Abstract
Rotational excitation of the carbon monosulphide (CS) molecule by thermal electron-impact is studied using the molecular R-matrix method combined with the adiabatic-nuclei-rotation (ANR) approximation. Rate coefficients are obtained for electron temperatures in the range 5-5000 K and for transitions involving levels up to J = 40. It is confirmed that dipole allowed transitions (Delta J = 1) are dominant and that the corresponding rate coefficients exceed those for excitation by neutrals by at least five orders of magnitude. As a result, the present rates should be included in any detailed population model of CS in sources where the electron fraction is larger than similar to 10(-5), in particular in diffuse molecular clouds and interstellar shocks.
Highlights
Carbon monosulphide (CS) was discovered in the interstellar medium by Penzias et al (1971) through the J = 3 → 2 millimeter emission line
The present rates should be included in any detailed population model of carbon monosulphide (CS) in sources where the electron fraction is larger than ∼10−5, in particular in diffuse molecular clouds and interstellar shocks
As explained by these authors, the CS abundance is consistent with the observed amounts of HCS+, but the abundances of the expected precurors of the ion are 1–2 orders of magnitude too low in diffuse clouds, where the electron fraction is large
Summary
Along with other sulphur-bearing molecules, CS is an interesting species because gas-phase chemistry models fail to reproduce its abundance by large factors, both in dark and diffuse gas (Lucas & Liszt 2002, and references therein). As explained by these authors, the CS abundance is consistent with the observed amounts of HCS+ (if CS is formed via dissociative recombination), but the abundances of the expected precurors of the ion are 1–2 orders of magnitude too low in diffuse clouds, where the electron fraction is large.
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