Efficient Production of S₈ in Interstellar Ices: The Effects of Cosmic-Ray-driven Radiation Chemistry and Nondiffusive Bulk Reactions

Abstract

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 Corresponding author: Christopher N. Shingledecker cns@mpe.mpg.de Shingledecker et al. et al. (2016). In that work, the authors concluded that this S 2 likely formed in the pre-solar nebula via the radiolysis of species such as H 2 S. In an attempt to improve how astrochemical models treat non-thermal processes, particularly those driven by cosmic rays, we have recently developed methods for including such radiation chemistry in rate-equation-based codes . Our preliminary findings are that the addition of these new mechanisms generally improves the agreement between models and observations (Shingledecker et al. 2018); however, given the novelty of our approach, no modeling studies have yet been done which focus on the effects of cosmic raybombardment on the abundances of sulfur-bearing species in ice mantles. CalmonteAnother major source of uncertainty in current simulations of grain chemistry concerns reactions within the bulk of dust-grain ice mantles. One significant source of uncertainty in such models involves whether or to what degree bulk diffusion is important, and if so, what the underlying mechanism behind this diffusion might be. Commonly, models today typically assume, for example, swapping (Öberg et al. 2009; Fayolle et al. 2011) or diffusion via interstitial sites (Lamberts et al. 2013; Chang & Herbst 2014; Shingledecker et al. 2017), where the energetic barriers to bulk diffusion, E bulk b, are taken to be some fraction of the desorption energy, E D , and are highly uncertain. In Shingledecker et al. (2019a), we attempted to reduce this ambiguity by simulating well-constrained experiments, rather than the ISM. In our preliminary studies reported there, we found that the assumption that radicals within ices react via thermal diffusion leads to generally poor agreement between calculated and empirical results, due to the much slower chemistry in the simulations than what is shown to occur in the lab. Conversely, good agreement with experimental data was achieved by assuming that radicals in the ice react predominantly with their nearest neighbors, i.e. nondiffusively. These results are in qualitative agreement with a recent study by Ghesquière et al. (2018), who concluded that true bulk diffusion does not occur, rather, as temperatures increase, bulk species can be "passively" transported due to structural changes such as pore collapse or crystallization, or can "actively" diffuse along internal surfaces or cracks.This work, therefore, is an attempt to build upon these recent investigations, and to examine what effect (a) cosmic ray-driven radiation chemistry, and (b) the fast, non-diffusive reaction of radicals in

The Effect of Cosmic Rays on Cometary Nuclei. II. Impact on Ice Composition and Structure

“…First, radicals and reactive species produced through the radiolysis of molecules by GCRs can diffuse before reacting with neighboring species. However, recent simulations of interstellar ice chemistry incorporating cosmic-ray-driven radiation chemistry indicate that radicals and reactive species quickly react with neighboring species in the ice (Shingledecker et al 2019(Shingledecker et al , 2020, in agreement with experiments that provided no evidence for true bulk diffusion (Ghesquière et al 2018). Then, once secondary species are formed, they can diffuse within the nucleus.…”

Efficient Methanol Production on the Dark Side of a Prestellar Core

“…According to modeling results of Laas & Caselli (2019), the principal sulfur-bearing species in the ice at advanced chemical stages are SO, OCS, HSO, and CS. Also, pure-sulfur molecules, in particular S 8 , have been suggested to be abundant on grains (Shingledecker et al 2020). Methanol and sulfur monoxide are not directly related.…”

Systematic investigation of CO2 : NH3 ice mixtures using mid-IR and VUV spectroscopy – part 1: thermal processing

“…In non-astrophysical solids, it is well known that diffusion along crystallite grain boundaries is generally enhanced compared to the crystal or mineral equivalent. 32,33 Experimental data 34 and astrochemical models 35,36 have suggested that structural diffusion within the bulk ice may actually enhance reactivity. This would suggest that CO 2 diffusion along the crystallite grain boundaries was higher in the NH 3 -rich mixtures which may also have increased reactivity.…”