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

TL;DR

This study investigates how doubly ionized sulfur chemistry influences the formation and destruction of SH+ and S^+2 in X-ray dominated regions, revealing significant increases in SH+ abundance and implications for astrophysical diagnostics.

Contribution

It introduces the impact of increased S^+2 + H2 reaction rates on sulfur chemistry in XDRs, highlighting the dominant pathway to SH+ formation and its observational consequences.

Findings

SH+ column density can increase by up to 2 dex near AGNs.

Doubly ionized sulfur chemistry can dominate SH+ formation if the branching ratio exceeds a few percent.

High S^+2 + H2 reaction rates lead to efficient destruction of S^+2 in molecular regions.

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.