Organization
School of Science Professor
Title
Professor
External link
NAKAMOTO Taishi
Degree
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博士(理学) ( 1993.3 東京大学 )
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理学修士 ( 1990.3 東京大学 )
Research Interests
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Planetary Sciences
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Meteorite Formation
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Star and Planetary System Formation
Research Areas
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Natural Science / Astronomy
Professional Memberships
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Meteoritical Society
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International Astronomical Union
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Japanese Society for Planetary Science
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Astronomical Society of Japan
Papers
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Oxygen isotope exchange between molten silicate spherules and ambient water vapor with nonzero relative velocity: Implication for chondrule formation environment Reviewed
Sota Arakawa, Daiki Yamamoto, Takayuki Ushikubo, Hiroaki Kaneko, Hidekazu Tanaka, Shigenobu Hirose, Taishi Nakamoto
ICARUS 405 2023.11
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Size Dependence of the Bouncing Barrier in Protoplanetary Dust Growth Reviewed
Sota Arakawa, Satoshi Okuzumi, Misako Tatsuuma, Hidekazu Tanaka, Eiichiro Kokubo, Daisuke Nishiura, Mikito Furuichi, Taishi Nakamoto
The Astrophysical Journal Letters 951 ( 1 ) L16 - L16 2023.7
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Cooling Rates of Chondrules after Lightning Discharge in Solid-rich Environments Reviewed
Hiroaki Kaneko, Kento Sato, Chihiro Ikeda, Taishi Nakamoto
The Astrophysical Journal 947 ( 1 ) 2023.4
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Erratum: “Compound Chondrule Formation in Optically Thin Shock Waves” (2019, ApJ, 877, 84) Reviewed
Sota Arakawa, Taishi Nakamoto
The Astrophysical Journal 935 ( 2 ) 2022.8
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Fine-grained Rim Formation via Kinetic Dust Aggregation in Shock Waves Around Evaporating Icy Planetesimals Reviewed
Sota Arakawa, Hiroaki Kaneko, Taishi Nakamoto
The Astrophysical Journal 927 ( 2 ) 188 - 188 2022.3
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Dependence of the initial internal structure of chondrule rim on dust size distribution Reviewed
Hiroaki Kaneko, Sota Arakawa, Taishi Nakamoto
Icarus 374 114726 - 114726 2022.3
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Geometrical structure and thermal conductivity of dust aggregates formed via ballistic cluster-cluster aggregation Reviewed
Arakawa, Sota, Takemoto, Masaki, Nakamoto, Taishi
Progress of Theoretical and Experimental Physics 2019 ( 9 ) 11dummy 2019.9
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Rocky Planetesimal Formation Aided by Organics Reviewed
Kazuaki A. Homma, Satoshi Okuzumi, Taishi Nakamoto, Yuta Ueda
The Astrophysical Journal 877 ( 2 ) 128 - 128 2019.6
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Compound Chondrule Formation in Optically Thin Shock Waves Reviewed
Sota Arakawa, Taishi Nakamoto
The Astrophysical Journal 877 ( 2 ) 84 - 84 2019.5
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Thermal conductivity and coordination number of compressed dust aggregates Reviewed
Arakawa, Sota, Tatsuuma, Misako, Sakatani, Naoya, Nakamoto, Taishi
Icarus 324 8 - 14 2019.5
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Shock-generating Planetesimals Perturbed by a Giant Planet in a Gas Disk Reviewed
M. Nagasawa, K. K. Tanaka, H. Tanaka, H. Nomura, T. Nakamoto, H. Miura
Astrophysical Journal 871 ( 1 ) 2019.1
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Collisional Growth of Icy Dust Aggregates in the Disk Formation Stage: Difficulties for Planetesimal Formation via Direct Collisional Growth outside the Snowline Reviewed
Kenji Homma, Taishi Nakamoto
The Astrophysical Journal 868 ( 2 ) 118 - 118 2018.11
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Earth's atmosphere's lowest layers probed during a lunar eclipse Reviewed
Kiyoe Kawauchi, Norio Narita, Bun'ei Sato, Teruyuki Hirano, Yui Kawashima, Taishi Nakamoto, Takuya Yamashita, Motohide Tamura
PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF JAPAN 70 ( 5 ) 2018.10
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Thermal conductivity of porous aggregates Reviewed
Sota Arakawa, Hidekazu Tanaka, Akimasa Kataoka, Taishi Nakamoto
ASTRONOMY & ASTROPHYSICS 608 L7(4pp) 2017.12
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Comprehensive Study of Thermal Desorption of Grain-surface Species by Accretion Shocks around Protostars Reviewed
Hitoshi Miura, Tetsuo Yamamoto, Hideko Nomura, Taishi Nakamoto, Kyoko K. Tanaka, Hidekazu Tanaka, Makiko Nagasawa
ASTROPHYSICAL JOURNAL 839 ( 1 ) 47 - 47 2017.4
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Compound chondrule formation via collision of supercooled droplets Reviewed
Sota Arakawa, Taishi Nakamoto
ICARUS 276 102 - 106 2016.9
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WATER ICE at the SURFACE of the HD 100546 DISK Reviewed
M. Honda, T. Kudo, S. Takatsuki, A. K. Inoue, T. Nakamoto, M. Fukagawa, M. Tamura, H. Terada, N. Takato
Astrophysical Journal 821 ( 1 ) 2016.4
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COMPARISON OF IMPLICIT SCHEMES TO SOLVE EQUATIONS OF RADIATION HYDRODYNAMICS WITH A FLUX-LIMITED DIFFUSION APPROXIMATION: NEWTON–RAPHSON, OPERATOR SPLITTING, AND LINEARIZATION Reviewed
Hiroyuki Tetsu, Taishi Nakamoto
The Astrophysical Journal Supplement Series 223 ( 1 ) 14 - 14 2016.3
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ROCKY PLANETESIMAL FORMATION VIA FLUFFY AGGREGATES of NANOGRAINS Reviewed
Sota Arakawa, Taishi Nakamoto
Astrophysical Journal Letters 832 ( 2 ) 2016.1
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THEORETICAL EMISSION SPECTRA OF ATMOSPHERES OF HOT ROCKY SUPER-EARTHS Reviewed
Yuichi Ito, Masahiro Ikoma, Hajime Kawahara, Hiroko Nagahara, Yui Kawashima, Taishi Nakamoto
The Astrophysical Journal 801 ( 2 ) 144 - 144 2015.3
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REVISITING JOVIAN-RESONANCE INDUCED CHONDRULE FORMATION Reviewed
M. Nagasawa, K. K. Tanaka, H. Tanaka, T. Nakamoto, H. Miura, T. Yamamoto
The Astrophysical Journal 794 ( 1 ) 2014.9
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Near-infrared brightness of the Galilean satellites eclipsed in Jovian shadow: A New Technique to Investigate Jovian Upper Atmosphere Reviewed
K. Tsumura, K. Arimatsu, E. Egami, Y. Hayano, C. Honda, J. Kimura, K. Kuramoto, S. Matsuura, Y. Minowa, K. Nakajima, T. Nakamoto, M. Shirahata, J. Surace, Y. Takahashi, T. Wada
Astrophysical Journal 789 ( 2 ) 2014.7
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Mass-loss evolution of close-in exoplanets: Evaporation of hot jupiters and the effect on population Reviewed
H. Kurokawa, T. Nakamoto
Astrophysical Journal 783 ( 1 ) 2014.3
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Symposium on Computational Planetary Science
Hayashi Y., Umeda T., Umemura M., Ogawa M., Kameyama M., Genda H., Kokubo E., Takahashi Y. O., Terada N., Nakamoto T., Nishizawa S., Makino J.
Planetary People - The Japanese Society for Planetary Sciences 23 ( 1 ) 64 - 69 2014
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PHOTOEVAPORATION OF CIRCUMSTELLAR DISKS REVISITED: THE DUST-FREE CASE Reviewed
Kei E. I. Tanaka, Taishi Nakamoto, Kazuyuki Omukai
ASTROPHYSICAL JOURNAL 773 ( 2 ) 2013.8
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Evaporation of icy planetesimals due to bow shocks Reviewed
Kyoko K. Tanaka, Tetsuo Yamamoto, Hidekazu Tanaka, Hitoshi Miura, Makiko Nagasawa, Taishi Nakamoto
Astrophysical Journal 764 ( 2 ) 120(11pp) 2013.2
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EFFECT OF PHOTODESORPTION ON THE SNOW LINES AT THE SURFACE OF OPTICALLY THICK CIRCUMSTELLAR DISKS AROUND HERBIG Ae/Be STARS Reviewed
Akinori Oka, Akio K. Inoue, Taishi Nakamoto, Mitsuhiko Honda
ASTROPHYSICAL JOURNAL 747 ( 2 ) 2012.3
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Effects of Atmospheric Absorption of Incoming Radiation on the Radiation Limit of the Troposphere Reviewed
Hiroyuki Kurokawa, Taishi Nakamoto
JOURNAL OF THE ATMOSPHERIC SCIENCES 69 ( 1 ) 403 - 413 2012.1
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Direct Stellar Radiation Pressure at the Dust Sublimation Front in Massive Star Formation: Effects of a Dust-free Disk Reviewed
Tanaka, Kei E. I., Nakamoto, Taishi
The Astrophysical Journal 739 ( 2 ) 2011.10
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EVOLUTION OF SNOW LINE IN OPTICALLY THICK PROTOPLANETARY DISKS: EFFECTS OF WATER ICE OPACITY AND DUST GRAIN SIZE Reviewed
Akinori Oka, Taishi Nakamoto, Shigeru Ida
ASTROPHYSICAL JOURNAL 738 ( 2 ) 2011.9
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Formation of cosmic crystals in highly supersaturated silicate vapor produced by planetesimal bow shocks Reviewed
H. Miura, K. K. Tanaka, T. Yamamoto, T. Nakamoto, J. Yamada, K. Tsukamoto, J. Nozawa
Astrophysical Journal 719 ( 1 ) 642 - 654 2010.10
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A New Mechanism for Mass Accretion Under Radiation Pressure in Massive Star Formation Reviewed
Tanaka, Kei E. I., Nakamoto, Taishi
The Astrophysical Journal 714 ( 1 ) 309 - 319 2010.5
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Compound chondrule formation in the shock-wave heating model: Three-dimensional hydrodynamics simulation of the disruption of a partially-molten dust particle Reviewed
Seiji Yasuda, Hitoshi Miura, Taishi Nakamoto
ICARUS 204 ( 1 ) 303 - 315 2009.11
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The escape of ionizing photons from supernova-dominated primordial galaxies Reviewed
Hidenobu Yajima, Masayuki Umemura, Masao Mori, Taishi Nakamoto
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 398 ( 2 ) 715 - 721 2009.9
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Effects of scattering and dust grain size on the temperature structure of protoplanetary discs: a three-layer approach Reviewed
Akio K. Inoue, Akinori Oka, Taishi Nakamoto
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 393 ( 4 ) 1377 - 1390 2009.3
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Detection of water ice grains on the surface of the circumstellar disk around HD 142527 Reviewed
M. Honda, A. K. Inoue, M. Fukagawa, A. Oka, T. Nakamoto, M. Ishii, H. Terada, N. Takato, H. Kawakita, Y. K. Okamoto, H. Shibai, M. Tamura, T. Kudo, Y. Itoh
Astrophysical Journal 690 ( 2 PART 2 ) 2009
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Crystallization of Silicate Particles in Circumstellar Disks by Shock Waves Reviewed
T. Nakamoto, H. Miura
EXOPLANETS AND DISKS: THEIR FORMATION AND DIVERSITY 1158 147 - + 2009
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Origin of three-dimensional shapes of chondrules - I. Hydrodynamics simulations of rotating droplet exposed to high-velocity rarefied gas flow Reviewed
Hitoshi Miura, Taishi Nakamoto, Masao Doi
ICARUS 197 ( 1 ) 269 - 281 2008.9
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Observational possibility of the "snow line" on the surface of circumstellar disks with the scattered light Reviewed
Akio K. Inoue, Mitsuhiko Honda, Taishi Nakamoto, Akinori Oka
PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF JAPAN 60 ( 3 ) 557 - 563 2008.6
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Fragment-collision model for compound chondrule formation: Estimation of collision probability Reviewed
Hitoshi Miura, Seiji Yasuda, Taishi Nakamoto
ICARUS 194 ( 2 ) 805 - 815 2008.4
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Shock-wave heating model for chondrule formation: Hydrodynamic simulation of molten droplets exposed to gas flows Reviewed
Hitoshi Miura, Taishi Nakamoto
ICARUS 188 ( 1 ) 246 - 265 2007.5
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Shock-wave heating model for chondrule formation: Prevention of isotopic fractionation Reviewed
Hitoshi Miura, Taishi Nakamoto
ASTROPHYSICAL JOURNAL 651 ( 2 ) 1272 - 1295 2006.11
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Cosmological radiative transfer codes comparison project - I. The static density field tests Reviewed
Iliev, Ilian T, Ciardi, Benedetta, Alvarez, Marcelo A, Maselli, Antonella, Ferrara, Andrea, Gnedin, Nickolay Y, Mellema, Garrelt, Nakamoto, Taishi, Norman, Michael L, Razoumov, Alexei O, Rijkhorst, Erik-Jan, Ritzerveld, Jelle, Shapiro, Paul R, Susa, Hajime, Umemura, Masayuki, Whalen, Daniel J
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 371 ( 3 ) 1057 - 1086 2006.9
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Maximal size of chondrules in shock wave heating model: Stripping of liquid surface in a hypersonic rarefied gas flow Reviewed
T Kato, T Nakamoto, H Miura
METEORITICS & PLANETARY SCIENCE 41 ( 1 ) 49 - 65 2006.1
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A shock-wave heating model for chondrule formation - II. Minimum size of chondrule precursors Reviewed
H Miura, T Nakamoto
ICARUS 175 ( 2 ) 289 - 304 2005.6
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Flow in a liquid sphere moving with a hypersonic velocity in a rarefied gas - An analytic solution of linearized equations Reviewed
M Sekiya, M Uesugi, T Nakamoto
PROGRESS OF THEORETICAL PHYSICS 109 ( 5 ) 717 - 728 2003.5
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A spectrophotometric method to determine the inclination of Class I objects Reviewed
T Nakazato, T Nakamoto, M Umemura
ASTROPHYSICAL JOURNAL 583 ( 1 ) 322 - 329 2003.1
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Deformation and internal flow of a chondrule-precursor molten sphere in a shocked nebular gas
Masayuki Uesugi, Minoru Sekiya, Taishi Nakamoto
Earth, Planets and Space 55 ( 8 ) 493 - 507 2003
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A shock-wave heating model for chondrule formation: Effects of evaporation and gas flows on silicate particles Reviewed
H Miura, T Nakamoto, H Susa
ICARUS 160 ( 2 ) 258 - 270 2002.12
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Disk-halo model for flat-spectrum T Tauri stars Reviewed
N Kikuchi, T Nakamoto, K Ogochi
PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF JAPAN 54 ( 4 ) 589 - 597 2002.8
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On the Maximal Size of Chondrules in Shock Wave Heating Model Reviewed
SUSA, H. and NAKAMOTO, T.
Astrophysical Journal ( vol. 567 ) L57-L60 2002
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A shock heating model for chondrule formation in a protoplanetary disk Reviewed
A Iida, T Nakamoto, H Susa, Y Nakagawa
ICARUS 153 ( 2 ) 430 - 450 2001.10
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The effects of radiative transfer on the reionization of an inhomogeneous universe Reviewed
Taishi Nakamoto, Masayuki Umemura, Hajime Susa
Monthly Notices of the Royal Astronomical Society 321 ( 4 ) 593 - 604 2001.3
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GROWTH OF PROTOPLANETARY DISKS AROUND YOUNG STELLAR OBJECTS Reviewed
T NAKAMOTO, Y NAKAGAWA
ASTROPHYSICAL JOURNAL 445 ( 1 ) 330 - 336 1995.5
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FORMATION, EARLY EVOLUTION, AND GRAVITATIONAL STABILITY OF PROTOPLANETARY DISKS Reviewed
T NAKAMOTO, Y NAKAGAWA
ASTROPHYSICAL JOURNAL 421 ( 2 ) 640 - 650 1994.2
Books
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Star Formation 1999
Nobeyama Radio Observatory 1999
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国際会議「Star Formation 1999」の集録「Star Formation 1999」の編集
1999
MISC
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ALMA observations of sulfur-bearing molecules in protoplanetary disks
H. Nomura, A. Higuchi, N. Sakai, S. Yamamoto, M. Nagasawa, K. K. Tanaka, H. Miura, T. Nakamoto, H. Tanaka, T. Yamamoto, C. Walsh, T. J. Millar
Proceedings of the International Astronomical Union 360 - 361 2020
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Meeting on master plan proposals
Nakamoto T., Kuramoto K., Sekine Y.
Planetary People - The Japanese Society for Planetary Sciences 27 ( 4 ) 341 - 343 2018
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Radiation Pressure and Photoionization on Accretion Disks in Massive Star Formation
Kei E. I. Tanaka, Taishi Nakamoto
FIRST STARS IV - FROM HAYASHI TO THE FUTURE 1480 427 - 429 2012
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Strategic study of lunar and planetary explorations in coming decade : Panel report on top sciences
Ohtani E., Kimura J., Takahashi Y., Nakajima K., Nakamoto T., Miyoshi Y., Kobayashi K., Yamagishi A., Namiki N., Kobayashi N., Demura H., Kuramoto K., Ohtsuki K., Imamura T., Terada N., Watanabe S., Arakawa M., Ito T., Yurimoto H., Watanabe J.
Planetary People - The Japanese Society for Planetary Sciences 20 ( 4 ) 349 - 365 2011
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OMOSHI Effect : A New Mechanism for Mass Accretion under the Radiation Pressure in Massive Star Formation
Kei Tanaka, Taishi Nakamoto
EXOPLANETS AND DISKS: THEIR FORMATION AND DIVERSITY 1158 167 - 168 2009
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Escape Fraction of Ionizing Photons from High-z Proto-Galaxy
Hidenobu Yajima, Masayuki Umemura, Masao Mori, Taishi Nakamoto
PANORAMIC VIEWS OF GALAXY FORMATION AND EVOLUTION, PROCEEDINGS 399 70 - + 2008
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原始太陽系における mm スケールの流体力学(乱流と輸送現象:コーヒーカップから宇宙まで)
三浦 均, 中本 泰史, 土居 政雄
数理解析研究所講究録 1567 47 - 55 2007.8
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Thermal history of chondrules during shock-wave heating
Miura Hitoshi, Nakamoto Taishi
Antarctic meteorite research 239 - 252 2005
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Chondrule Age Distribution and Rate of Heating Events for Chondrule Formation
Nakamoto Taishi, Kita Noriko T., Tachibana Shogo
Antarctic Meteorite Research 253 - 272 2005
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衝撃波加熱コンドリュール形成：形成現場の光学的厚さに関する制限
遊星人 2005
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Crystallization of silicate particles by shock waves
Taishi Nakamoto, Hitoshi Miura
HIGHLIGHTS OF ASTRONOMY, VOL 13 13 525 - 527 2005
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The Epoch of Cosmic Reionization
Umemura, Masayuki, Nakamoto, Taishi, Susa, Hajime
Studies of Galaxies in the Young Universe with New Generation Telescope 77-82 2004.1
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Chondrule radius and evaporate fraction in viewpoint of shock-wave heating model
H Miura, T Nakamoto
GEOCHIMICA ET COSMOCHIMICA ACTA 67 ( 18 ) A295 - A295 2003.9
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Chondrule Formation by Gaseous Shock Waves in the Solar Nebula: A Possibility that Evaporation Determines the Minimum Size of Chondrules
Proceedings of 35th ISAS Lunar and Planetary Symposium 68 - 71 2002
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コンドリュール？ ショック！（共著）
遊・星・人（日本惑星科学会学会誌） ( Vol. 11 ) 150 - 157 2002
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402 Chondrule Formation due to Shock Waves in Solar Nebula : Compound Chondrule Formation
Abstracts Fall Meeting of the Japanese Society for Planetary Sciences 2001 50 - 50 2001.10
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Cosmic Reionization and Galaxy Formation
Umemura, M, Nakamoto, Taishi, Susa, Hajime
The Physics of Galaxy Formation, ASP Conference Proceedings, Vol. 222 222 109 2001.1
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非一様宇宙における再電離(基研短期研究会「宇宙における第一世代天体形成」,研究会報告)
中本 泰史, 梅村 雅之, 須佐 元
素粒子論研究 100 ( 3 ) C104 - C107 1999
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"超並列計算機のためのCommodity Networkに基づく並列入出力・可視化システム"(共著)
信学技法 CPSY98-161 81 - 88 1999
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Cosmic Reionization and High Redshift QSO Absorption Line Systems
UMEMURA Masayuki, NAKAMOTO Taishi, SUSA Hajime
Soryushiron Kenkyu Electronics 100 ( 3 ) C108 - C113 1999
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NAKAMOTO Taishi
IPSJ SIG Notes 98-HPC-74 ( 115 ) 43 - 48 1998
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"幅射流体力学による宇宙物理"(共著)
日本流体力学会誌「ながれ」 15 457 - 461 1996
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"Active Diskとしての原始惑星系円盤"
日本惑星科学会誌「遊星人」 4 239 - 247 1995
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151 Non-linear Growth of Protoplanetary Disks around Protostars
Abstracts Fall Meeting of the Japanese Society for Planetary Sciences 1994 151 - 151 1994.10
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"Disk Self-Gravity Effects on the Time Evolution of the Solar Nebula"(共著)
Lunar and Planetary Science Conference 21 841 1990
Research Projects
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Meteoritic and theoretical studies on the origin of isotopic dichotomy in meteorites
Grant number：23H00143 2023.4 - 2027.3
Japan Society for the Promotion of Science Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (A)
Grant amount：\47060000 （ Direct Cost: \36200000 、 Indirect Cost：\10860000 ）
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Simultaneous Formation of Planetesimals and Chondrules: Lightning Model
Grant number：22K03698 2022.4 - 2025.3
Japan Society for the Promotion of Science Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (C)
Grant amount：\4290000 （ Direct Cost: \3300000 、 Indirect Cost：\990000 ）
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コンドリュール形成：新しい雷モデルの可能性
Grant number：18K03721 2018.4 - 2023.3
日本学術振興会 科学研究費助成事業 基盤研究(C)
中本 泰史
Grant amount：\4290000 （ Direct Cost: \3300000 、 Indirect Cost：\990000 ）
本研究では，3つの小課題に分割して研究を進めてきた：(課題1) 原始太陽系星雲中での雷発生の環境を調べる，(課題2) 雷によって加熱を受けたダスト粒子の温度変化を調べ，コンドリュールが形成され得るかどうかを明らかにする，(課題3) 雷によるコンドリュール形成と原始太陽系星雲中ダストの進化および微惑星形成の関係を明らかにする，である。
課題1に関し，原始太陽系星雲中で起こり得る帯電機構として，岩石微粒子と金属鉄微粒子の衝突による摩擦帯電を考えた。課題3とも関連させ，具体的状況として重力不安定により形成される微惑星内部の固体微粒子の沈殿を考えた。できたての微惑星はまだ大きく広がっており，その内部のガス中を固体微粒子が沈殿していく。この際，粒子サイズが異なるとガス抵抗が異なり，沈殿速度に違いが生じる。岩石粒子は大きく金属鉄粒子は小さいのでこれらは衝突し，それぞれ正と負に帯電（摩擦帯電）する。各粒子は沈殿を続け，大局的に正負の電荷分離が起こり，電場が生成される。本研究により，この電場は十分に強くなり，水素ガスの絶縁破壊が起こることがわかった。すなわち，固体微粒子が沈殿中の形成間もない微惑星の内部で，雷は発生し得る。
課題2に関しては，雷が発生して電流が流れ高温になったガスからの熱放射と熱伝導により前駆体ダスト粒子が加熱される様子を数値シミュレーションによって調べた。その結果，コンドリュールの加熱・溶融が起こる条件が明らかになった。また，温度の冷却率についても，分析から推定されている値に近い値となる条件がわかった。これについては，論文にまとめて学術誌に投稿した。
以上のように，重力不安定で誕生した微惑星内部で固体微粒子が沈殿して雷が起こり，コンドリュールが形成されるという従来にない全く新しいコンドリュール形成モデルを得ることができた。 -
Decoding the origin of solar system from the isotope anomalies of heavy elements in meteorites
Grant number：16H04081 2016.4 - 2019.3
Japan Society for the Promotion of Science Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (B)
Yokoyama Tetsuya
Grant amount：\17680000 （ Direct Cost: \13600000 、 Indirect Cost：\4080000 ）
The aim of this research is to derive a new planet formation theory by measuring isotope anomalies of heavy elements in meteorites and constraining the dynamic history of precursor materials of planetesimals. The isotopic compositions of Sr, Nd, and Mo in meteorites revealed the presence of two isotopically distinct reservoirs in the early solar system where parent bodies of meteorites were formed. In particular, the outer asteroid belt was found to be enriched in components derived from supernovae. Such isotopic variability is caused by the heterogeneous distribution of s- and r-process nuclides in the early solar system, which was triggered by the selective destruction of dust grains by nebular thermal processing in the inner solar system, followed by the transportation of recrystallized-silicate dusts into the outer solar system.
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Study of Shock Wave Heating Chondrule Formation Model: Bow-Shocks in Dust-Rich Regions
Grant number：15K05266 2015.4 - 2019.3
Japan Society for the Promotion of Science Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (C)
Nakamoto Taishi
Grant amount：\4550000 （ Direct Cost: \3500000 、 Indirect Cost：\1050000 ）
We examined a possibility that chondrules, which are one of the main components of meteorites, are formed by bow shocks around fast moving planetesimals in the solar nebula. Especially, we explored cases where dust-to-gas mass ratio is high in the chondrule precursor region. According to our numerical simulations, we found that even in high dust-to-gas mass ratio cases, it seems difficulty for bow shocks to meet the chondrule formation conditions inferred from meteorite observations. Thus, chondrules seem to be formed by mechanisms other than the bow shock.
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Study of Chondrule Formation Processes: An Approach Combining Theoretical Calculations and Measurements of Shape, Composition, and Texture of Chondrules
Grant number：24340102 2012.4 - 2015.3
Japan Society for the Promotion of Science Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (B)
NAKAMOTO Taishi, YOKOYAMA Tetsuya
Grant amount：\18850000 （ Direct Cost: \14500000 、 Indirect Cost：\4350000 ）
We have investigated the external shape and internal structure of chondrules contained in Allende meteorite, to reveal the chondrule formation mechanism. We measured about 100 chondrules and found 15 chondrules, which can be considered to be completely molten once. We compare the obtained relationship between the size and the shape of those chondrules with the theoretical relation that is obtained assuming the gas-frictional heating. Then, we have found that those two relations are consistent each other. This implies that the chondrules in Allende meteorite were exposed to the gas flow when they were molten.
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Formation theory of habitable terrestrial planets
Grant number：23103005 2011.4 - 2016.3
Japan Society for the Promotion of Science Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research on Innovative Areas (Research in a proposed research area)
IDA SHIGERU
Grant amount：\141960000 （ Direct Cost: \109200000 、 Indirect Cost：\32760000 ）
Regarding formation of protostars and protoplanetary disks, we have constructed a new scenario that a disk is formed at the stage of a Jupiter-mass star and the disk fragment into clumps to fall onto the star. Regarding planetesimal formation, through analysis of simultaneous evolution of dust size, porosity and orbit, we found that icy planetesimals are successfully formed. Regarding comparison of theory with observed data, we performed Monte Carlo simulations to reproduce the orbital eccentricity distribution of exoplanets. Our study of planetary atmosphere acquired from the disk could explain diversity of bulk density of extrasolar super-Earths. We also proposed two theoretical models to explain the ring structure in the disks that were revealed by ALMA observations.
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Origin of heterogeneous isotope distribution for heavy elements in the early solar system
Grant number：23340171 2011.4 - 2014.3
Japan Society for the Promotion of Science Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (B)
YOKOYAMA Tetsuya, HIRATA Takahumi, NAKAMOTO Taishi
Grant amount：\20150000 （ Direct Cost: \15500000 、 Indirect Cost：\4650000 ）
We have investigated the origin of heterogeneous isotope distribution for heavy elements in the early solar system. Highly precise isotope analyses of Sr, Nd, Te, and Mo were conducted using TIMS for bulk rocks of chondrites and nonchondrites, as well as for chondrite components (e.g., CAIs, chondrules, acid leachates). We found that planetary scale isotope heterogeneities exist only for moderately refractory elements with 50% condensation temperature between 1300K to 1600K (e.g., Cr, Mo, Sr). This implies that isotope heterogeneities for moderately refractory elements were created by a dynamical history of dust grains related to thermal processing, material transportation, and mixing in the protoplanetary disk prior to planetesimal formation, rather than assuming processes such as supernova injection and/or grain-size sorting.
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Study of optical processes in the atmospheres of exoplanets and their feasibility of observations
Grant number：23654065 2011 - 2012
Japan Society for the Promotion of Science Grants-in-Aid for Scientific Research Grant-in-Aid for Challenging Exploratory Research
YAMAMOTO Tetsuo, NAKAMOTO Taishi
Grant amount：\3510000 （ Direct Cost: \2700000 、 Indirect Cost：\810000 ）
We formulated light propagation in the atmospheres of exoplanets in transit and derived a formula of an angle of refraction for light rays propagating through the atmosphere. The formula provides precise values even for the rays incident with small impact parameters onto the atmosphere. We proposed a method of estimating a scale height of an atmosphere from a light curve at a transition of an exoplanet in front of a central star. Systematic calculations of ray tracing are carried out for the rays passing through model atmospheres under isothermal hydrostatic equilibrium with varying wavelength of the light rays, and discussion was made on apparent increase in the radius of a planet in transition. The results are applied to the observation of Galilean satellites, which are observed through the atmosphere of Jupiter. Analysis of the observation indicates that the theory roughly explains the observation but suggests the presence of some effect that is not taken into account in the theory.
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Ultra-high Speed Crystallization in the Primitive Solar System
Grant number：22244066 2010.4 - 2014.3
Japan Society for the Promotion of Science Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (A)
TSUKAMOTO Katsuo, NAKAMURA Tomoki, NAKAMOTO Taishi, INATOMI Yuko, NAKAMURA Norihiro, KIMURA Yuki, MIRUA Hitoshi
Grant amount：\45370000 （ Direct Cost: \34900000 、 Indirect Cost：\10470000 ）
In-situ observations of crystallization from chondrule melts have successfully been conducted using gas get levitation method. It was found that hypercooling of 300-1100K is needed for the reproduction of various chondrule textures and the crystallization finishes within a few seconds. Based on the experimental data, a new kinetic model could be proposed for the chondrule texture formation.
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A New Development of Chondrule Formation Theory : Based on a Comparison with Cosmic Spherules
Grant number：21540434 2009 - 2011
Japan Society for the Promotion of Science Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (C)
NAKAMOTO Taishi
Grant amount：\4420000 （ Direct Cost: \3400000 、 Indirect Cost：\1020000 ）
Formation mechanism of chondrules, which are included in most of meteorites, has not been understood clearly. To elucidate the formation process, we approach to the formation process of cosmic spherules that are silicate small particles similar to chondrules. First, we measure the shape, chemical composition, and texture of each cosmic spherule. We find a clear correlation among them. Also, we find that the correlation can be understood quantitatively using a theory of the motion of the particle in the Earth atmosphere and the heating/cooling to the particle. Based on these understandings, we expect new studies on the formation mechanism of chondrules should be developed.
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Chondrule Formation from A Levitated Hypercooled Melt Droplet
Grant number：19204052 2007 - 2008
Japan Society for the Promotion of Science Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (A)
TSUKAMTO Katsuo, NAKAMURA Tomoki, NAKAMOTO Taishi, KOBATAKE Hidekazu, NAGASHIMA Ken, YOKOYAMA Etsuro, MIURA Hitoshi
Grant amount：\44850000 （ Direct Cost: \34500000 、 Indirect Cost：\10350000 ）
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Heating of Dust Particles and Chondrule Formation Induced by Shock-Waves in Upper Layer of Protoplanetary Disks Generated by X-Ray Flares
Grant number：17540211 2005 - 2007
Japan Society for the Promotion of Science Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (C)
NAKAMOTO Taishi
Grant amount：\3040000 （ Direct Cost: \2800000 、 Indirect Cost：\240000 ）
We studied the phenomen that a magnetic bubble caused by X-Ray flare of the central star generates the shock waves at the surface of protoplanetary disks around the central star. And we looked at the characteristics of shock waves to examine if they can heat dust particles in the disk enough to melt them and form chondrules based on the shock-wave heating model for chondrule formation. Especially, we focused on following three points : (1) if such an phenomen may take place, (2) if it occurs what kind of effects they may bring to materials in the disk, and (3) if we can know the presence of the phenomen by making observation astronomically and meteoritically. To approach these problems, we have conducted Magneto-Hydrodynamics (MHD) numerical simulations, radiative transfer numerical simulations, analytic investigations, and so forth.
Our MHD simulations have revealed that shock waves can be formed by the magnetic bubbles associated with X-Ray flares. And some of those shock waves turned out to be appropriate for chondrule formation and crystallization of dust particles. Namely, (a) dust particles in the disk region closer than 3 AU from the central star can be heated enough, and (b) it is unlikely that dust particle in a region further than about 5 AU are heated by the shock waves because the bubble would not reach that region due to the collimation.
On the other hand, thanks to the detailed simulations, we can now discuss places where dust particles are heated to form chondrules and/or crystallize precisely. And we can discuss the possibility that heated and crystallized dust particles may be incorporated in comets. This discussion implies that crystallized dust particles in observed comets should be transferred from the inner region of the solar system, because the shock wave heating can only take place in a region closer than 5 AU.
Some possible transfer mechanisms of dust particles have been proposed so far, but we need to investigate those mechanisms in detail in the future. -
ダストの沈殿と成長に伴う原始惑星系円盤の物理構造と化学組成およびスペクトルの変化
Grant number：17039008 2005 - 2006
日本学術振興会 科学研究費助成事業 特定領域研究
相川 祐理, 中本 泰史, 野村 英子
Grant amount：\2400000 （ Direct Cost: \2400000 ）
本研究では原始惑星系円盤におけるダストの沈殿、成長、およびそれに伴う円盤構造(温度・密度・化学組成分布)の変化を数値計算で調べる。今年度はまずX線による円盤の構造変化やイオン化率を調べた。X線は10AU以内では主要な加熱源となりうるが、遠方では紫外線による加熱の方が効くことが分かった。次にモデル円盤における水素分子の励起状態と輝線強度を調べた。その結果、中心星からの紫外線がある程度強い場合は水素輝線強度比が円盤内の微小ダスト存在最の指標となることを明らかにした。
円盤の一酸化炭素分子輝線観測においては、輝線強度比から求めた温度が一酸化炭素の昇華温度よりも低い場合があることが指摘されている。我々は円盤モデルの鉛直温度勾配に注目し、乱流拡散によって観測可能な量の一酸化炭素が凍結前に低温領域に運ばれること、一酸化炭素輝線強度比から得られる温度が円盤内のダストサイズ分布に依存することを示した。
惑星は固体物質の集積により形成されるため、円盤内の氷の存在量は惑星系形成に大きな影響を与える。本研究では円盤内での氷境界(水が固体として存在する領域の境界)を求めるため、円盤の輻射輸送計算コードの開発を行った。この計算では円盤を局所的には鉛直方向の1次元平行平板とし、それが動径方向に並んでいるという近似を用いた。この近似により円盤内温度を精度良く求めることが出来る。今年度の研究では、円盤が光学的に厚い時は氷境界が中心星の近く(約0.7AU)に位置し,光学的に薄くなると2.7AU程度まで後退することがわかった。氷境界位置の時間変化は多くの要因に依存するため、今後さらに研究を進める必要がある。また円盤内の固体物質の進化に関しても研究を行つた。特に衝撃波によるダスト粒子への加熱現象に若目し、ダスト粒子の変化を理論的に調べ、幅射輸送を通した天文学的な観測可能性も検討した。 -
Elucidation on the Origin of First Generation Objects by HMCS-E (Heterogeneous MultiComputer System-Embedded)
Grant number：16002003 2004 - 2007
Japan Society for the Promotion of Science Grants-in-Aid for Scientific Research Grant-in-Aid for Specially Promoted Research
UMEMURA Masayuki, NAKAMOTO Taishi, BOKU Taisuke, TAKAHASHI Daisuke, SUSA Hajime, MORI Masao
Grant amount：\382980000 （ Direct Cost: \294600000 、 Indirect Cost：\88380000 ）
The formation of first generation objects in the universe is an fundamental issue to clarify the cosmic evolution, the galaxy formation, and the generation of heavy elements. The objective of the project is to elucidate the origin of first generation objects in the universe by performing ultra-high resolution simulations. For the purpose, the special purpose board for gravity calculations "Blade-GRAPE" has been newly developed, and Cosmo Simulator "FIRST", which is a PC cluster system with embedded Blade-GRAPE in each node, has been completed. This is the collaboration between experts on astrophysical simulations and those on computer technology. FIRST is composed of 256 nodes and 496CPUs, and possesses two file servers. The peak performance of FIRST is 33TFLOPS in total, where the host system is 3.1 TFLOPS and the part of Blade-GRAPEs is 33TFLOPS. Also, Gfarm file system, which is a scalable virtual file system federating local file systems of cluster nodes, is providing a 22TB file system. The development of such a hybrid computer system is quite unique in the world.
Large-scale simulations have been performed with FIRST, and a variety of results are obtained as follows : (1) discovery of the dependence on dark matter cusps of the formation of first generation objects, (2) derivation of the criteria for radiative feedback by first stars by radiation hydrodynamic simulations, (3) proposal of a new picture of globular cluster formation by radiation hydrodynamic and N-body simulations on the formation of star clusters in ultraviolet background radiation. (4) derivation of the escape probability of ionizing photons from primeval galaxies, (5) effects of non-equilibrium atomic processes on the collision of galaxy clusters, (6) proposal of a new picture of 'Andromeda tears' through the collision with a satellite galaxy. In particular, the result (1) has been obtained by raising the mass resolution by more than two order of magnitude compared to previous simulations. This gives a novel picture of first generation objects. -
Study on Early Evolution of Protostars based on Tree-dimensional Radiative Magnetohydrodynamical Numeroca ; l Simulations
Grant number：15340062 2003 - 2004
Japan Society for the Promotion of Science Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (B)
HANAWA Tomoyuki, TOMISAKA Kohji, MATSUMOTO Tomoaki, INUTSUKA Shu-ichiro, MAKINO Junichiro, NAKAMOTO Taishi
Grant amount：\16700000 （ Direct Cost: \16700000 ）
This project is aimed to study evolution of protostars associated with disks and companions on the basis of numerical simulation in which self-gravity, magnetic field, and radiative transfer are taken into account. We have constructed PC cluster, developed numerical simulation codes employing AMR(adaptive mesh refinement), and published manuals on numerical schemes and data analysis, in order to maintain the basis for our numerical simulation studies. Main scientific achievements are as follows.
1.We have performed three-dimensional numerical simulations of gravitational collapse of rotating magnetized molecular cloud with the nested grid for the first time. We have found that the total of the magnetic and centrifugal forces converges at a certain value at the epoch of the protostar formation irrespectively of the initial magnetic field and rotation. When the centrifugal force dominates over the centrifugal force, the cloud tends to fragment to form binary.
2.We have derived minimum spatial resolution to solve the thermal evolution of two phase gases. The spatial resolution should be smaller than a third of the typical scale length for the heat transfer.
3.We have shown that the primary tends to accrete more gas than the companion. This denies the conventional idea that gas accretion should increase the mass ratio.
4.We have followed gravitational collapse of primordial gas with three-dimensional numerical simulations. The simulations suggest that frequency of binary formation should be high also in population III stars.
5.We have performed one-dimensional numerical simulations of supernova explosion. They show that the supernova explosions promote formation of the second generation stars within 10 thousands years through compression of the surrounding interstellar gas. -
第一世代天体形成の包括的研究
Grant number：15340060 2003 - 2004
日本学術振興会 科学研究費助成事業 基盤研究(B)
梅村 雅之, 中本 泰史, 佐藤 潤一, 西 亮一, 中村 文隆, 須佐 元
Grant amount：\3100000 （ Direct Cost: \3100000 ）
宇宙第一世代天体形成と,これが放射する紫外線によって引き起こされる宇宙再電離,さらに紫外線輻射場中で銀河形成の問題について,取り組みを行った。第一世代天体形成に関しては,種族III星の誕生を,3次元階層格子流体計算によって調べ,多くのものが連星系として誕生する可能性を指摘した。これは,電離光子の放射源として重要な役割を果たす。宇宙再電離については,6次元輻射輸送計算によってACDM宇宙における非一様宇宙の再電離過程を計算し,WMAPで示唆される早期再電離に必要な紫外線強度とその進化を見積もった。その結果,赤方偏移20前後の時期に,赤方偏移5の宇宙と同程度の背景紫外線が必要であることがわかった。また,紫外線輻射場中で銀河形成の問題を,長特性線法輻射輸送計算とSPHを結合した数値計算コードによる輻射流体力学計算によって調べた。計算の結果,10^8M_【of sun】以下のゆらぎは,バリオン成分のほとんどが背景紫外線によって蒸発してしまい銀河にはなりえないことがわかった。一方で,10^<8-9>M_【of sun】のゆらぎは,背景紫外線の中にあっても,高密度ピークが自己遮蔽され,これによって星が誕生し,矮小銀河形成につながることがわかった。この結果は,天の川銀河の周りに存在する矮小銀河の数が,ACDM宇宙で予言される数よりも少なくなっているという問題に対して,解決の糸口を与えるものである。
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原始太陽系星雲中の衝撃波によるコンドリュール形成過程の研究
Grant number：14740284 2002 - 2004
日本学術振興会 科学研究費助成事業 若手研究(B)
中本 泰史
Grant amount：\2400000 （ Direct Cost: \2400000 ）
本年度は,高速気流中での液滴の力学的安定性の解析,コンドリュール形成にとって適切な熱履歴を再現できる衝撃波の光学的条件,などの点についての研究を進めた.
コンドリュールの衝撃波加熱形成モデルでは,衝撃波面通過後,ダスト粒子は表面から順に加熱され溶融する.したがって一時的に,表面が液体,内部が固体という構造になることが期待される.このような部分溶融状態の液滴が,高速気流中で力学的に安定に存在出来るのかどうかという問題を,線型化した流体力学方程式を解析的に解くことによって調べた.まず,液体マントル-固体コア構造の粒子のうち,液体マントルの内部流や変形についての解析的な解を求めた.次にこの解を用いて,液体マントルがはぎ取られる条件を調べた.その結果,一般に,粒子のサイズが大きい場合には剥ぎ取りが起きることがわかった.さらに,衝撃波加熱モデルで粒子を加熱できる条件を満たす衝撃波において剥ぎ取りが起きない最大サイズを求めたところ,約1mm程度となることがわかった.これは実際に観測されているコンドリュールの最大サイズとほぼ一致している.したがって,コンドリュールの最大サイズは液滴部の剥ぎ取りによって決まったと考えられる.
一方,コンドリュールを形成するためには適切な熱履歴が必要であると考えられている.従来は,冷却率だけが問題になっていたが,最近,加熱率にも観測的に制限が与えられた.本研究では,冷却率と加熱率の2つが同時に満たされる条件を調べた.その結果,密度が比較的低く衝撃波速度が大きい衝撃波の場合にのみ,両方の条件が満たされることがわかった.これは,コンドリュールを形成をした衝撃波の生成条件を明らかにする上で,極めて重要な知見であると思われる. -
多次元輻射輸送計算による原始星系の現象論的解明
Grant number：12740118 2000 - 2001
日本学術振興会 科学研究費助成事業 奨励研究(A)
中本 泰史
Grant amount：\2100000 （ Direct Cost: \2100000 ）
本研究の目的は,2次元および3次元の輻射輸送計算を行って複雑な密度分布を持つ系からの輻射輸送を正確に求め,それを利用して観測される原始星の性質を明らかにすることであった.この目的のためには,まず,計算手法を確立させる必要があった.
本研究において,2次元の輻射平衡計算コードが完成し,実用に供されるようになった.このコードを用いてシミュレーションを行った結果,次のような新たな知見が得られた.
1.原始星の真の光度と観測される光度とは異なるものであること.そしてそれは,天体の観測角度に依存していること.またさらに,それらの依存性も定量的に明らかにした.
2.観測されている天体のスペクトルを用いて,天体の観測角度を推定する手法を考案した.
3.従来知られていた原始星の現象論的な分類と,天体の実際の物理的状態との間の関係が,単純ではないことを見出した.これは,物理的には異なる天体が,観測的には同じカテゴリの天体として現象論的に分類されている可能性があることを示唆している.
以上の成果をふまえ,今後はさらに定量的に,観測される情報から各天体の真の物理的な姿を明らかにする手法を開発することを試みたい.また,その手法を多数の原始星候補天体に適用し,原始星の真の姿を解明すること,およびそれらの統計的な性質を明らかにすることが必要であると思われる.
なお,本研究の副産物として,多次元輻射輸送計算の手法を3次元非一様密度分布初期宇宙における光再電離過程に応用した研究においても成果が得られたことを特に付記しておきたい.またさらに,原始星において形成されると予想されている原始惑星系円盤内の現象により,現在隕石に見られる特徴的な構造(コンドリュール)が形成される可能性があることも明らかになった. -
The Study of QSO Absorption Lines using 3D Radiative Transfer
Grant number：11640225 1999 - 2001
Japan Society for the Promotion of Science Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (C)
UMEMURA Masayuki, NAKAMOTO Taishi
Grant amount：\3600000 （ Direct Cost: \3600000 ）
Recently, it has been considered that the Lya absorption systems in QSO spectra can be accounted for in terms of the absorption by intergalactic overdense fluctuations. However, all of previous works have been based upon optically-thin or local optical-depth approximations. Here, we present 3D radiative transfer calculations on the cosmic reionization and the formation of QSO absorption line systems.
The resultant ionization degrees in the universe are different from place to place by more than three orders of magnitude. Due to such inhomogeneous ionization structure, relatively low ionization regions could produce strong absorption in QSO spectra. To make a direct comparison with the observations, we have simulated absorption lines. First, we focus on Lya absorption. It has turned out that the so-called Gunn-Peterson effect appears as the overlapping effect of absorption lines just when low dispersion spectroscopy is made. In order to compare the simulations quantitatively with observations, we have assessed the continuum depression, D_A. It is found that any model with a constant UV intensity does not match the observed trend that D_A tends to grow quickly at higher redshifts up to 5. This implies that the metagalactic UV intensity must decrease rapidly with z at z > 5 by two orders of magnitude at least. If the well fitted value of UV intensity at z = 5 is unchanged at higher redshifts, the reionization epoch is estimated to be z_<reion> > 7- However, the Lya absorption is not appropriate to determine the reionization epoch more accurately, because Lyα is too strongly depleted even if the mean neutral fraction is less than 10^<-2>.
Therefore, three conditions are required for a line in order to investigate the universe at z > 5 : (1) line emission is detectable, (2) it has lower line opacity than Lya, and (3) it has lower extinction against dust because young star-forming galaxies are often dust-enshrouded. The most favorable solution is Ha absorption lines. Actually, we have found that the Ha continuum depression traces the reionization history more accurately. Therefore, it is concluded that Ha forest is a more powerful tool to probe the universe at z > 5. The wavelengths of Ha forest drop on 3μm【less than or similar】λ_<Hα>【less than or similar】7μm at 4【less than or similar】z【less than or similar】10. Thus, the observations can be done with Subaru IRCS. -
3次元輻射輸送を考慮した銀河形成過程の解明
Grant number：10740093 1998 - 1999
日本学術振興会 科学研究費助成事業 奨励研究(A)
中本 泰史
Grant amount：\2000000 （ Direct Cost: \2000000 ）
宇宙の再電離過程は,宇宙で最初に形成された天体から放射された紫外線により引き起こされると考えられる.その紫外線が周囲に伝播していきガスを電離するという過程が,周囲のガスのその後の進化を左右する.このため,従来から紫外線の輻射輸送過程が調べられてきた.しかし輻射輸送は一般に計算が難しいので,従来の計算は1次元平板や1次元球対称などの簡単な幾何学的形状の場合がおもであった.いっぽう実際の初期宇宙においては密度が3次元的揺らぎを持っており,それらの密度揺らぎの程度・進化を明らかにすることが,その後の天体形成過程を解明するためには必要である.そこで本研究では,宇宙誕生後10億年程度の時期における宇宙の再電離過程を,3次元空間内の輻射輸送を正確に解くことによって調べた.その結果,宇宙の再電離は従来考えられていたよりもゆっくりとした現象として進行することがわかった.そしてこれは,3次元的に分布している密度の高い領域による紫外線の遮蔽効果によるものであることがわかった.
さらには,観測されるライマンアルファ線の分布と数値シミュレーション結果を比較することにより,宇宙の再電離の時期およびそのときの紫外線強度の関係を明らかにすることが出来た.それによれば,赤方偏移z=7からz=5にかけて紫外線強度が2桁以上にわたって強くなったこと,そしてそれによって宇宙の電離が進行することがわかった.これにより,宇宙の最初の紫外線源の生成時期およびその進化を明らかにするための重要な手がかりが得られた.
また一方,ここで開発した輻射輸送の数値計算法を他の天体の研究にも応用した.星形成の初期段階におけるフィラメント状星間雲の重力収縮過程を輻射流体力学的に数値計算し,最初の静水圧平衡天体の形成過程を明らかにした.また,原始星やTタウリ型星周囲の密度構造を,観測されるスペクトルと2次元軸対称輻射平衡計算との比較により求めることに成功した. -
輻射流体力学計算法の開発と星・惑星系形成問題への応用
Grant number：08740168 1996
日本学術振興会 科学研究費助成事業 奨励研究(A)
中本 泰史
Grant amount：\900000 （ Direct Cost: \900000 ）
2次元軸対称および3次元の流れを計算できる,多次元輻射流体力学計算コードの開発を行った.本研究で確立を目指した計算法は従来行われていた近似法とは全く異なり,光学的に厚い状況から薄い状況まで,均一のアルゴリズムで計算することが出来る計算法である.従って星・惑星系形成問題のように,初期には全領域で光学的に薄い状況ながら後期には一部で光学的に厚くなり一部は光学的に薄いままであるような問題を計算するのに,最も適した計算法である.
本奨励研究に支援された今年度の研究では,計算法の骨格部分のコーディング及び試験,ベクトル並列型大型計算機および超並列型大型計算機への実装および試験,などが行われた.これらのコーディングと試験の結果,本計算法は当初の予想通り,光学的厚さに関して広い状況にわたって精度の良い結果を与えることが確認できた.
一方,本計算法は非常に多くの計算量とメモリを消費する.これらを低減することが,実際の計算を大量に行う際には必要となってくるため,計算コードの効率化にも取り組んだ.その結果,計算法の中で最も計算量を必要とする部分については,ベクトル並列型大型計算機,超並列型大型計算機それぞれに対して,非常に効率の良い計算アルゴリズムを発見することが出来た.今後は他の部分についても効率化を図っていく予定である.
計算コードはほぼ完成しつつある段階であるが,それを科学的問題に応用した計算は,残念ながらまだなされていない.しかし,本計算コードを応用して計算できる興味深い問題は,星・惑星系形成問題に限らず,種々の分野に渡って数多くあることも明らかになってきた.今後はそういった問題に積極的に挑戦していきたいと考えている. -
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