The Effects of Doubly Ionized Chemistry on SH + and S +2 Abundances in X-Ray-dominated Regions
- 11 February 2008
- journal article
- Published by American Astronomical Society in The Astrophysical Journal
- Vol. 675 (2), L81-L84
- https://doi.org/10.1086/533465
Abstract
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.Comment: 19 pages, 3 figures, Accepted for Publication in ApJ LetterKeywords
This publication has 27 references indexed in Scilit:
- Mid‐Infrared Fine‐Structure Line Ratios in Active Galactic Nuclei Observed with theSpitzerIRS: Evidence for Extinction by the TorusThe Astrophysical Journal, 2007
- The H ii Region/PDR Connection: Self‐consistent Calculations of Physical Conditions in Star‐forming RegionsThe Astrophysical Journal Supplement Series, 2005
- Charge transfer between and He, and CO at electron-volt energiesPhysical Review A, 2003
- Thermal energy charge transfer betweenS2+andH2,N2,and COPhysical Review A, 2003
- CLOUDY 90: Numerical Simulation of Plasmas and Their SpectraPublications of the Astronomical Society of the Pacific, 1998
- An Atlas of Computed Equivalent Widths of Quasar Broad Emission LinesThe Astrophysical Journal Supplement Series, 1997
- High-Resolution Optical and Ultraviolet Absorption-Line Studies of Interstellar GasAnnual Review of Astronomy and Astrophysics, 1986
- The formation of molecules in interstellar clouds from singly and multiply ionized atomsThe Astrophysical Journal, 1978
- A survey of interstellar H I from L-alpha absorption measurements. IIThe Astrophysical Journal, 1978
- The interstellar molecules CH and CH+Published by Elsevier BV ,1976