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}(n{{He}}/n{{H}})=-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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- April 2016
- DOI:
- 10.3847/2041-8205/821/1/L13
- arXiv:
- arXiv:1604.03158
- Bibcode:
- 2016ApJ...821L..13N
- Keywords:
-
- binaries: spectroscopic;
- Galaxy: halo;
- stars: atmospheres;
- stars: horizontal-branch;
- stars: kinematics and dynamics;
- subdwarfs;
- Astrophysics - Solar and Stellar Astrophysics
- E-Print:
- Accepted for publication in the Astrophysical Journal Letters