Reconstructing the history of water ice formation from HDO/H2O and D2O/HDO ratios in protostellar cores
Reconstructing the history of water ice formation from HDO/H2O and D2O/HDO ratios in protostellar cores
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从原恒星核心 HDO/H2O 和 D2O/HDO 比率重建水冰形成历史
DOI：
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发表时间：
2015
期刊：
影响因子：
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通讯作者：
Yuri Aikawa
中科院分区：
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作者：
Kenji Furuya;E. Dishoeck;Yuri Aikawa
最近的干涉仪观测发现，在低质量原恒星NGC 1333-IRAS 2A附近，水冰已经升华，D2 O/HDO丰度比HDO/H2O高出约一个数量级。以往的实验室和理论研究表明，如果HDO和D2 O冰与H2O冰同时形成，则D2 O/HDO冰比应低于HDO/H2O冰比。在这项工作中，我们提出观察到的特征，D2 O/HDO > HDO/H2O，是低质量星星形成早期冷阶段化学演化的自然结果：1）大部分氧气被锁定在分子云中的水冰和其他分子中，其中水氘化效率低下，2）在冷的星前/原恒星核心中，水冰的形成继续进行，但效率大大降低，由于H2的邻对位比下降、紫外辐射场较弱等原因，氘化过程高度增强。使用简单的分析模型和气冰天体化学模拟追踪从分子云形成到原恒星核心的演变，我们表明，提出的情景可以定量解释观察到的HDO/H2O和D2 O/HDO比。我们还发现，大多数的HDO和D2 O冰可能形成在寒冷的星前/原恒星的核心，而不是在分子云，其中大多数的H2O冰形成。这项工作展示了权力的组合的HDO/H2O和D2 O/HDO的比率作为一种工具，以揭示过去的历史，水冰形成的早期冷阶段的星星形成时，氘富集在大部分的水发生。需要进一步的观测来探索D2 O/HDO > HDO/H2O的关系是否在低质量的原恒星源中普遍存在。
Recent interferometer observations have found that the D2O/HDO abundance ratio is higher than that of HDO/H2O by about one order of magnitude in the vicinity of low-mass protostar NGC 1333-IRAS 2A, where water ice has sublimated. Previous laboratory and theoretical studies show that the D2O/HDO ice ratio should be lower than the HDO/H2O ice ratio, if HDO and D2O ices are formed simultaneously with H2O ice. In this work, we propose that the observed feature, D2O/HDO > HDO/H2O, is a natural consequence of chemical evolution in the early cold stages of low-mass star formation: 1) majority of oxygen is locked up in water ice and other molecules in molecular clouds, where water deuteration is not efficient, and 2) water ice formation continues with much reduced efficiency in cold prestellar/protostellar cores, where deuteration processes are highly enhanced due to the drop of the ortho-para ratio of H2, the weaker UV radiation field, etc. Using a simple analytical model and gas-ice astrochemical simulations tracing the evolution from the formation of molecular clouds to protostellar cores, we show that the proposed scenario can quantitatively explain the observed HDO/H2O and D2O/HDO ratios. We also find that the majority of HDO and D2O ices are likely formed in cold prestellar/protostellar cores rather than in molecular clouds, where the majority of H2O ice is formed. This work demonstrates the power of the combination of the HDO/H2O and D2O/HDO ratios as a tool to reveal the past history of water ice formation in the early cold stages of star formation and when the enrichment of deuterium in the bulk of water occurred. Further observations are needed to explore if the relation, D2O/HDO > HDO/H2O, is common in low-mass protostellar sources.