Modeling sulfur depletion in interstellar clouds

Abstract

Context. The elemental depletion of interstellar sulfur from the gas phase has been a recurring challenge for astrochemical models. Observations show that sulfur remains relatively non-depleted with respect to its cosmic value throughout the diffuse and translucent stages of an interstellar molecular cloud, but its atomic and molecular gas-phase constituents cannot account for this cosmic value towards lines of sight containing higher-density environments. Aims. We have attempted to address this issue by modeling the evolution of an interstellar cloud from its pristine state as a diffuse atomic cloud to a molecular environment of much higher density, using a gas/grain astrochemical code and an enhanced sulfur reaction network. Methods. A common gas/grain astrochemical reaction network has been systematically updated and greatly extended based on previous literature and previous sulfur models, with a focus on the grain chemistry and processes. A simple astrochemical model was used to benchmark the resulting network updates, and the results of the model were compared to typical astronomical observations sourced from the literature. Results. Our new gas/grain astrochemical model is able to reproduce the elemental depletion of sulfur, whereby sulfur can be depleted from the gas-phase by two orders of magnitude, and that this process may occur under dark cloud conditions if the cloud has a chemical age of at least 10 6 years. The resulting mix of sulfur-bearing species on the grain ranges across all the most common chemical elements (H/C/N/O), not dissimilar to the molecules observed in cometary environments. Notably, this mixture is not dominated simply by H 2 S, unlike all other current astrochemical models. Conclusions. Despite our relatively simple physical model, most of the known gas-phase S-bearing molecular abundances are accurately reproduced under dense conditions, however they are not expected to be the primary molecular sinks of sulfur. Our model predicts that most of the "missing" sulfur is in the form of organo-sulfur species trapped on grains.

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

“…C.N.S. thanks the Alexander von Humboldt Foundation for their generous support and both K. Laas & Caselli (2019) and Semenov et al (2010). Table 7.…”

“…In order to investigate the role of radiation chemistry and non-diffusive bulk reactions on the abundances of sulfur species in dense interstellar regions, we have run simulations of generic cold cores using physical conditions and initial elemental abundances taken from Laas & Caselli (2019), and listed in Tables 2 and 3. We have carried out five sets of simulations, as listed in Table 4.…”

Gas phase Elemental abundances in Molecular cloudS (GEMS)

“…Sulphur abundance. A large theoretical and observational effort has been done in the last five years to understand the sulphur chemistry that is progressively leading to a new paradigm (Vidal et al 2017;Fuente et al 2016Fuente et al , 2019Le Gal et al 2019;Laas & Caselli 2019). Gas-grain models provide a new vision of the interstellar medium in which both gaseous and ice species are predicted.…”

“…One first result is that our models overestimate by more than one order of magnitude the CS abundance in TMC 1 and Barnard 1b. The problem of the overestimation of the CS abundance was already pointed out by Vidal et al (2017) and Laas & Caselli (2019). Vidal et al (2017) suggested that it could come from wrong branching ratios of the dissociative recombination of HCS + or a potentially new sink for CS that might be the O + CS reaction whose reaction rate has not been measured at low temperatures.…”

Rotational spectrum of isotopic methyl mercaptan, 13CH3SH, in the laboratory and towards Sagittarius B2(N2)

“…However, the chemistry in star-forming regions has been poorly understood at least until quite recently because the abundances of the observed sulphurbearing molecules in these sources accounted only for a small fraction of the total sulphur budget whose depletion into dust grains has been poorly constrained [5−7]. Considerable progress has been made over the years, particularly in a recent contribution [8].…”