Volatile Chemical Evolution in Planet-Forming Disks

The volatile composition of a planet is determined by the inventory of gas and ice in the parent disk. The evolution from an embedded (protostellar) disk to a protoplanetary disk is accompanied by large changes in the disk physics, which should in turn have important consequences for the chemistry. However, the chemical effects of disk evolution are poorly constrained observationally. I will present ALMA observations of molecular tracers (CO, C₂H, and HCN isotopologues) towards a sample of 19 disks spanning ages of ~0.2-14 Myr, offering a view of volatile chemical evolution over the disk lifetime. I will discuss the timescales for CO depletion from the disk surface layers inferred from our observations, and how the relationship between C₂H, HCN, and CO can shed light on the C/O ratio in the disk molecular layer. I will also highlight how the volatile chemistry is impacted by the distinct physical environment in embedded sources compared to more evolved disks. Understanding such chemical evolution is an important step in predicting how the volatile composition of planet-forming material changes at different stages in the disk lifetime.