An evolutionary study of volatile chemistry in protoplanetary disks

TL;DR

This study uses ALMA observations to investigate how volatile chemistry in protoplanetary disks evolves over time, revealing rapid CO depletion early in disk development and chemical differences between early and later stages that influence planet formation.

Contribution

It provides the first observational evidence of volatile chemical evolution across different disk stages, highlighting the impact of envelope shielding and sublimation on disk chemistry.

Findings

Rapid CO depletion within 0.5-1 Myr of disk age

No enhancement of C2H and HCN formation despite CO depletion

Chemical differences between Class 0/I and Class II disks due to envelope effects

Abstract

The volatile composition of a planet is determined by the inventory of gas and ice in the parent disk. The volatile chemistry in the disk is expected to evolve over time, though this evolution is poorly constrained observationally. We present ALMA observations of C18O, C2H, and the isotopologues H13CN, HC15N, and DCN towards five Class 0/I disk candidates. Combined with a sample of fourteen Class II disks presented in Bergner et al. (2019b), this data set offers a view of volatile chemical evolution over the disk lifetime. Our estimates of C18O abundances are consistent with a rapid depletion of CO in the first ~0.5-1 Myr of the disk lifetime. We do not see evidence that C2H and HCN formation are enhanced by CO depletion, possibly because the gas is already quite under-abundant in CO. Further CO depletion may actually hinder their production by limiting the gas-phase carbon supply. The embedded sources show several chemical differences compared to the Class II stage, which seem to arise from shielding of radiation by the envelope (impacting C2H formation and HC15N fractionation) and sublimation of ices from infalling material (impacting HCN and C18O abundances). Such chemical differences between Class 0/I and Class II sources may affect the volatile composition of planet-forming material at different stages in the disk lifetime.