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: $T_{\mathrm{eff}}=30600\pm 500$ K, $\log g=5.57\pm 0.06$ cm s${}^{-2}$ and He abundance $\log (n\mathrm{He}/n\mathrm{H})=-3.0\pm 0.2$. 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 $[\mathrm{Fe}/\mathrm{H}]=-1.3$. 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 $d=5.5\pm 0.5$ 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 it was accreted from the tidal debris of a dwarf galaxy by the Milky Way.

Accepted for publication in the Astrophysical Journal Letters

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• A proto-helium white dwarf stripped by a substellar companion via common-envelope ejection -- Uncovering the true nature of a candidate hypervelocity B-type star
• Tidal breakup of triple stars in the Galactic Centre
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• A spectroscopic and kinematic survey of fast hot subdwarfs
• Hypervelocity binaries from close encounters with a SMBH-IMBH binary: orbital properties and diagnostics
• Tidal breakup of quadruple stars in the Galactic Centre
• Runaway Dwarf Carbon Stars as Candidate Supernova Ejecta
• Binary Natal Kicks in the Galactic Center: X-ray Binaries, Hypervelocity Stars, and Gravitational Waves