Keyte, Luke; (2024) Abundance and Distribution of Volatiles in Planet-Forming Disks. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis explores the fundamental relationship between the physical and chemical properties of protoplanetary disks and the exoplanetary systems that form within them. High-resolution submillimeter observations from Atacama Large Millimeter/submillimeter Array (ALMA) have revolutionised our ability to study these disks in unprecedented detail. By leveraging ALMA observations and sophisticated thermochemical modelling, this work constrains the composition of gas potentially accreted by nascent planets, establishing a direct link between their atmospheric composition and birth environment. Firstly, the abundances and distribution of volatile carbon and oxygen in the HD 100546 protoplanetary disk are characterised. Analysis of ALMA observations of emission from CS and SO is used to trace the C/O ratio of the gas, a well-established metric for tracing a planet’s formation location. Azimuthal variations in the emission from these sulfur-bearing molecules are shown to originate from an azimuthal variation in C/O ratio itself. This finding contributes a new layer of complexity to the understanding of factors influencing exoplanetary atmospheric composition. Next, ALMA observations of sulfur-bearing species in HD 100546 are again utilised, this time to tightly constrain the total abundance of volatile sulfur at scales relevant to planetary formation. This work addresses the longstanding ‘missing sulfur problem’ in planet-forming environments, where significant depletion of gas-phase sulfur is observed. A depletion factor of ∼ 1000 for gas-phase sulfur is revealed in this system, with radial variations exceeding three orders of magnitude. These variations are linked to the location of prominent dust rings and gaps within the disk. It is further shown that the vast majority of elemental sulfur is sequestered in refractory materials, with less ≲ 5% residing in the gas and ices combined. Finally, the volatile composition of the planet-hosting disk HD 169142 is investigated. Carbon and oxygen abundances are shown to be generally not depleted compared to other protoplanetary disk systems. Depletion of sulfur is observed at a level comparable to HD 100546. Significantly, the detection of SiS emission from this disk constitutes the major finding of this work. The detection of SiS is indicative of shocked gas near the embedded protoplanet, potentially providing crucial insights into the dynamical processes shaping the early environment of this planetary system.

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