Updated on 2023/02/05

写真a

 
NISHI Ryoichi
 
Organization
Academic Assembly Institute of Science and Technology Fundamental Sciences Associate Professor
Faculty of Science Associate Professor
Graduate School of Science and Technology Fundamental Sciences Associate Professor
Title
Associate Professor
External link

Degree

  • 博士(理学) ( 1992.3   京都大学 )

Research Areas

  • Natural Science / Astronomy

  • Natural Science / Theoretical studies related to particle-, nuclear-, cosmic ray and astro-physics

Research History

  • Niigata University   Graduate School of Science and Technology Fundamental Sciences   Associate Professor

    2010.4

  • Niigata University   Faculty of Science Department of Physics   Associate Professor

    2010.4 - 2017.3

  • Niigata University   Graduate School of Science and Technology Fundamental Sciences   Associate Professor

    2004.9 - 2010.3

  • Niigata University   Associate Professor (as old post name)

    2002.4 - 2004.8

Professional Memberships

 

Papers

  • EVIDENCE FOR CLOUD-CLOUD COLLISION AND PARSEC-SCALE STELLAR FEEDBACK WITHIN THE L1641-N REGION Reviewed

    Fumitaka Nakamura, Tomoya Miura, Yoshimi Kitamura, Yoshito Shimajiri, Ryohei Kawabe, Toshiya Akashi, Norio Ikeda, Takashi Tsukagoshi, Munetake Momose, Ryoichi Nishi, Zhi-Yun Li

    ASTROPHYSICAL JOURNAL   746 ( 1 )   25 - 38   2012.2

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    Language:English   Publishing type:Research paper (scientific journal)   Publisher:IOP PUBLISHING LTD  

    We present high spatial resolution (CO)-C-12 (J = 1-0) images taken by the Nobeyama 45 m telescope toward a 48' x 48' area, including the L1641-N cluster. The effective spatial resolution of the maps is 21 '', corresponding to 0.04 pc at a distance of 400 pc. A recent 1.1 mm dust continuum map reveals that the dense gas is concentrated in several thin filaments. We find that a few dust filaments are located at the parts where (CO)-C-12 (J = 1-0) emission drops sharply. Furthermore, the filaments have two components with different velocities. The velocity difference between the two components is about 3 km s(-1), corresponding to a Mach number of 10, significantly larger than the local turbulent velocity in the cloud. These facts imply that the collision of the two components (hereafter, the cloud-cloud collision) possibly contributed to the formation of these filaments. Since the two components appear to overlap toward the filaments on the plane of the sky, the collision may have occurred almost along the line of sight. Star formation in the L1641-N cluster was probably triggered by such a collision. We also find several parsec-scale CO shells whose centers are close to either the L1641-N cluster or the V 380 Ori cluster. We propose that these shells were created by multiple winds and/or outflows from cluster young stellar objects, i.e., "protocluster winds." One exceptional dust filament located at the western cloud edge lies along a shell; it is presumably part of the expanding shell. Both the cloud-cloud collision and protocluster winds are likely to influence the cloud structure and kinematics in this region.

    DOI: 10.1088/0004-637X/746/1/25

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  • THE MOLECULAR OUTFLOWS IN THE rho OPHIUCHI MAIN CLOUD: IMPLICATIONS FOR TURBULENCE GENERATION Reviewed

    Fumitaka Nakamura, Yuhei Kamada, Takeshi Kamazaki, Ryohei Kawabe, Yoshimi Kitamura, Yoshito Shimajiri, Takashi Tsukagoshi, Kengo Tachihara, Toshiya Akashi, Kenta Azegami, Norio Ikeda, Yasutaka Kurono, Zhi-Yun Li, Tomoya Miura, Ryoichi Nishi, Tomofumi Umemoto

    ASTROPHYSICAL JOURNAL   726 ( 1 )   1 - 15   2011.1

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    Language:English   Publishing type:Research paper (scientific journal)   Publisher:IOP PUBLISHING LTD  

    We present the results of CO (J = 3 - 2) and CO (J = 1 - 0) mapping observations toward the active cluster-forming clump, L1688, in the rho Ophiuchi molecular cloud. From the CO (J = 3 - 2) and CO (J = 1 - 0) data cubes, we identify five outflows, whose driving sources are VLA 1623, EL 32, LFAM 26, EL 29, and IRS 44. Among the identified outflows, the most luminous outflow is the one from the prototypical Class 0 source, VLA 1623. We also discover that the EL 32 outflow located in the Oph B2 region has very extended blueshifted and redshifted lobes with wide opening angles. This outflow is most massive and has the largest momentum among the identified outflows in the CO (J = 1 - 0) map. We estimate the total energy injection rate due to the molecular outflows identified by the present and previous studies to be about 0.2 L-circle dot, larger than or at least comparable to the turbulence dissipation rate [approximate to(0.03 - 0.1)L-circle dot]. Therefore, we conclude that the protostellar outflows are likely to play a significant role in replenishing the supersonic turbulence in this clump.

    DOI: 10.1088/0004-637X/726/1/46

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  • PHYSICAL PROPERTIES OF DENSE CORES IN THE rho OPHIUCHI MAIN CLOUD AND A SIGNIFICANT ROLE OF EXTERNAL PRESSURES IN CLUSTERED STAR FORMATION Reviewed

    Hajime Maruta, Fumitaka Nakamura, Ryoichi Nishi, Norio Ikeda, Yoshimi Kitamura

    ASTROPHYSICAL JOURNAL   714 ( 1 )   680 - 698   2010.5

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    Language:English   Publishing type:Research paper (scientific journal)   Publisher:IOP PUBLISHING LTD  

    Using the archive data of the H(13)CO(+) (J = 1-0) line emission taken with the Nobeyama 45 m radio telescope with a spatial resolution of similar to 0.01 pc, we have identified 68 dense cores in the central dense region of the rho Ophiuchi main cloud. The H13CO+ data also indicate that the fractional abundance of H(13)CO(+) relative to H(2) is roughly inversely proportional to the square root of the H2 column density with a mean of 1.72 x 10(-11). The mean radius, FWHM line width, and LTE mass of the identified cores are estimated to be 0.045 /- 0.011 pc, 0.49 +/- 0.14 km s(-1), and 3.4 +/- 3.6 M(circle dot), respectively. The majority of the identified cores have subsonic internal motions. The virial ratio, the ratio of the virial mass to the LTE mass, tends to decrease with increasing LTE mass and about 60% of the cores have virial ratios smaller than 2, indicating that these cores are not transient structures but self-gravitating. The detailed virial analysis suggests that the surface pressure often dominates over the self-gravity and thus plays a crucial role in regulating core formation and evolution. By comparing the rho Oph cores with those in the Orion A molecular cloud observed with the same telescope, we found that the statistical properties of the core physical quantities are similar between the two clouds if the effect of the different spatial resolutions is corrected. The line widths of the rho Oph cores appear to be nearly independent of the core radii over the range of 0.01-0.1 pc and deviate upward from the Heyer & Brunt relation. This may be evidence that turbulent motions are driven by protostellar outflows in the cluster environment.

    DOI: 10.1088/0004-637X/714/1/680

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  • Formation Process of First Generation Luminous Objects and Their Observational Possibility Invited

    Ryoichi Nishi

    The 16th Workshop on General Relativity and Gravitation   2006.1

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    Language:English   Publishing type:Research paper (international conference proceedings)  

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  • Primordial molecular emission in population III galaxies Reviewed

    H Mizusawa, K Omukai, R Nishi

    PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF JAPAN   57 ( 6 )   951 - 967   2005.12

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    Language:English   Publishing type:Research paper (scientific journal)   Publisher:OXFORD UNIV PRESS  

    We consider the formation of molecules in primordial prestellar clumps and evaluate the line luminosities to assess detectability by next-generation observational facilities. If the initial H-2 fraction is sufficiently high, HD becomes an important coolant in the clumps. The luminosity from such HD cooling clumps is lower than that from H-2 cooling ones because of the lower temperature (< 100K). As for Li reactions, we include the three-body LiH formation approximately. The Li molecular fraction remains very low (< 10(-3)) throughout the evolution, owing to the high dissociative reaction rate of LiH + H -> Li + H-2. LiH does not become an important coolant in any density range. The luminous emission lines from the prestellar cores include H-2 rovibrational lines [1-0 Q(1), 1-0 O(3), 1-0 0(5)] and pure rotational lines [0-0 S(3), 0-0 S(4), 0-0 S(5)]. The next-generation facilities SPICA and JWST are able to detect H-2 emission in a large pre-galactic cloud that forms metal-free stars at a high rate of similar to 10(3) M-circle dot yr(-1) at a redshift of z < 10. We also derive an analytical expression for the luminosity that reproduces the numerical results.

    DOI: 10.1093/pasj/57.6.951

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  • H2 line emission associated with the formation of the first stars Reviewed

    Hiromi Mizusawa, Ryoichi Nishi, Kazuyuki Omukai

    Publications of the Astronomical Society of Japan   56 ( 3 )   487 - 495   2004

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    Language:English   Publishing type:Research paper (scientific journal)   Publisher:Oxford University Press  

    Molecular hydrogen (H2) line radiation emitted in the formation events of first-generation stars is evaluated in a discussion of its detectability by future observational facilities. H2 luminosity evolution from the onset of prestellar collapse until the formation of a ∼ 100 M⊙ protostar is followed. Calculations are extended not only to the early phase of the runaway collapse, but also to the later phase of accretion, whose observational features have not been studied before. Contrary to the runaway collapse phase, where the pure-rotational lines are always dominant, in the accretion phase rovibrational line emission becomes prominent. The maximum luminosity is also attained in the accretion phase for strong emission lines. The peak intensity of the strongest rovibrational line reaches ∼ 10-29 W m-2, corresponding to the flux density of 10-5 μJy, for a source at the typical redshift of first-generation star formation, 1 + z = 20. Although the redshifted rovibrational H2 emission from such an epoch falls in the wavelength range of the next-generation infrared satellite, Space Infrared Telescope for Cosmology and Astrophysics, for exceeding the detection threshold, 107 such protostars are required to reach the maximum luminosity simultaneously in a pregalactic cloud. It is improbable that this condition is satisfied in a realistic scenario of early structure formation.

    DOI: 10.1093/pasj/56.3.487

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  • Star Formation in the Primordial Gas Clouds

    Ryoichi Nishi

    Progress of Theoretical Physics Supplement   147   1 - 10   2003.12

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  • On the Feedback Effects of the First Generation Stars

    Ryoichi Nishi

    Progress of Theoretical Physics Supplement   147 ( 147 )   181 - 198   2003.12

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    Language:English   Publishing type:Research paper (scientific journal)   Publisher:Progress of Theoretical Physics  

    DOI: 10.1143/PTPS.147.181

    CiNii Article

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    Other Link: https://projects.repo.nii.ac.jp/?action=repository_uri&item_id=240452

  • On the Fragment Mass Scale of Primordial Gas Clouds

    Ryoichi Nishi

    Progress of Theoretical Physics Supplement   147   85 - 98   2003.12

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  • Microlensing optical depth toward the galactic bulge from microlensing observations in astrophysics group observations during 2000 with difference image analysis Reviewed

    T Sumi, F Abe, IA Bond, RJ Dodd, JB Hearnshaw, M Honda, M Honma, Y Kan-ya, PM Kilmartin, K Masuda, Y Matsubara, Y Muraki, T Nakamura, R Nishi, S Noda, K Ohnishi, OKL Petterson, NJ Rattenbury, M Reid, T Saito, Y Saito, H Sato, M Sekiguchi, J Skuljan, DJ Sullivan, M Takeuti, PJ Tristram, S Wilkinson, T Yanagisawa, PCM Yock

    ASTROPHYSICAL JOURNAL   591 ( 1 )   204 - 227   2003.7

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    Language:English   Publishing type:Research paper (scientific journal)   Publisher:UNIV CHICAGO PRESS  

    We analyze the data of the gravitational microlensing survey carried out by the Microlensing Observations in Astrophysics (MOA) group during 2000 toward the Galactic bulge ( GB). Our observations are designed to detect efficient high-magnification events with faint source stars and short-timescale events, by increasing the sampling rate up to similar to6 times per night and using Difference Image Analysis (DIA). We detect 28 microlensing candidates in 12 GB fields corresponding to 16 deg(2). We use Monte Carlo simulations to estimate our microlensing event detection efficiency, where we construct the I-band extinction map of our GB fields in order to find dereddened magnitudes. We find a systematic bias and large uncertainty in the measured value of the timescale t(E,out) in our simulations. They are associated with blending and unresolved sources, and are allowed for in our measurements. We compute an optical depth tau = 2.59(-064)(+0.84) x 10(-6) toward the GB for events with timescales 0.3 < t(E) < 200 days. We consider disk- disk lensing, and obtain an optical depth tau(bulge) = 3.36(-0.81)(+1/11) x 10(-6)[0.77/(1 - f(disk))] for the bulge component assuming a 23% stellar contribution from disk stars. These observed optical depths are consistent with previous measurements by the MACHO and OGLE groups, and still higher than those predicted by existing Galactic models. We present the timescale distribution of the observed events, and find there are no significant short events of a few days, in spite of our high detection efficiency for short-timescale events down to t(E) similar to 0.3 days. We find that half of all our detected events have high magnification (>10). These events are useful for studies of extrasolar planets.

    DOI: 10.1086/375212

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  • Mechanism of magnetic flux loss in molecular clouds Reviewed

    Takenori Nakano, Ryoichi Nishi, Toyoharu Umebayashi

    Astrophysical Journal   573 ( 1 I )   199 - 214   2002.7

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    Language:English   Publishing type:Research paper (scientific journal)   Publisher:Institute of Physics Publishing  

    We investigate the detailed processes at work in the drift of magnetic fields in molecular clouds. To the frictional force, whereby the magnetic force is transmitted to neutral molecules, ions contribute more than half only at cloud densities nH ≲ 104 cm-3, and charged grains contribute more than about 90% at nH gsim
    10 6 cm-3. Thus, grains play a decisive role in the process of magnetic flux loss. Approximating the flux loss time tB by a power law tB ∝ B-γ, where B is the mean field strength in the cloud, we find γ ≈ 2, characteristic of ambipolar diffusion, only at nH ≲ 107 cm-3, at which ions and the smallest grains are pretty well frozen to the magnetic fields. At nH &gt
    107 cm-3, γ decreases steeply with nH, and finally at nH ≈ ndec ≈ a few × 1011 cm-3, at which the magnetic fields effectively decouple from the gas, γ ≫ 1 is attained, reminiscent of Ohmic dissipation, although flux loss occurs about 10 times faster than by pure Ohmic dissipation. Because even ions are not very well frozen at nH &gt
    107 cm-3, ions and grains drift slower than the magnetic fields. This insufficient freezing makes t B more and more insensitive to B as nH increases. Ohmic dissipation is dominant only at nH ≳ 1 × 1012 cm-3. While ions and electrons drift in the direction of the magnetic force at all densities, grains of opposite charges drift in opposite directions at high densities, at which grains are major contributors to the frictional force. Although magnetic flux loss occurs significantly faster than by Ohmic dissipation even at very high densities, such as nH ≈ ndec, the process going on at high densities is quite different from ambipolar diffusion, in which particles of opposite charges are supposed to drift as one unit.

    DOI: 10.1086/340587

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Research Projects

  • 位置天文衛星を用いたオリオン領域の大質量星形成過程の研究

    2014.4 - 2019.3

    日本学術振興会  科学研究費助成事業  基盤研究(C)

    西亮一

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    Authorship:Principal investigator  Grant type:Competitive

    Grant amount:\2300000 ( Direct Cost: \2300000 )

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Teaching Experience

  • 理学基礎演習

    2022
    Institution name:新潟大学

  • 理学スタディ・スキルズ

    2022
    Institution name:新潟大学

  • 日本事情自然系A

    2022
    Institution name:新潟大学

  • 宇宙物理学特論II

    2021
    Institution name:新潟大学

  • 先端科学技術総論

    2021
    Institution name:新潟大学

  • 現代物理学セミナーB

    2020
    Institution name:新潟大学

  • 現代物理学セミナーA

    2020
    Institution name:新潟大学

  • 物理学特論V

    2020
    Institution name:新潟大学

  • 専門力アクティブ・ラーニング

    2018
    -
    2021
    Institution name:新潟大学

  • 電磁気学IB

    2017
    Institution name:新潟大学

  • 電気力学A

    2017
    Institution name:新潟大学

  • 電磁気学IA

    2017
    Institution name:新潟大学

  • 電気力学B

    2017
    Institution name:新潟大学

  • 理学スタディ・スキルズ

    2017
    -
    2020
    Institution name:新潟大学

  • 物理数学演習II

    2017
    Institution name:新潟大学

  • 物理学への招待B

    2015
    -
    2016
    Institution name:新潟大学

  • 課題探索特講

    2013
    -
    2016
    Institution name:新潟大学

  • 数理物質科学演習Ⅱ(物理学)

    2013
    -
    2015
    Institution name:新潟大学

  • 数理物質科学特定研究ⅡA(物理学)

    2013
    -
    2015
    Institution name:新潟大学

  • 数理物質科学特定研究ⅡB(物理学)

    2013
    -
    2015
    Institution name:新潟大学

  • 物理学基礎D

    2012
    Institution name:新潟大学

  • 数理物質科学演習Ⅰ(物理学)

    2012
    -
    2015
    Institution name:新潟大学

  • 数理物質科学特定研究Ⅰ(物理学)

    2012
    -
    2015
    Institution name:新潟大学

  • コラボレーション演習

    2012
    -
    2015
    Institution name:新潟大学

  • 天文学特論

    2012
    Institution name:新潟大学

  • プラズマ物理学特論

    2012
    Institution name:新潟大学

  • 宇宙物理学特論Ⅲ

    2011
    Institution name:新潟大学

  • 宇宙物理学講究Ⅲ

    2011
    -
    2012
    Institution name:新潟大学

  • 宇宙物理学特論

    2011
    Institution name:新潟大学

  • 宇宙物理学特論IV

    2010
    -
    2011
    Institution name:新潟大学

  • 物理学最前線入門

    2010
    Institution name:新潟大学

  • 宇宙物理学

    2008
    Institution name:新潟大学

  • 宇宙物理学特論Ⅱ

    2008
    -
    2019
    Institution name:新潟大学

  • 宇宙物理学講究Ⅱ

    2008
    -
    2016
    Institution name:新潟大学

  • 宇宙物理学特論II

    2007
    -
    2021
    Institution name:新潟大学

  • 物理数学III

    2007
    -
    2016
    Institution name:新潟大学

  • 電気力学

    2007
    -
    2016
    Institution name:新潟大学

  • 物理学基礎A II

    2007
    -
    2011
    Institution name:新潟大学

  • 宇宙物理学講究II

    2007
    Institution name:新潟大学

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