open access

https://hdl.handle.net/2115/84249

Rapid Ortho-to-para Nuclear Spin Conversion of H2 on a Silicate Dust Surface

英語

2021-02-26

The American Astronomical Society

The Astrophysical Journal

908(2)

234

The H2 molecule has two nuclear spin isomers, the so-called ortho and para isomers. Nuclear spin conversion (NSC) between these states is forbidden in the gas phase. The energy difference between the lowest ortho and para states is as large as 14.7 meV, corresponding to ~170 K. Therefore, each state of H2 differently affects not only the chemistry but also the macroscopic gas dynamics in space; thus, the ortho-to-para abundance ratio (OPR) of H2 has significant impacts on various astronomical phenomena. For a long time, the OPR of nascent H2 upon formation on dust grains has been assumed to have a statistical value of 3 and to gradually equilibrate in the gas phase at the temperature of the circumstances. Recently, the NSC of H2 was experimentally revealed to occur on water ice at very low temperatures and thus incorporated into gas–dust chemical models. However, H2 molecules should form well before dust grains are coated by water ice. Information about how the OPR of H2 behaves on bare silicate dust before ice-mantle formation is lacking. Knowing the influence of the OPR of H2 is desirable, if the OPR changes even on a bare silicate surface within an astronomically meaningful timescale. We report the first laboratory measurements of the NSC of H2 physisorbed on amorphous silicate (Mg2SiO4) at temperatures up to 18 K. The conversion was found to occur very rapidly.

学術雑誌論文

Accepted Manuscript

Creative Commons Attribution 4.0 International

This is the Accepted Manuscript version of an article accepted for publication in Astrophysical journal. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at 10.3847/1538-4357/abd9c0.

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