双电离化学对X射线优势区SH +和S^+2丰度的影响
The Effects of Doubly Ionized Chemistry on SH+ and S^+2 Abundances in X-ray Dominated Regions
摘要：
最近对S^+2 + H2反应的实验室测量发现，总速率系数明显大于以前在X射线主导区（XDR）理论模型中使用的速率系数。虽然产物的分支比是未知的，但一种能量上可能的途径导致SH+分子，一种已知的XDR诊断剂。在这项工作中，我们研究了S^+2对XDR中SH+的形成和S^+2的破坏的影响。我们发现预测的SH+柱密度的分子气体周围的活动星系核（AGN）增加多达2倍。只要S^+2 + H2 -> SH+ + H+的分支比超过百分之几，双电离化学将成为形成SH+的主要途径，然后引发其他含硫分子的形成。我们还发现，一旦H2形成，S^+2 + H2的高速率会有效地破坏S^+2，而S^+2的丰度在氢原子区域仍然很高。我们讨论了氢原子区的S^+2对中红外诊断的可能影响。SH+丰度的增加对XDR的研究具有重要意义，而我们对S^+2的结论可能会影响斯皮策和SOFIA观测结果的解释。
英文摘要：
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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