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Kohei Hattori's Home Page


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) (Last updated November 2019)

Research Interests

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

First-author papers

  1. Action-based distribution function modelling for constraining the shape of the Galactic dark matter halo (Hattori, Valluri, Vasiliev 2020, MNRAS submitted)
  2. Origin of a massive hyper-runaway subgiant star LAMOST-HVS1 -- implication from Gaia and follow-up spectroscopy (Hattori et al. 2019b, ApJ, 873, 116)
  3. Metallicity dependence of the Hercules stream in Gaia/RAVE data -- explanation by non-closed orbits (Hattori et al. 2019a, MNRAS, 484, 4540)
  4. Constraining Solar position and velocity with a Nearby Hypervelocity Star (Hattori, Valluri, Castro 2018b, ApJ, 869, 33)
  5. Old, Metal-Poor Extreme Velocity Stars in the Solar Neighborhood (Hattori et al. 2018a, ApJ, 866, 121)
  6. Reliability of the Measured Velocity Anisotropy of the Milky Way Stellar Halo (Hattori et al. 2017, ApJ, 841, 91)
  7. Shepherding tidal debris with the Galactic bar: the Ophiuchus stream (Hattori, Erkal, Sanders 2016, MNRAS, 460, 497)
  8. Vertical kinematics of the thick disc at 4.5 < R < 9.5 kpc (Hattori & Gilmore 2015, MNRAS, 454, 649)
  9. Possible Evidence for Metal Accretion onto the Surfaces of Metal-poor Main-sequence Stars (Hattori et al. 2014, ApJ, 784, 153)
  10. Very Metal-poor Outer-halo Stars with Round Orbits (Hattori et al. 2013 ApJ Letter, 763, 17)
  11. The orbital eccentricity distribution of solar-neighbourhood halo stars (Hattori & Yoshii 2011, MNRAS, 418, 2481)
  12. 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. 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, acceepted) [5th author]
  2. 591 high velocity stars in the Galactic halo selected from LAMOST DR7 and Gaia DR2 Li et al. (2020, ApJS, accepted)
  3. The Southern Stellar Stream Spectroscopic Survey (S5): Chemical Abundances of Seven Stellar Streams Ji et al. (2020, ApJ, 160, id.181)
  4. Broken into Pieces: ATLAS and Aliqa Uma as One Single Stream Li et al. (2020, ApJ, submitted)
  5. 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 Ji et al. (2020, ApJ, 898, 40)
  6. Dynamical Relics of the Ancient Galactic Halo Yuan et al. (2020, ApJ, 891, id.39) [8th author]
  7. 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]
  8. Mass Spectroscopy of the Milky Way Dey, Arjun et al. (2019, BAAS, 51, 489) [18th author]
  9. 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]
  10. 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.

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. 2019A: Subaru Telescope, 1.5 nights (January 2020)
  2. 2019A: Magellan Telescope, 1 night (October and December 2019)
  3. 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)



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)
Link to researchmap (日本語/Japanese) (服部公平)