Bottlenecks to interstellar sulfur chemistry

Hydride molecules lie at the base of interstellar chemistry, but synthesis sulfuretted hydrides is poorly understood and their abundances often crudely constrained. Motivated by new observations Orion Bar photodissociation region (PDR) – 1? resolution ALMA images SH + ; IRAM 30 m detections bright H 2 32 S, 34 33 S lines; 3 (upper limits); SOFIA/GREAT limits) we perform a systematic study chemistry sulfur-bearing hydrides. We self-consistently determine column densities using coupled excitation, radiative transfer as well chemical formation destruction models. revise some key gas-phase reactions that lead to synthesis. This includes ab initio quantum calculations vibrational-state-dependent ( v ) ? H. find UV-pumped ? 2) with ions explain presence in high thermal-pressure gas component, P th ? k ? 10 8 cm ?3 K, close dissociation front (at A V < mag). These PDR layers are characterized no or very little depletion elemental sulfur from gas. However, subsequent hydrogen abstraction , atoms vibrationally excited fail form enough ultimately observed density (~2.5 × 14 ?2 an ortho-to-para ratio 2.9 ± 0.3; consistent high-temperature statistical value). To overcome these bottlenecks, build models include simple network grain surface leading solid (s-H S). The higher adsorption binding energies suggested recent studies imply adsorb on grains (and s-H S) warmer dust temperatures T d 50 K) closer UV-illuminated edges molecular clouds. show everywhere mantles form(ed), emission lines will be detectable. Photodesorption and, lesser extent, desorption, produce roughly same (a few abundance peak ?8 nearly independently n G 0 . agrees dark clouds without invoking substantial abundances.