US 708 – an unbound hyper-velocity subluminous O star

By H. Hirsch, U. Heber, S. O'Toole, F. Bresolin. 2005

142 citations recorded by the source corpus.

We report the discovery of an unbound hyper-velocity star, US 708, in the Milky Way halo, with a heliocentric radial velocity of $+708\pm15$ ${\rm km \, s^{-1}}$. A quantitative NLTE model atmosphere analysis of optical spectra obtained with LRIS at the Keck I telescope shows that US 708 is an extremely helium-rich ( N He /$N_{\rm H}=10$) subluminous O type star with T eff = $44\,500$ K, $\log g=5.23$ at a distance of 19 kpc. Its Galactic rest frame velocity is at least 751 ${\rm km \, s^{-1}}$, much higher than the local Galactic escape velocity indicating that the star is unbound to the Galaxy. It has been suggested that such hyper-velocity stars can be formed by the tidal disruption of a binary through interaction with the super-massive black hole (SMBH) at the Galactic centre (GC). Numerical kinematical experiments are carried out to reconstruct the path from the GC. US 708 needs about 32 Myrs to travel from the GC to its present position, less than its evolutionary lifetime. Its predicted proper motion $\mu_\alpha \cos{\delta} = -2.3$ ${\rm mas\,yr^{-1}}$ and $\mu_\delta = -2.4$ ${\rm mas\,yr^{-1}}$ should be measurable by future space missions. We conjecture that US 708 is formed by the merger of two helium white dwarfs in a close binary induced by the interaction with the SMBH in the GC and then escaped.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/c7c82335d1b7fc2b9967cd83c20d8f1eef361d55

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In Astronomy and Astrophysics

Citations in the corpus

Interaction of Massive Black Hole Binaries with Their Stellar Environment. I. Ejection of Hypervelocity Stars (2006, background): “This and five more HVSs, recently discovered by Hirsch et al. (2005) and Brown et al. (2006a, 2006b), are all consistent with a Galactic center origin, while an ejection from the LMC is more plausible for the seventh HVS currently known (Edelmann et al. 2005).”
Massive Perturber-driven Interactions between Stars and a Massive Black Hole (2006, background): “04 pc of the GC (Eisenhauer et al. 2005), and at the same time can account for the observed HVSs well on their way out of the Galaxy (Brown et al. 2005; Edelmann et al. 2005; Hirsch et al. 2005; Brown et al. 2006a,b; Bromley et al. 2006).”
Mass Deficits, Stalling Radii, and the Merger Histories of Elliptical Galaxies (2006, background): “A few hypervelocity stars have been detected in the halo of the Milky Way (Brown et al. 2005, 2006; Hirsch et al. 2005), and a possible model for their origin would postulate a binary SBH at the Galactic center, either currently or in the recent past.”
Hot Subdwarf Stars (2009, unknown): “Such a hyper-velocity star (HVS), US708, was discovered in 2005 (Hirsch et al., 2005).”
Three Hypervelocity White Dwarfs in Gaia DR2: Evidence for Dynamically Driven Double-degenerate Double-detonation Type Ia Supernovae (2018, background): “Our three hypervelocity candidates, GD 492 (Vennes et al. 2017), US 708 (Hirsch et al. 2005; Geier et al. 2015), and the three peculiar SDSS WDs from Figure 4 are shown as colored symbols.”
The RAVE Survey: Constraining the Local Galactic Escape Speed (2006, background): “There are currently seven known ‘hypervelocity’, probably unbound, stars in our Galaxy (Edelmann et al. 2005; Hirsch et al. 2005; Brown et al. 2006).”
LAMOST Experiment for Galactic Understanding and Exploration (LEGUE) — The survey's science plan (2012, background): “Hirsch et al. (2005) speculated that US 708 was formed by the merger of two He WDs in a close binary induced by the interaction with the SMBH in the GC and then escaped.”
The Progenitors of Type Ia Supernovae (2012, background): “The first three HVSs have only recently been discovered serendipitously (e.g. Brown et al., 2005; Hirsch et al., 2005; Edelmann et al., 2005).”
Milky Way mass models for orbit calculations (2012, background): “…(Altmann & de Boer 2000; Kaempf et al. 2005), subluminous B stars (Altmann et al. 2004; Tillich et al. 2011), halo stars (Schuster et al. 2012; Pereira et al. 2012), runaway stars (Irrgang et al. 2010; Silva & Napiwotzki 2011), and hypervelocity stars (Hirsch et al. 2005; Edelmann et al. 2005).”