Efficient Production of S$_8$ in Interstellar Ices: The effects of
  cosmic ray-driven radiation chemistry and non-diffusive bulk reactions
  In this work, we reexamine sulfur chemistry occurring on and in the ice
mantles of interstellar dust grains, and report the effects of two new
modifications to standard astrochemical models; namely, (a) the incorporation
of cosmic ray-driven radiation chemistry and (b) the assumption of fast,
non-diffusive reactions for key radicals in the bulk. Results from our models
of dense molecular clouds show that these changes can have a profound influence
on the abundances of sulfur-bearing species in ice mantles, including a
reduction in the abundance of solid-phase H$_2$S and HS, and a significant
increase in the abundances of OCS, SO$_2$, as well as pure allotropes of
sulfur, especially S$_8$. These pure-sulfur species - though nearly impossible
to observe directly - have long been speculated to be potential sulfur
reservoirs and our results represent possibly the most accurate estimates yet
of their abundances in the dense ISM. Moreover, the results of these updated
models are found to be in good agreement with available observational data.
Finally, we examine the implications of our findings with regard to the
as-yet-unknown sulfur reservoir thought to exist in dense interstellar
environments.
