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.

中文翻译：

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原行星盘挥发性化学的演化研究

行星的挥发性成分由母盘中的气体和冰的存量决定。盘中的挥发性化学物质预计会随着时间的推移而演变，尽管这种演变在观察上受到的限制很差。我们展示了 ALMA 对 C18O、C2H 和同位素体 H13CN、HC15N 和 DCN 对五个 0/I 类磁盘候选者的观察结果。结合 Bergner 等人提出的 14 个 II 类磁盘样本。(2019b)，该数据集提供了磁盘寿命期间挥发性化学演化的视图。我们对 C18O 丰度的估计与在磁盘寿命的第一个 ~0.5-1 Myr 中快速消耗 CO 一致。我们没有看到证据表明 CO 消耗会增强 C2H 和 HCN 的形成，这可能是因为气体中的 CO 已经相当不足。进一步的 CO 消耗实际上可能会通过限制气相碳供应来阻碍它们的生产。与 II 级阶段相比，嵌入的源显示出一些化学差异，这似乎是由于外壳对辐射的屏蔽（影响 C2H 形成和 HC15N 分馏）和来自落入材料的冰升华（影响 HCN 和 C18O 丰度）。0/I 类和 II 类源之间的这种化学差异可能会影响行星形成材料在磁盘寿命不同阶段的挥发性成分。这似乎是由于外壳对辐射的屏蔽（影响 C2H 形成和 HC15N 分馏）和冰从下落物质升华（影响 HCN 和 C18O 丰度）。0/I 类和 II 类源之间的这种化学差异可能会影响行星形成材料在磁盘寿命不同阶段的挥发性成分。这似乎是由于外壳对辐射的屏蔽（影响 C2H 形成和 HC15N 分馏）和冰从下落物质升华（影响 HCN 和 C18O 丰度）。0/I 类和 II 类源之间的这种化学差异可能会影响行星形成材料在磁盘寿命不同阶段的挥发性成分。