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ORIGIN AND STATUS OF LOW-MASS CANDIDATE HYPERVELOCITY STARS

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

Abstract

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.

Keywords

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

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Acknowledgement

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