Yeom, Bum-Suk;Lee, Young Sun;Koo, Jae-Rim;Beers, Timothy C.;Kim, Young Kwang

  • Received : 2019.03.26
  • Accepted : 2019.05.17
  • Published : 2019.06.30


We present an analysis of the chemical abundances and kinematics of six low-mass dwarf stars, previously claimed to be candidate hypervelocity stars (HVSs). We obtained moderate-resolution (R ~ 6000) spectra of these stars to estimate the abundances of several chemical elements (Mg, Si, Ca, Ti, Cr, Fe, and Ni), and derived their space velocities and orbital parameters using proper motions from the Gaia Data Release 2. All six stars are shown to be bound to the Milky Way, and in fact are not even considered high-velocity stars with respect to the Galactic rest frame. Nevertheless, we attempt to characterize their parent Galactic stellar components by simultaneously comparing their element abundance patterns and orbital parameters with those expected from various Galactic stellar components. We find that two of our program stars are typical disk stars. For four stars, even though their kinematic probabilistic membership assignment suggests membership in the Galactic disk, based on their distinct orbital properties and chemical characteristics, we cannot rule out exotic origins as follows. Two stars may be runaway stars from the Galactic disk. One star has possibly been accreted from a disrupted dwarf galaxy or dynamically heated from a birthplace in the Galactic bulge. The last object may be either a runaway disk star or has been dynamically heated. Spectroscopic follow-up observations with higher resolution for these curious objects will provide a better understanding of their origin.


methods: data analysis;techniques: spectroscopic;Galaxy: disk;stars: abundances


  1. Abadi, M. G., Navarro, J. F., & Steinmetz, M. 2009, An Alternative Origin for Hypervelocity Stars, ApJL, 691, L63
  2. Allende Prieto, C., Sivarani, T., Beers, T. C., et al. 2008, The SEGUE Stellar Parameter Pipeline. III. Comparison with High-Resolution Spectroscopy of SDSS/SEGUE Field Stars, AJ, 136, 2070
  3. Alves-Brito, A., Melendez, J., Asplund, M., et al. 2010, Chemical Similarities between Galactic Bulge and Local Thick Disk Red Giants: O, Na, Mg, Al, Si, Ca, and Ti, A&A, 513, 35
  4. Battistini, C., & Bensby, T. 2015, The Origin and Evolution of the Odd-Z Iron-Peak Elements Sc, V, Mn, and Co in the Milky Way Stellar Disk, A&A, 577, 9
  5. Beers, T. C., Carollo, D., Ivezic, Z., et al. 2012, The Case for the Dual Halo of the Milky Way, ApJ, 746, 34
  6. Beers, T. C., Chiba, M., Yoshii, Y., et al. 2000, Kinematics of Metal-poor Stars in the Galaxy. II. Proper Motions for a Large Nonkinematically Selected Sample, AJ, 119, 2866
  7. Bensby, T., Feltzing, S., & Lundstrom, I. 2003, Elemental Abundance Trends in the Galactic Thin and Thick Disks as Traced by Nearby F and G Dwarf Stars, A&A, 410, 527
  8. Bensby, T., Feltzing, S., & Lundstrom, I. 2005, ${\alpha}$-, r-, and s-Process Element Trends in the Galactic Thin and Thick Disks, A&A, 433, 185
  9. Blaauw, A. 1961, On the Origin of the O- and B-Type Stars with High Velocities (the "Run-Away" Stars), and Some Related Problems, BAN, 15, 265
  10. Boeche, C., & Grebel, E. K. 2016, SP Ace: a New Code to Derive Stellar Parameters and Elemental Abundances, A&A, 587, 2
  11. Boeche, C., Siebert, A., Williams, M., et al. 2011, The RAVE Catalog of Stellar Elemental Abundances: First Data Release, AJ, 142, 193
  12. Boubert, D., & Evans, N. W. 2016, A Dipole on the Sky: Predictions for Hypervelocity Stars from the Large Magellanic Cloud, ApJL, 825, L6
  13. Boubert, D., Guillochon, J., Hawkins, K., et al. 2018, Revisiting Hypervelocity Stars after Gaia DR2, MNRAS, 479, 2789.
  14. Bovy, J. 2015, galpy: A Python Library for Galactic Dynamics, ApJs, 216, 29
  15. Bromley, B. C., Kenyon, S. J., Brown,W. R., & Geller, M. J. 2009, Runaway Stars, Hypervelocity Stars, and Radial Velocity Surveys, ApJ, 706, 925
  16. Brown, W. R. 2015, Hypervelocity Stars, ARA&A, 53, 15
  17. Brown, W. R., Anderson, J., Gnedin, O. Y., et al. 2015, Proper Motions and Trajectories for 16 Extreme Runaway and Hypervelocity Stars, ApJ, 804, 49
  18. Brown, W. R., Geller, M. J., & Kenyon, S. J. 2014, MMT Hypervelocity Star Survey. III. The Complete Survey, ApJ, 787, 89
  19. Brown, W. R., Geller, M. J., Kenyon, S. J., & Kurtz, M. J. 2005, Discovery of an Unbound Hypervelocity Star in the Milky Way Halo, ApJ, 622, L33
  20. Castelli, F., & Kurucz, R. L. 2003, Modelling of Stellar Atmospheres, ed. N. Piskunov, W.W. Weiss, and D.F. Gray (Published on behalf of the IAU by the ASP, 20)
  21. Cenarro, A. J., Peletier, R. F., Sanchez-Blazquez, P., et al. 2007, Medium-Resolution Isaac Newton Telescope Library of Empirical Spectra - II. The Stellar Atmospheric Parameters, MNRAS, 374, 664
  22. Cui, X.-Q., Zhao, Y.-H., Chu, Y.-Q., et al. 2012, The Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST), Res. Astron. Astrophys., 12, 1197
  23. Delgado Mena, E. G., Israelian, J. I., Gonzalez Hernandez, S. G., et al. 2014, Li Depletion in Solar Analogues with Exoplanets. Extending the Sample, A&A, 562, 92
  24. Freeman, K., & Bland-Hawthorn, J. 2002, The New Galaxy: Signatures of Its Formation, ARA&A, 40, 487
  25. Fulbright, J. P. 2000, Abundances and Kinematics of Field Halo and Disk Stars. I. Observational Data and Abundance Analysis, AJ, 120, 1841
  26. Gaia Collaboration, Brown, A. G. A., Vallenari, A., et al. 2018, Gaia Data Release 2. Summary of the Contents and Survey Properties, A&A, 616, A1.
  27. Gies, D. R. 1987, The Kinematical and Binary Properties of Association and Field O Stars, ApJS, 64, 545
  28. Hawkins, K., & Wyse, R. F. G. 2018, The Fastest Travel Together: Chemical Tagging of the Fastest Stars in Gaia DR2 to the Stellar Halo, MNRAS, 481, 1028
  29. Hills, J. G. 1988, Hyper-Velocity and Tidal Stars from Binaries Disrupted by a Massive Galactic Black Hole, Nature, 331, 687
  30. Hogg, D. W., Blanton, M. R., Roweis, S. T., et al. 2005, Modeling Complete Distributions with Incomplete Observations: The Velocity Ellipsoid from Hipparcos Data, ApJ, 629, 268
  31. Houk, N., & Swift, C. 2000, VizieR Online Data Catalog: Michigan Catalogue of HD stars, Vol. 5 (Houk+, 1999), VizieR Online Data Catalog, 3214
  32. Johnson, C. I., Rich, R. M., Kobayashi, C., Kunder, A., & Koch, A. 2014, Light, Alpha, and Fe-peak Element Abundances in the Galactic Bulge, AJ, 148, 67
  33. Kollmeier, J. A., & Gould, A. 2007, Where Are the Old-Population Hypervelocity Stars?, ApJ, 664, 343
  34. Kollmeier, J. A., Gould, A., Knapp, G., & Beers, T. C. 2009, Old-Population Hypervelocity Stars from the Galactic Center: Limits from the Sloan Digital Sky Survey, ApJ, 697, 1543
  35. Kurtz, M. J. & Mink, D. J. 1998, RVSAO 2.0: Digital Redshifts and Radial Velocities, PASP, 110, 934
  36. Lawrence, A., Warren, S. J., Almaini, O., et al. 2007, The UKIRT Infrared Deep Sky Survey (UKIDSS), MNRAS, 379, 1599
  37. Lee, Y. S., Beers, T. C., Prieto, C. A., et al. 2011, The SEGUE Stellar Parameter Pipeline. V. Estimation of Alpha-element Abundance Ratios from Low-resolution SDSS/SEGUE Stellar Spectra, AJ, 141, 90
  38. Lee, Y. S., Beers, T. C., Sivarani, T., et al. 2008a, The SEGUE Stellar Parameter Pipeline. I. Description and Comparison of Individual Methods, AJ, 136, 2022
  39. Lee, Y. S., Beers, T. C., Sivarani, T., et al. 2008b, The SEGUE Stellar Parameter Pipeline. II. Validation with Galactic Globular and Open Clusters, AJ, 136, 2050
  40. Li, Y., Luo, A., Zhao, G., et al. 2012, Metal-Poor Hypervelocity Star Candidates from the Sloan Digital Sky Survey, ApJL, 744, 24
  41. Li, Y.-B., Luo, A.-L., Zhao, G., et al. 2015, 19 Low Mass Hypervelocity Star Candidates from the First Data Release of the LAMOST Survey, Res. Astron. Astrophys., 15, 1364.
  42. Mishenina, T. V., Pignatari, M., Korotin, S. A., Soubiran, C., et al. 2013, Abundances of Neutron-Capture Elements in Stars of the Galactic Disk Substructures, A&A, 552, 128
  43. Munn, J. A., Monet, D. G., Levine, S. E., et al. 2004, An Improved Proper-Motion Catalog Combining USNO-B and the Sloan Digital Sky Survey, AJ, 127, 3034
  44. Munn, J. A., Monet, D. G., Levine, S. E., et al. 2008, Erratum: "An Improved Proper-Motion Catalog Combining Usno-B and the Sloan Digital Sky Survey" (2004, AJ, 127, 3034), AJ, 136, 895
  45. Palladino, L. E., Schlesinger, K. J., Holley-Bockelmann, K., et al. 2014, Hypervelocity Star Candidates in the SEGUE G and K Dwarf Sample, ApJ, 780, 7
  46. Piffl, T., Scannapieco, C., Binney, J., et al. 2014, The RAVE Survey: the Galactic Escape Speed and the Mass of the Milky Way, A&A, 562, A91
  47. Poveda, A., Ruiz, J., & Allen, C. 1967, Run-away Stars as the Result of the Gravitational Collapse of Protostellar Clusters, Boletin de los Observatorios Tonantzintla y Tacubaya, 4, 86
  48. Prugniel, Ph., Vauglin, I., & Koleva, M. 2011, The Atmospheric Parameters and Spectral Interpolator for the MILES Stars, A&A, 531, 165
  49. Reddy, B. E., Lambert, D. L., & Allende Prieto, C. 2006, Elemental Abundance Survey of the Galactic Thick Disc, MNRAS, 367, 1329
  50. Sneden, C. 1973, Carbon and Nitrogen Abundances in Metal-Poor Stars, PhD thesis, Univ. Texas at Austin
  51. Steinmetz, M., Zwitter, T., Siebert, A., et al. 2006, The Radial Velocity Experiment (RAVE): First Data Release, AJ, 132, 1645
  52. Stone, R. C. 1991, The Space Frequency and Origin of the Runaway O and B Stars, AJ, 102, 333
  53. Takeda, Y., & Honda, S. 2005, Photospheric CNO Abundances of Solar-Type Stars, PASJ, 57, 65
  54. Tauris, T. M. 2015, Maximum Speed of Hypervelocity Stars Ejected from Binaries, MNRAS, 448, 6
  55. Tauris, T. M., Takens, R. J. 1998, Runaway Velocities of Stellar Components Originating from Disrupted Binaries via Asymmetric Supernova Explosions, A&A, 330, 1047
  56. Tetzlaff, N., Neuhauser, R., & Hohle, M. M. 2011, A Catalogue of Young Runaway Hipparcos Stars within 3 kpc from the Sun, MNRAS, 410, 190
  57. Tinsley, B. M. 1979, Stellar Lifetimes and Abundance Ratios in Chemical Evolution, ApJ, 229, 1046
  58. Van der Swaelmen, M., Hill, V., Primas, F., et al. 2013, Chemical Abundances in LMC Stellar Populations. II. The Bar Sample, A&A, 560, A44
  59. Venn, K. A., Irwin, M., Shetrone, M. D., Tout, C. A., Hill, V., Tolstoy, E. 2004, Stellar Chemical Signatures and Hierarchical Galaxy Formation, AJ, 128, 1177
  60. Watkins, L. L., Evans, N.W., & An, J. H. 2010, The Masses of the Milky Way and Andromeda Galaxies, MNRAS, 406, 264
  61. Xue, X. X., Rix, H. W., Zhao, G., et al. 2008, The Milky Way's Circular Velocity Curve to 60 kpc and an Estimate of the Dark Matter Halo Mass from the Kinematics of -2400 SDSS Blue Horizontal-Branch Stars, ApJ, 684, 1143
  62. Yanny, B., Rockosi, C., Newberg, H. J., et al. 2009, SEGUE: A Spectroscopic Survey of 240,000 Stars with g = 14-20, AJ, 137, 4377
  63. York, D. G., Adelman, J., Anderson, J. E., Jr., et al. 2000, The Sloan Digital Sky Survey: Technical Summary, AJ, 120, 1579
  64. Yu, Q., & Tremaine, S. 2003, Ejection of Hypervelocity Stars by the (Binary) Black Hole in the Galactic Center, ApJ, 599, 1129
  65. Zhong, J., Chen, L., Liu, C., et al. 2014, The Nearest High-velocity Stars Revealed by LAMOST Data Release 1, ApJL, 789, L2
  66. Ziegerer, E., Volkert, M., Heber, U., et al. 2015, Candidate Hypervelocity Stars of Spectral Type G and K Revisited, A&A, 576, 14


Supported by : National Research Foundation (NRF), National Science Foundation of the USA