Background
Sulfur is one of the most abundant elements in the Universe and plays a crucial role in both astrochemistry and biology. Yet observations reveal a puzzling trend: while gas-phase sulfur exists at near-cosmic levels in the interstellar medium, it becomes severely depleted in colder, denser star-forming environments. This "missing sulfur" represents one of the major outstanding questions in astrochemistry.
Protoplanetary disks offer a unique laboratory for investigating this depletion process. However, quantifying the gas-phase sulfur content and identifying the major molecular carriers in these environments has proven challenging, requiring both high-sensitivity observations and sophisticated chemical modelling.
Case Study: HD 100546
To investigate the missing sulfur problem, we conducted a detailed analysis of the protoplanetary disk around HD 100546 - a young star where planets are currently being formed. By combining observations from ALMA and APEX telescopes, we mapped the distribution of multiple sulfur-bearing molecules throughout the disk.
Our observations, interpreted through sophisticated thermochemical models, revealed a dramatic depletion of gas-phase sulfur. Compared to the interstellar medium, 99.9% of the sulfur appears to be missing from the gas phase. Even more intriguingly, we found that this depletion isn't uniform - the abundance of gas-phase sulfur varies by over three orders of magnitude between different regions of the disk.
This complex chemical structure is illustrated in the Figure below, which presents modelled abundances for eight key sulfur-bearing molecules across the disk. The abundance patterns show clear correlations with disk features like dust rings and gaps, suggesting a potential connection between the disk's physical evolution and its sulfur chemistry.
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Figure 1: Modelled gas-phase abundances for 8 key sulfur-bearing molecules in HD 100546
Understanding the Sulfur Reservoirs
Our modelling approach allowed us to disentangle how sulfur is distributed between different physical states in the disk. By extracting molecular abundances from our chemical models, we could directly measure sulfur in both gas and ice phases. The remaining "missing" sulfur - the difference between these volatile components and the cosmic abundance - was presumed to be locked in refractory materials like iron sulfide.
As shown in the Figure below, this analysis revealed striking vertical and radial patterns. In the disk midplane, almost all volatile sulfur exists as ices beyond 30 AU, with gas-phase species becoming dominant only in the warm inner cavity. The disk's atmosphere shows a different pattern - while most material remains in refractory form, there's a transition from gas to ice around 60 AU, driven by changes in UV irradiation and temperature. In total, we found that volatile sulfur (gas + ice) accounts for only 0.01-5.3% of the total sulfur budget, depending on location, with the vast majority presumably locked away in refractory materials.
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Figure 2: Split between gas/ice/refractory sulfur as a function of radius, for the midplane (top panel) and disk atmosphere (bottom panel)
Broader Implications
Our study provides new insight into a long-standing puzzle in astrochemistry. The extreme depletion we observed - with only ~0.01-5.3% of sulfur in volatile form - combined with its correlation with disk structure, suggests that the incorporation of sulfur into planetary atmospheres and cores is highly complex. While interpreting sulfur abundances in exoplanet atmospheres (as observed by JWST) remains challenging, the dramatic radial variations we uncovered suggest that atmospheric sulfur content could serve as a powerful diagnostic of where and how a planet formed within its natal disk.
Future Directions
Our work with HD 100546 represents just one piece of the missing sulfur puzzle, but opens several promising avenues for future investigation: - Extending this analysis to a larger sample of protoplanetary disks - Understanding the chemical pathways that transform volatile sulfur into refractory forms - Connecting disk sulfur content to the atmospheric composition of exoplanets
This research was originally published in MNRAS (Keyte et al. 2024; "Spatially resolving the volatile sulfur abundance in the HD 100546 protoplanetary disc")