Journal of The Korean Astronomical Society (천문학회지)

The Korean Astronomical Society (한국천문학회)
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ORIGIN AND STATUS OF LOW-MASS CANDIDATE HYPERVELOCITY STARS

  • Yeom, Bum-Suk (Department of Astronomy and Space Science, Chungnam National University) ;
  • Lee, Young Sun (Department of Astronomy and Space Science, Chungnam National University) ;
  • Koo, Jae-Rim (Department of Astronomy and Space Science, Chungnam National University) ;
  • Beers, Timothy C. (Department of Physics and JINA Center for the Evolution of the Elements, University of Notre Dame) ;
  • Kim, Young Kwang (Department of Astronomy and Space Science, Chungnam National University)
  • Received : 2019.03.26
  • Accepted : 2019.05.17
  • Published : 2019.06.30
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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

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Figure 1. Spectra of standard stars in the wavelength range used to derive stellar parameters and chemical abundances.

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Figure 2. Same as in Figure 1, but for our program stars.

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Figure 3. Differences (residuals) between estimated values and reference values for three stellar parameters of our comparison stars, as a function of S/N. The blue star symbol with an error bar indicates the median value and its median absolute deviation (MAD) calculated in each S/N range. There are four S/N ranges shown (< 50, 50 – 100, 100 – 150, and > 150), separated by the vertical dotted lines.

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Figure 4. Same as in Figure 3, but for the chemical abundances of Mg, Si, Ca, Ti, Cr, and Ni. Note that the lower number of points for S/N < 50 results from a lower number of abundance measurements, due to absorption features being too weak.

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Figure 5. Toomre diagram for our sample of stars. The blue symbols indicate the velocities calculated with the SDSS proper motions, while the red symbols are based on the Gaia DR2 proper motions. The black and red circles roughly represent the kinematic boundaries of the thin and thick disks, at a constant velocity of 70 km s−1 and 180 km s−1 (Venn et al. 2004), respectively. The blue line indicates the local Galactic escape speed of Vesc = 533 km s−1 (Piffl et al. 2014). The numbers besides each star are the sample ID used by Pal14.

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Figure 6. Projected orbits for our program stars over 2 Gyr from the present, in the planes spanned by Z and R (left panels) and X and Y (right panels), respectively. Z is the distance from the Galactic plane, while R is the distance from the Galactic center projected onto the Galactic plane. X and Y are based on the Cartesian reference system, in which the center of the Galaxy is located at (0, 0) kpc and the Sun is located at (X,Z) = (8.0, 0.0) kpc. A filled circle indicates the current location of a star.

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Figure 7. Abundance ratios of Mg, Si, Ca, Ti, Cr, and Ni, as a function of [Fe/H], for stars in the Galactic bulge (green triangles; Alves-Brito et al. 2010; Johnson et al. 2014), the thick disk (filled blue circles; Alves-Brito et al. 2010; Bensby et al. 2003; Reddy et al. 2006), the thin disk (red circles; Alves-Brito et al. 2010; Bensby et al. 2003; Reddy et al. 2006), the halo (filled orange squares; Alves-Brito et al. 2010; Reddy et al. 2006), and the LMC (black dots; Van der Swaelmen et al. 2013). Our program stars are displayed as red star symbols along their Pal14 IDs.

Table 1 Details of spectroscopic observations

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Table 2 Stellar parameters and chemical abundances of the comparison stars adopted from the literature

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Table 3 Stellar parameters and chemical abundances derived for our program stars

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Table 4 Proper motions of our program stars from different catalogs

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Table 5 Velocities and orbital parameters of our program stars

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Table 6 Possible origins of our program stars

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