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Research

I work on the inter-disciplinary field called astrostatistics, a new research field that combines astrophysics and statistics.

On the astrophysics side, my main research topic is to understand the structure and formation history of our Milky Way by using stellar dynamics and chemistry. Recently, I am also interested in the so-called hypervelocity stars ejected from the supermassive black hole at the Galactic center. I have been extensively using the big data from Gaia for these studies.

On the statistics side, I am interested in data mining, hierarchical Bayesian inference, and Markov Chain Monte Carlo analysis. Data mining is becoming increasingly important with the emergence of Peta-byte scale data sets available from recent and future surveys such as Gaia, DESI, and LSST.

In 2018, I discovered some hypervelocity star candidates from Gaia data, and I am developing a code to more efficiently find hypervelocity stars. Recently I used one of these hypervelocity stars to constrain the shape of the Galactic dark matter halo.

Curriculum Vitae (CV)

Research Interests

(The paper ID is from the list of lead-author papers shown below.)

Lead-author papers

  1. [arXiv] Metallicity and $α$-abundance for 48 million stars in low-extinction regions in the Milky Way (Hattori 2024, arXiv2404.01269) Catalog data
  2. Finding r-II sibling stars in the Milky Way with the Greedy Optimistic Clustering algorithm (Hattori, Okuno, Roederer 2023, ApJ, 946 48)
  3. Probability of forming gaps in the GD-1 stream by close encounters of globular clusters (Doke & Hattori 2022, ApJ, 941 129) [corresponding author]
  4. Action-based distribution function modelling for constraining the shape of the Galactic dark matter halo (Hattori, Valluri, Vasiliev 2021, MNRAS, 508, 5468)
  5. Origin of a massive hyper-runaway subgiant star LAMOST-HVS1 -- implication from Gaia and follow-up spectroscopy (Hattori et al. 2019b, ApJ, 873, 116)
  6. Metallicity dependence of the Hercules stream in Gaia/RAVE data -- explanation by non-closed orbits (Hattori et al. 2019a, MNRAS, 484, 4540)
  7. Constraining Solar position and velocity with a Nearby Hypervelocity Star (Hattori, Valluri, Castro 2018b, ApJ, 869, 33)
  8. Old, Metal-Poor Extreme Velocity Stars in the Solar Neighborhood (Hattori et al. 2018a, ApJ, 866, 121)
  9. Reliability of the Measured Velocity Anisotropy of the Milky Way Stellar Halo (Hattori et al. 2017, ApJ, 841, 91)
  10. Shepherding tidal debris with the Galactic bar: the Ophiuchus stream (Hattori, Erkal, Sanders 2016, MNRAS, 460, 497)
  11. Vertical kinematics of the thick disc at 4.5 < R < 9.5 kpc (Hattori & Gilmore 2015, MNRAS, 454, 649)
  12. Possible Evidence for Metal Accretion onto the Surfaces of Metal-poor Main-sequence Stars (Hattori et al. 2014, ApJ, 784, 153)
  13. Very Metal-poor Outer-halo Stars with Round Orbits (Hattori et al. 2013 ApJ Letter, 763, 17)
  14. The orbital eccentricity distribution of solar-neighbourhood halo stars (Hattori & Yoshii 2011, MNRAS, 418, 2481)
  15. A new test for the Galactic formation and evolution -- prediction for the orbital eccentricity distribution of the halo stars (Hattori & Yoshii 2010, MNRAS, 408, 2137)

Selected conference proceedings

  1. The shape of the dark matter halo revealed from a hypervelocity star (Hattori & Valluri 2019)

Co-author papers

  1. [arXiv] JASMINE: Near-Infrared Astrometry and Time Series Photometry Science Kawata et al. (2023, PASJ accepted, arXiv2307.05666) [19th author]
  2. Search for a Black Hole Binary in Gaia DR3 Astrometric Binary Stars with Spectroscopic Data Tanikawa, Hattori et al. (2023, ApJ, 946, 79) [2nd author]
  3. [arXiv] A Greedy and Optimistic Approach to Clustering with a Specified Uncertainty of Covariates Okuno & Hattori. (2022, arXiv2204:08205)
  4. SIRIUS Project - V. Formation of off-centre ionized bubbles associated with Orion Nebula Cluster Fujii, Hattori et al. (2022, MNRAS, 514, 43) [2nd author]
  5. High-precision chemical abundances of Galactic building blocks. II. Revisiting the chemical distinctness of the Helmi streams Matsuno et al. (2022, AA, 665, 46) [9th author]
  6. The R-Process Alliance: A Nearly Complete R-Process Abundance Template Derived from Ultraviolet Spectroscopy of the R-Process-Enhanced Metal-Poor Star HD 222925 Roederer et al. (2022, ApJS, 260, 27) [10th author]
  7. Dippers from TESS Full-frame Images. II. Spectroscopic Characterization of Four Young Dipper Kasagi et al. (2022, ApJS, 259, 40) [8th author]
  8. A Very Metal-poor RR Lyrae Star with a Disk Orbit Found in the Solar Neighborhood Matsunaga et al. (2022, ApJ, 925, 10) [3rd author]
  9. High-precision chemical abundances of Galactic building blocks. The distinct chemical abundance sequence of Sequoia Matsuno et al. (2022, AA, 661, 103) [8th author]
  10. Dynamically Tagged Groups of Metal-Poor Stars from the Best and Brightest Survey Shank et al. (2022, ApJ, 926, 26) [11th author]
  11. Broken into Pieces: ATLAS and Aliqa Uma as One Single Stream Li et al. (2021, ApJ, 911, 149) [18th author]
  12. Dippers from the TESS Full-Frame Images I: The Results of the first 1 year data and Discovery of A Runaway dipper Tajiri et al. (2020, ApJS, 251, 18) [5th author]
  13. 591 high velocity stars in the Galactic halo selected from LAMOST DR7 and Gaia DR2 Li et al. (2021, ApJS, 252, 3)
  14. The Southern Stellar Stream Spectroscopic Survey (S5): Chemical Abundances of Seven Stellar Streams Ji et al. (2020, ApJ, 160, 181)
  15. The R-Process Alliance: A Very Metal-poor, Extremely r-process-enhanced Star with [Eu/Fe] = + 2.2, and the Class of r-III Stars Cain et al. (2020, ApJ, 898, 40)
  16. Dynamical Relics of the Ancient Galactic Halo Yuan et al. (2020, ApJ, 891, 39) [8th author]
  17. On the estimation of the local dark matter density using the rotation curve of the Milky Way de Salas et al. (2019, JCAP, 10, 037) [4th author]
  18. Mass Spectroscopy of the Milky Way Dey, Arjun et al. (2019, BAAS, 51, 489) [18th author]
  19. Beta Dips in the Gaia Era: Simulation Predictions of the Galactic Velocity Anisotropy Parameter (β) for Stellar Halos Loebman, Valluri, Hattori et al. (2018, ApJ, 853, 196) [3rd author]
  20. Kinematics of Highly r-process-enhanced Field Stars: Evidence for an Accretion Origin and Detection of Several Groups from Disrupted Satellites Roederer, Hattori, Valluri (2018, ApJ, 156, 179) [2nd author]

Publication list in ADS

First-author, refereed papers.
All the papers.

Selected presentations

  1. NAOJ symposium (2021). Data Science in Astronomy

Student supervision

  1. Yuka Doke (University of Tokyo) -- Summer student in 2021. Probability of forming gaps in the GD-1 stream by close encounters of globular clusters (Doke & Hattori 2022, ApJ, 941, 129)

Press releases

  1. U-M researchers confirm massive hyper-runaway star ejected from the Milky Way Disk (2019, University of Michigan)
  2. Are stars putting on makeup? -- Stellar surface iron abundances enhanced by metal accretion from surrounding gas (2014, University of Tokyo)

Accepted observational proposals (as the PI)

  1. 2022B: Subaru Telescope, 4 half nights (December 2022)
  2. 2019A: Subaru Telescope, 1.5 nights (January 2020)
  3. 2019A: Magellan Telescope, 1 night (October and December 2019)
  4. 2017B: Magellan Telescope, 2 hours (January 2018), Observation of LAMOST-HVS1

Accepted computational grants (as a co-I)

  1. 2019: NASA ATP grant (as a co-I) (PI: Monica Valluri)
  2. 2019: MICDE Catalyst grant Determining the 3D shape of Milky Way’s Dark Matter Halo (as a co-I) (PI: Monica Valluri)

Grants (as the PI)

  1. 2024.4-2028.3: JSPS KAKENHI Grant Number JP24K07101. (3,400k JPY = 34k USD) # 基盤C
  2. 2021.4-2023.3: JSPS KAKENHI Grant Number JP21H00053. (2,000k JPY = 20k USD) # 新学術
  3. 2021.4-2024.3: JSPS KAKENHI Grant Number JP21K13965. (3,600k JPY = 36k USD) # 若手
  4. 2014.4-2016.3: JSPS Fellowship for Research Abroad. (10,400k JPY = 104k USD) # 海外学振
  5. 2011.4-2014.3: JSPS Fellowship DC1. 9,100k JPY = 91k USD) # 学振特別研究員DC1


Useful links

  1. Visibility of stars (Magellan observatory: 289.3074 -29.0146 2380) http://catserver.ing.iac.es/staralt/

Contact

Kohei Hattori
(1) Institute of Statistical Mathematics
10-3 Midoricho, Tachikawa, Tokyo 190-0014, Japan
(2) National Astronomical Observatory of Japan
2-21-1 Osawa, Mitaka, Tokyo 181-0015, Japan
<khattori(-[.AT.]-)ism.ac.jp>

Link to ORCID (0000-0001-6924-8862)
Link to Google Scholar (Kohei Hattori)
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