An evolutionary study of volatile chemistry in protoplanetary disks
  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.
