The Effects of Doubly Ionized Chemistry on SH+ and S^+2 Abundances in X-ray Dominated Regions

Details The Effects of Doubly Ionized Chemistry on SH+ and S^+2 Abundances in X-ray Dominated Regions The Effects of Doubly Ionized Chemistry on SH+ and S^+2 Abundances in X-ray Dominated Regions

2008-01-24

arxiv.org/abs/0801.3833v1

Astronomy and Astrophysics

Astrophysics

19 pages, 3 figures, Accepted for Publication in ApJ Letters

Scientific paper

10.1086/533465

Recent laboratory measurements for the S^+2 + H2 reaction find a total rate coefficient significantly larger than previously used in theoretical models of X-ray dominated regions (XDRs). While the branching ratio of the products is unknown, one energetically possible route leads to the SH+ molecule, a known XDR diagnostic. In this work, we study the effects of S^+2 on the formation of SH+ and the destruction of S^+2 in XDRs. We find the predicted SH+ column density for molecular gas surrounding an Active Galactic Nucleus (AGN) increases by as much as 2 dex. As long as the branching ratio for S^+2 + H2 -> SH+ + H+ exceeds a few percent, doubly ionized chemistry will be the dominant pathway to SH+, which then initiates the formation of other sulfur-bearing molecules. We also find that the high rate of S^+2 + H2 efficiently destroys S^+2 once H2 forms, while the S^+2 abundance remains high in the atomic hydrogen region. We discuss the possible consequences of S^+2 in the atomic hydrogen region on mid-infrared diagnostics. The enhanced SH+ abundance has important implications in the study of XDRs, while our conclusions for S^+2 could potentially impact the interpretation of Spitzer and SOFIA observations.

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