低温科学 = Low Temperature Science;第78巻

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宇宙と太陽系の水の起源と水の核スピン異性体

羽馬, 哲也;香内, 晃;渡部, 直樹

Permalink : http://hdl.handle.net/2115/77762
JaLCDOI : 10.14943/lowtemsci.78.11
KEYWORDS : 彗星;星間分子雲;原始惑星系円盤;水;核スピン;comet;interstellar molecular cloud;proto-planetary disk;water;nuclear spin

Abstract

H2O 氷は太陽系をはじめとする惑星系の重要な材料物質である.それにもかかわらず,太陽系に存 在するH2O 氷が過去に宇宙でいつ,どこで,どのようにしてできたのかは不明な点が多い.1980 年 代以降,彗星コマに含まれるH2O の核スピン異性体比を観測することで「彗星の水は46 億年前に30 K で生成した」という主張がなされてきた.しかし,この仮説に対する実験的検証は今まで行われて おらず,宇宙のH2O の核スピン異性体比が何を意味しているのかはよくわかっていない.本稿では, H2O の核スピン異性体についてのこれまでの観測研究を紹介し,「H2O の核スピン異性体比」を正し く解釈するために北大低温研でおこなってきた実験研究を紹介する.
Water (H2O) has two identical protons in a symmetrical geometry. Because a proton is a fermion with a nuclear spin of I = 1/2, there are two nuclear spin isomers for H2O: ortho (I = 1, triplet) and para (I = 0, singlet). Observational studies of cometary comae and protoplanetary disks have reported the existence of gaseous H2O with anomalous ortho-to-para ratios (OPRs) with statistical values less than 3, which has often been considered to represent the formation temperature of ice on cold interstellar dust. This hypothesis assumes that the OPR of H2O desorbed from ice is related to the ice formation temperature on the dust. However, the OPR of gaseous H2O desorbed from the ice has yet to be directly measured in a laboratory. Consequently, we still do not understand the origin of the anomalous OPRs measured for interstellar H2O. In this review, we summarize recent laboratory studies aimed at understanding the origin of the anomalous OPRs measured for interstellar H2O. The results of this study indicate that the OPR of H2O desorbed from interstellar ice should have a statistical value of 3, regardless of the formation process of the ice. This indicates that the OPR of interstellar gaseous H2O cannot be used to deduce ice formation temperature. This study also highlights the importance of interstellar gas-phase processes in understanding the anomalous OPRs of interstellar H2O.

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