The Galactic Center S-Stars and the Hypervelocity Stars in the Galactic Halo: Two Faces of the Tidal Breakup of Stellar Binaries by the Central Massive Black Hole?

Abstract

In this paper, we investigate the link between the hypervelocity stars (HVSs) discovered in the Galactic halo and the Galactic center (GC) S-stars, under the hypothesis that they are both the products of the tidal breakup of the same population of stellar binaries by the central massive black hole (MBH). By adopting several hypothetical models for binaries to be injected into the vicinity of the MBH and doing numerical simulations, we realize the tidal breakup processes of the binaries and their follow-up dynamical evolution. We find that many statistical properties of the detected HVSs and GC S-stars could be reproduced under some binary injecting models, and their number ratio can be reproduced if the stellar initial mass function is top-heavy (e.g., with slope ∼−1.6). The total number of the captured companions is ∼50 that have masses in the range ∼3-7 M and semimajor axes 4000 AU and survive to the present within their main-sequence lifetime. The innermost one is expected to have a semimajor axis ∼300-1500 AU and a pericenter distance ∼10-200 AU, with a significant probability of being closer to the MBH than S2. Future detection of such a close star would offer an important test to general relativity. The majority of the surviving ejected companions of the GC S-stars are expected to be located at Galactocentric distances 20 kpc, and have heliocentric radial velocities ∼−500-1500 km s −1 and proper motions up to ∼5-20 mas yr −1 . Future detection of these HVSs may provide evidence for the tidal breakup formation mechanism of the GC S-stars.

Relaxation near Supermassive Black Holes Driven by Nuclear Spiral Arms: Anisotropic Hypervelocity Stars, S-stars, and Tidal Disruption Events

“…The much higher rates found by the latter authors could be explained by noting that Zhang et al (2013) assumed an IMF and not the PDMF. In addition, Zhang et al (2013) also considered top-heavy IMFs.…”

“…Zhang et al (2013) found rates of ∼ 10 −4 to 10 −5 yr −1 , for various binary injection models and stellar masses between 3 and 15 M ⊙ . The much higher rates found by the latter authors could be explained by noting that Zhang et al (2013) assumed an IMF and not the PDMF. In addition, Zhang et al (2013) also considered top-heavy IMFs.…”

An artificial neural network to discover hypervelocity stars: candidates in Gaia DR1/TGAS

“…Hypervelocity stars are rare objects, that occur in the Galaxy at an uncertain rate roughly between 10 −5 −10 −4 yr −1 (Hills 1988;Perets et al 2007;Zhang et al 2013;Brown et al 2014). Considering the magnitude limit of Gaia and different assumptions on the population of binaries in the GC, such a rate implies only ∼ 0.1 − 1 HVSs for every 10 6 stars in the final Gaia catalogue (Marchetti et al in preparation).…”

On the Existence of Pulsars in the Vicinity of the Massive Black Hole in the Galactic Center

“…(2003, see also Ginsburg & Loeb 2006;Löckmann et al 2008;Perets 2009;Antonini & Merritt 2013;Zhang et al 2013). The other components of those broken-up binaries may be ejected to the Galactic halo as hypervelocity stars (HVSs; e.g., Hills 1988;Yu & Tremaine 2003;Brown et al 2005;Edelmann et al 2005;Hirsch et al 2005;Brown et al 2012).…”