Pure sulfur is known to form several allotropes, which have lately been proposed as possible sulfur reservoir in cold clouds, where gas-phase sulfur depletion is observed. Indeed, larger species show stronger interactions with the water ice mantles of dust grains. However, their spectroscopic features are more suitable to their detection in terrestrial laboratories rather than in the interstellar medium.
In the denser and colder regions of the interstellar medium (ISM), gas-phase sulfur is depleted by 2 or 3 orders of magnitude with respect to its cosmic abundance. Thus, which species are the main carriers of sulfur is an open question. Recent studies have proposed Sn species as potential sulfur reservoirs. Among the various sulfur allotropes, the most stable one is the S8 ring, detected in the asteroid Ryugu and Orgueil meteorite. Shorter species, namely S3 and S4, have been found in the comet 67P/C-G, but their presence in the ISM remains elusive. In this study, we compute the binding energies (BEs) of Sn (n = 1–8) species on an amorphous water-ice surface model and analyze their infrared (IR) and Raman spectral features to provide data for their identification in the ISM. Our computations reveal that these species exhibit lower BEs than previously assumed and that their spectral features experience minimal shifts when adsorbed on water ice, because of the weak and nonspecific Sn–ice interactions. Furthermore, these species display very low IR band intensities and, therefore, very accurate instruments operating in the mid-IR range are required for detecting the presence of these species in dense interstellar environments.
This work has been published in The Astrophysical Journal.
Link to the article as open access in ApJ: https://iopscience.iop.org/article/10.3847/1538-4357/ad5548