The origin of interstellar water and nuclear spin isomers of water

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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
  • ウチュウ ト タイヨウケイ ノ ミズ ノ キゲン ト ミズ ノ カク スピン イセイタイ

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Abstract

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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  • 低温科学

    低温科学 78 11-26, 2020-05-01

    低温科学第78巻編集委員会

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