An extremely fast halo hot subdwarf star in a wide binary system
Abstract
New spectroscopic observations of the halo hyper-velocity star candidate SDSS J121150.27+143716.2 (V = 17.92 mag) revealed a cool companion to the hot subdwarf primary. The components have a very similar radial velocity and their absolute luminosities are consistent with the same distance, confirming the physical nature of the binary, which is the first double-lined hyper-velocity candidate. Our spectral decomposition of the Keck/ESI spectrum provided an sdB+K3V pair, analogous to many long-period subdwarf binaries observed in the Galactic disk. We found the subdwarf atmospheric parameters: T_(eff) = 30 600 ± 500 K, log g = 5.57 ± 0.06 cm s^(−2), and He abundance log (nHe/nH) = - 3.0 ± 0.2. Oxygen is the most abundant metal in the hot subdwarf atmosphere, and Mg and Na lines are the most prominent spectral features of the cool companion, consistent with a metallicity of [Fe/H] = - 1.3. The non-detection of radial velocity variations suggest the orbital period to be a few hundred days, in agreement with similar binaries observed in the disk. Using the SDSS-III flux calibrated spectrum we measured the distance to the system d = 5.5 ± 0.5 kpc, which is consistent with ultraviolet, optical, and infrared photometric constraints derived from binary spectral energy distributions. Our kinematic study shows that the Galactic rest-frame velocity of the system is so high that an unbound orbit cannot be ruled out. On the other hand, a bound orbit requires a massive dark matter halo. We conclude that the binary either formed in the halo or was accreted from the tidal debris of a dwarf galaxy by the Milky Way.
Additional Information
© 2016 The American Astronomical Society. Received 2016 February 9; accepted 2016 March 21; published 2016 April 11. P.N. and E.Z. were supported by the Deutsche Forschungsgemeinschaft (DFG) through grants HE1356/49-2 and HE1356/45-2, respectively. T.K. acknowledges support by the Netherlands Research School for Astronomy (NOVA). This work is based on observations at the La Silla-Paranal Observatory of the European Southern Observatory for program number 093.D-0127(A). This work is based on observations obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. This research has used the services of astroserver.org.Attached Files
Published - apjl_821_1_L13.pdf
Submitted - 1604.03158v1.pdf
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Additional details
- Eprint ID
- 66574
- Resolver ID
- CaltechAUTHORS:20160502-084108701
- HE1356/49-2
- Deutsche Forschungsgemeinschaft (DFG)
- HE1356/45-2
- Deutsche Forschungsgemeinschaft (DFG)
- Nederlandse Onderzoekschool voor de Astronomie (NOVA)
- W. M. Keck Foundation
- Created
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2016-05-02Created from EPrint's datestamp field
- Updated
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2021-11-10Created from EPrint's last_modified field