Knowledge Bank

A. Dalgarno first suggested thirty-two years ago that doubly-ionized $X2+$ elements reacting with H$2$ could lead to the formation of molecules in regions with a significant X-ray flux. However, few investigations of the effects of $X2+$ on the formation of molecules have actually been conducted. Recent studies have found the reaction rate coefficients to be orders of magnitude higher than previously thought. This fact, combined with a renewed interest in the physics and chemistry of X-ray Dominated Regions (XDRs), points to a need to study the effects of $X2+$ on molecule formation. We investigated how S$\mathrm{<mrow\; data-mjx-texclass="ORD">2+</mrow>}$ + H$2$ affects the formation of SH$\mathrm{<mrow\; data-mjx-texclass="ORD">+</mrow>}$ for atomic gas exposed to an X-ray continuum typical of an Active Galactic Nucleus (AGN). As long as the fraction of S$\mathrm{<mrow\; data-mjx-texclass="ORD">2+</mrow>}$ + H$2$ reactions which lead to SH$\mathrm{<mrow\; data-mjx-texclass="ORD">+</mrow>}$ formation exceeds a few percent, this process will be the dominant formation pathway for SH$+$. This potentially has major consequences for our understanding of sulfur chemistry in galaxies where the energetics is dominated by AGN activity.