Missing interstellar sulfur in inventories of polysulfanes and molecular octasulfur crowns

Introduction

Over the last decades, the interstellar sulfur depletion problem, i.e., astronomical observations of lower-than-expected sulfur abundances in dense molecular clouds, represents a fundamental, unresolved dilemma in the fields of astronomy and astrochemistry^1,2. Although the exact degree of sulfur depletion in interstellar regions is still a matter of vigorous debate^3,4, in molecular clouds such as the Taurus Molecular Cloud (TMC-1), sulfur is observed at fractional abundances some three orders of magnitude less in comparison to cosmic abundances^2,5,6. However, in the diffuse regions of space, sulfur is only mildly depleted by ~30% such as toward GX 5-1, a low mass X-ray binary near the galactic center, or the observed amounts of sulfur match cosmic abundances, such as those observed in the local part of our Galaxy^{3,5,7, 8–9}. Astronomers and astrochemists theorize that this missing gas-phase sulfur is likely deposited onto interstellar nanoparticles, also referred to as carbonaceous and silicate dust particles (or simply “interstellar grains”)^8,10, but both observational information and experimental evidence are lacking.

Since the detection of the very first interstellar sulfur-containing molecule, carbon monosulfide (CS) in 1971¹¹, ~40 sulfur-containing molecules have been identified in the interstellar medium (ISM) (Fig. 1)¹². In the gas phase, these molecules range from simple triatomics such as hydrogen sulfide (H₂S) to thiols (RSH), thioaldehydes (HCSR), thioketenes (RCCS), thioacids (HSOCR), and thioamides (RCSNR’) with R and R’ being organic side chains. Recent mid-infrared observations with the James Webb Space Telescope (JWST) provided fundamental compositional insights of the icy grains in dense molecular clouds, such as in pristine cloud ices toward two background stars NIR38 and J110621¹³, where carbonyl sulfide (OCS) and possibly sulfur dioxide (SO₂) have been detected. Water is ubiquitously found in the interstellar ices amongst the other major ice species: carbon monoxide (CO), carbon dioxide (CO₂) and ammonia (NH₃)¹⁴. Although the simplest sulfur-containing molecule: hydrogen sulfide (H₂S) has not been observed yet in interstellar ices via infrared observation^{13, 14–15}, chemical models predict that hydrogen sulfide is present in these ices¹⁶. Since infrared interstellar ice analyses displayed evidence for compounds more volatile than hydrogen sulfide (such as...