Spatially resolving the volatile sulfur abundance in the HD 100546 protoplanetary disc

Abstract

Volatile elements play a crucial role in the formation of planetary systems. Their abundance and distribution in protoplanetary disks provide vital insights into the connection between formation processes and the atmospheric composition of individual planets. Sulfur, being one of the most abundant elements in planet-forming environments, is of great significance, and now observable in exoplanets with JWST. However, planetary formation models currently lack vital knowledge regarding sulfur chemistry in protoplanetary disks. Developing a deeper understanding of the major volatile sulfur carriers in disks is essential to building models that can meaningfully predict planetary atmospheric composition, and reconstruct planetary formation pathways. In this work, we combine archival observations with new data from ALMA and APEX, covering a range of sulfur-bearing species/isotopologs. We interpret this data using the dali thermo-chemical code, for which our model is highly refined and disk-specific. We find that volatile sulfur is heavily depleted from the cosmic value by a factor of ∼1000, with a disk-averaged abundance of S/H ∼10−8. We show that the gas-phase sulfur abundance varies radially by ≳ 3 orders of magnitude, with the highest abundances inside the inner dust ring and coincident with the outer dust ring at r ∼ 150 to 230 au. Extracting chemical abundances from our models, we find OCS, H2CS, and CS to be the dominant molecular carriers in the gas phase. We also infer the presence of a substantial OCS ice reservoir. We relate our results to the potential atmospheric composition of planets in HD 100546, and the wider exoplanet population.