2024MNRAS.534.3576K

The composition of a planet's atmosphere is intricately linked to the chemical makeup of the protoplanetary disc in which it formed. Determining the elemental abundances from key volatiles within discs is therefore essential for establishing connections between the composition of discs and planets. The disc around the Herbig Ae star HD 169142 is a compelling target for such a study due to its molecule-rich nature and the presence of a newly forming planet between two prominent dust rings. In this work, we probe the chemistry of the HD 169142 disc at small spatial scales, drawing links between the composition of the disc and the planet-accreted gas. Using thermochemical models and archival data, we constrain the elemental abundances of volatile carbon, oxygen, and sulfur. Carbon and oxygen are only moderately depleted from the gas phase relative to their interstellar abundances, with the inner ∼60 au appearing enriched in volatile oxygen. The carbon-to-oxygen ratio is approximately solar within the inner disc (∼0.5) and rises above this in the outer disc (>0.5), as expected across the H$_2$O snowline. The gas-phase sulfur abundance is depleted by a factor of ∼1000, consistent with a number of other protoplanetary discs. Interestingly, the observed SiS emission near the HD 169142 b protoplanet vastly exceeds chemical model predictions, supporting previous hypotheses suggesting its origin in shocked gas or a localized outflow. We contextualize our findings in terms of the potential atmospheric composition of the embedded planet, and highlight the utility of sulfur-bearing molecules as probes of protoplanetary disc chemistry.