Bottlenecks to interstellar sulfur chemistry: Sulfur-bearing hydrides in
  UV-illuminated gas and grains
  Hydride molecules lie at the base of interstellar chemistry, but the
synthesis of sulfuretted hydrides is poorly understood. Motivated by new
observations of the Orion Bar PDR - 1'' resolution ALMA images of SH+; IRAM 30m
detections of H2S, H2S34, and H2S33; H3S+ (upper limits); and SOFIA
observations of SH - we perform a systematic study of the chemistry of
S-bearing hydrides. We determine their column densities using coupled
excitation, radiative transfer as well as chemical formation and destruction
models. We revise some of the key gas-phase reactions that lead to their
chemical synthesis. This includes ab initio quantum calculations of the
vibrational-state-dependent reactions SH+ + H2 <-> H2S+ + H and S + H2 <-> SH +
H. We find that reactions of UV-pumped H2 (v>1) with S+ explain the presence of
SH+ in a high thermal-pressure gas component, P_th~10^8 cm^-3 K, close to the
H2 dissociation front. However, subsequent hydrogen abstraction reactions of
SH+, H2S+, and S with vibrationally excited H2, fail to ultimately explain the
observed H2S column density (~2.5x10^14 cm^-2, with an ortho-to-para ratio of
2.9+/-0.3). To overcome these bottlenecks, we build PDR models that include a
simple network of grain surface reactions leading to the formation of solid H2S
(s-H2S). The higher adsorption binding energies of S and SH suggested by recent
studies imply that S atoms adsorb on grains (and form s-H2S) at warmer dust
temperatures and closer to the UV-illuminated edges of molecular clouds.
Photodesorption and, to a lesser extent, chemical desorption, produce roughly
the same H2S column density (a few 10^14 cm-^2) and abundance peak (a few
10^-8) nearly independently of n_H and G_0. This agrees with the observed H2S
column density in the Orion Bar as well as at the edges of dark clouds without
invoking substantial depletion of elemental sulfur abundances.
