2024/05/23 更新

写真b

ナカヤマ アキフミ
中山 陽史
NAKAYAMA Akifumi
*大学が定期的に情報更新している項目(その他は、researchmapの登録情報を転載)
所属*
理学研究科 物理学専攻 博士課程前期課程
理学研究科 物理学専攻 博士課程後期課程
職名*
特任准教授
研究キーワード
  • 系外地球型惑星

  • 惑星気候

  • ハビタブル惑星

  • 学内職務経歴*
    • 2022年4月 - 現在 
      理学研究科   物理学専攻 博士課程前期課程   特任准教授
    • 2022年4月 - 現在 
      理学研究科   物理学専攻 博士課程後期課程   特任准教授
     

    研究分野

    • 自然科学一般 / 宇宙惑星科学  / 系外惑星科学

    経歴

    • 2022年4月 - 現在 
      立教大学   理学部物理学科   特任准教授

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    • 2020年4月 - 2022年3月 
      東京大学   大学院理学系研究科   特任研究員

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      国名:日本国

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    学歴

    • 2013年4月 - 2020年3月 
      東京大学   大学院理学系研究科   地球惑星科学専攻

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      国名: 日本国

      備考: 修士課程

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    • 2009年4月 - 2013年3月 
      北海道大学   理学部   地球科学科

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    論文

    • Numerical Performance of Correlated-k Distribution Method in Atmospheric Escape Simulation

      Yuichi Ito, Tatsuya Yoshida, Akifumi Nakayama

      The Astrophysical Journal962 ( 2 ) 106 - 106   2024年2月1日

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      掲載種別:研究論文(学術雑誌)   出版者・発行元:American Astronomical Society  

      Abstract

      Atmospheric escape is crucial to understanding the evolution of planets in and out of the solar system and to interpreting atmospheric observations. While hydrodynamic escape simulations have been actively developed incorporating detailed processes such as UV heating, chemical reactions, and radiative cooling, the radiative cooling by molecules has been treated as emission from selected lines or rotational/vibrational bands to reduce its numerical cost. However, ad hoc selections of radiative lines would risk estimating inaccurate cooling rates because important lines or wavelengths for atmospheric cooling depend on emitting conditions such as temperature and optical thickness. In this study, we apply the correlated-k distribution (CKD) method to cooling rate calculations for H<sub>2</sub>-dominant transonic atmospheres containing H<sub>2</sub>O or CO as radiative species, to investigate its numerical performance and the importance of considering all lines of the molecules. Our simulations demonstrate that the sum of weak lines, which provides only 1% of the line emission energy in total at optically thin conditions, can become the primary source of radiative cooling in optically thick regions, especially for H<sub>2</sub>O-containing atmospheres. Also, in our hydrodynamic simulations, the CKD method with a wavelength resolution of 1000 is found to be effective, allowing the calculation of escape rate and temperature profiles with acceptable numerical cost. Our results show the importance of treating all radiative lines and the usefulness of the CKD method in hydrodynamic escape simulations. It is particularly practical for heavy-element-enriched atmospheres considered in small exoplanets, including super-Earths, without any prior selections for effective lines.

      DOI: 10.3847/1538-4357/ad187f

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      その他リンク: https://iopscience.iop.org/article/10.3847/1538-4357/ad187f/pdf

    • Survival of Terrestrial N2–O2 Atmospheres in Violent XUV Environments through Efficient Atomic Line Radiative Cooling 査読有り

      Akifumi Nakayama, Masahiro Ikoma, Naoki Terada

      The Astrophysical Journal937 ( 2 ) 72 - 72   2022年10月1日

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      掲載種別:研究論文(学術雑誌)   出版者・発行元:American Astronomical Society  

      Abstract

      Atmospheres play a crucial role in planetary habitability. Around M dwarfs and young Sun-like stars, planets receiving the same insolation as the present-day Earth are exposed to intense stellar X-rays and extreme-ultraviolet (XUV) radiation. This study explores the fundamental question of whether the atmosphere of present-day Earth could survive in such harsh XUV environments. Previous theoretical studies suggest that stellar XUV irradiation is sufficiently intense to remove such atmospheres completely on short timescales. In this study, we develop a new upper-atmospheric model and re-examine the thermal and hydrodynamic responses of the thermospheric structure of an Earth-like N<sub>2</sub>–O<sub>2</sub> atmosphere, on an Earth-mass planet, to an increase in the XUV irradiation. Our model includes the effects of radiative cooling via electronic transitions of atoms and ions, known as atomic line cooling, in addition to the processes accounted for by previous models. We demonstrate that atomic line cooling dominates over the hydrodynamic effect at XUV irradiation levels greater than several times the present level of the Earth. Consequentially, the atmosphere’s structure is kept almost hydrostatic, and its escape remains sluggish even at XUV irradiation levels up to a thousand times that of the Earth at present. Our estimates for the Jeans escape rates of N<sub>2</sub>–O<sub>2</sub> atmospheres suggest that these 1 bar atmospheres survive in early active phases of Sun-like stars. Even around active late M dwarfs, N<sub>2</sub>–O<sub>2</sub> atmospheres could escape significant thermal loss on timescales of gigayears. These results give new insights into the habitability of terrestrial exoplanets and the Earth’s climate history.

      DOI: 10.3847/1538-4357/ac86ca

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      その他リンク: https://iopscience.iop.org/article/10.3847/1538-4357/ac86ca/pdf

    • Two Bright M Dwarfs Hosting Ultra-Short-Period Super-Earths with Earth-like Compositions* 査読有り

      Teruyuki Hirano, John H. Livingston, Akihiko Fukui, Norio Narita, Hiroki Harakawa, Hiroyuki Tako Ishikawa, Kohei Miyakawa, Tadahiro Kimura, Akifumi Nakayama, Naho Fujita, Yasunori Hori, Keivan G. Stassun, Allyson Bieryla, Charles Cadieux, David R. Ciardi, Karen A. Collins, Masahiro Ikoma, Andrew Vanderburg, Thomas Barclay, C. E. Brasseur, Jerome P. de Leon, John P. Doty, René Doyon, Emma Esparza-Borges, Gilbert A. Esquerdo, Elise Furlan, Eric Gaidos, Erica J. Gonzales, Klaus Hodapp, Steve B. Howell, Keisuke Isogai, Shane Jacobson, Jon M. Jenkins, Eric L. N. Jensen, Kiyoe Kawauchi, Takayuki Kotani, Tomoyuki Kudo, Seiya Kurita, Takashi Kurokawa, Nobuhiko Kusakabe, Masayuki Kuzuhara, David Lafrenière, David W. Latham, Bob Massey, Mayuko Mori, Felipe Murgas, Jun Nishikawa, Taku Nishiumi, Masashi Omiya, Martin Paegert, Enric Palle, Hannu Parviainen, Samuel N. Quinn, George R. Ricker, Richard P. Schwarz, Sara Seager, Motohide Tamura, Peter Tenenbaum, Yuka Terada, Roland K. Vanderspek, Sébastien Vievard, Noriharu Watanabe, Joshua N. Winn

      The Astronomical Journal162 ( 4 ) 161 - 161   2021年10月1日

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      掲載種別:研究論文(学術雑誌)   出版者・発行元:American Astronomical Society  

      DOI: 10.3847/1538-3881/ac0fdc

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      その他リンク: https://iopscience.iop.org/article/10.3847/1538-3881/ac0fdc/pdf

    • Runaway climate cooling of ocean planets in the habitable zone: a consequence of seafloor weathering enhanced by melting of high-pressure ice 査読有り

      Akifumi Nakayama, Takanori Kodama, Masahiro Ikoma, Yutaka Abe

      Monthly Notices of the Royal Astronomical Society488 ( 2 ) 1580 - 1596   2019年7月1日

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      掲載種別:研究論文(学術雑誌)   出版者・発行元:Oxford University Press (OUP)  

      Terrestrial planets covered globally with thick oceans (termed ocean planets)
      in the habitable zone were previously inferred to have extremely hot climates
      in most cases. This is because ${\rm H_2O}$ high-pressure (HP) ice on the
      seafloor prevents chemical weathering and, thus, removal of atmospheric CO$_2$.
      Previous studies, however, ignored melting of the HP ice and horizontal
      variation in heat flux from oceanic crusts. Here we examine whether high heat
      fluxes near the mid-ocean ridge melts the HP ice and thereby removes
      atmospheric ${\rm CO_2}$. We develop integrated climate models of an Earth-size
      ocean planet with plate tectonics for different ocean masses, which include the
      effects of HP ice melting, seafloor weathering, and the carbonate-silicate
      geochemical carbon cycle. We find that the heat flux near the mid-ocean ridge
      is high enough to melt the ice, enabling seafloor weathering. In contrast to
      the previous theoretical prediction, we show that climates of terrestrial
      planets with massive oceans lapse into extremely cold ones (or snowball states)
      with CO$_2$-poor atmospheres. Such extremely cold climates are achieved mainly
      because the HP ice melting fixes seafloor temperature at the melting
      temperature, thereby keeping a high weathering flux regardless of surface
      temperature. We estimate that ocean planets with oceans several tens of the
      Earth's ocean mass no longer maintain temperate climates. These results suggest
      that terrestrial planets with extremely cold climates exist even in the
      habitable zone beyond the solar system, given the frequency of water-rich
      planets predicted by planet formation theories.

      DOI: 10.1093/mnras/stz1812

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      その他リンク: http://arxiv.org/pdf/1907.00827v3

    所属学協会

    共同研究・競争的資金等の研究

    • 低温度星周りの地球型惑星が温暖環境を保持する条件の解明

      日本学術振興会  科学研究費助成事業 

      中山 陽史

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      2023年4月 - 2026年3月

      課題番号:23K13161

      配分額:4550000円 ( 直接経費:3500000円 、 間接経費:1050000円 )

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