Theoretical modelling of the adsorption of neutral and charged sulphur-bearing species on to olivine nanoclusters

Abstract

Sulphur depletion in the interstellar medium (ISM) is a long-standing issue, as only 1% of its cosmic abundance is detected in dense molecular clouds (MCs), while it does not appear to be depleted in other environments. In addition to gas phase species, MCs also contain interstellar dust grains, which are irregular, micron-sized, solid aggregates of carbonaceous materials and/or silicates. Grains provide a surface where species can meet, accrete, and react. Although freeze-out of sulphur onto dust grains could explain its depletion, only OCS and, tentatively, SO${}_2$ were observed on their surfaces. Therefore, it is our aim to investigate the interaction between sulphur-containing species and the exposed mineral core of the grains at a stage prior to when sulphur depletion is observed. Here, the grain core is represented by olivine nanoclusters, one of the most abundant minerals in the ISM, with composition Mg${}_4$Si${}_2$O${}_8$ and Mg${}_3$FeSi${}_2$O${}_8$. We performed a series of quantum mechanical calculations to characterize the adsorption of 9 S-bearing species, both neutral and charged, onto the nanoclusters. Our calculations reveal that the Fe-S interaction is preferred to Mg-S, causing sometimes the chemisorption of the adsorbate. These species are more strongly adsorbed on the bare dust grain silicate cores than on water ice mantles, and hence therefore likely sticking on the surface of grains forming part of the grain core. This demonstrates that the interaction of bare grains with sulphur species in cloud envelopes can determine the S-depletion observed in dense molecular clouds.

9 pages, 4 figures, Accepted for publication in Monthly Notices of the Royal Astronomical Society

References in corpus (11)

• A Unified Representation of Gas-Phase Element Depletions in the Interstellar Medium
• An Ice Age JWST inventory of dense molecular cloud ices
• Modeling Sulfur Depletion in Interstellar Clouds
• UCLCHEM: A Gas-Grain Chemical Code
• Sensitivity analysis of grain surface chemistry to binding energies of ice species
• Sulphur-bearing molecules in diffuse molecular clouds: new results from SOFIA/GREAT and the IRAM 30 m telescope
• A new study of an old sink of sulfur in hot molecular cores: the sulfur residue
• Bottlenecks to interstellar sulfur chemistry: Sulfur-bearing hydrides in UV-illuminated gas and grains
• Diffusion of CH${}_4$ in amorphous solid water
• Dust charge distribution in the interstellar medium
• APEX and NOEMA observations of H${}_2$S in nearby luminous galaxies and the ULIRG Mrk~231 -- Is there a relation between dense gas properties and molecular outflows?