction; Molecular cloud; Dynamic model; Sulfur; Shock wave; Interstellar molecule
• ISSN: 0035-8711; 1365-8711; 1365-2966; 1365-2966
• DOI: 10.1093/mnras/223.4.743

The column densities of sulphur-bearing molecules in diffuse clouds subject to shock heating have been calculated. Both one-fluid hydrodynamic and three-fluid magnetohydrodynamic models have been considered. It is found that observable colum densities of SH+ and possibly other sulphur-bearing species should be present in MHD shocks with velocities $u_\text s\approx12\enspace\text{km}\enspace\text s^{-1}$ and transverse magnetic field strengths ≈5 µG. On the other hand, the predicted column densities of SH+ in one-fluid hydrodynamic shocks are below the limit of detectability. It follows that the observation of SH+ along lines of sight where CH+ is already observed, with column densities $N(\text{CH}^+)\approx10^{13}\enspace\text {cm}^{-2}$, should provide discrimination between possible modes of shock propagation. Photoionization and photodissociation reactions are found to have a profound influence on the column densities of sulphur-bearing molecular species. Many of the rates of the photoprocesses are poorly known or estimated, and calculations or measurements of the relevant cross-sections are required.