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RE: Hypervelocity Stars
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Title: 'Slow' and Fast Rotators among Hypervelocity Stars
Authors: Mercedes Lopez-Morales, Alceste Z. Bonanos

We measure the rotational velocities of the late B-type hypervelocity stars HVS7 and HVS8 from high resolution spectroscopy to be 60 +/- 17 km/s and 260 +/- 70 km/s. The 'slow' rotation of HVS7 is consistent with an origin in a binary system, however, the fast rotation of HVS8 is more common of single B-type stars. Our results suggest that HVS8 could have been ejected by a mechanism other than that proposed by Hills. We also estimate the effective temperatures and surface gravities for HVS7 and HVS8 and obtain an additional measurement of their radial velocities. We find evidence in support of a blue horizontal branch nature for HVS7, and a main sequence nature for HVS8.

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Title: Runaway and hypervelocity stars in the Galactic halo: Binary rejuvenation and triple disruption
Authors: Hagai B. Perets

Young stars observed in the distant Galactic halo are usually thought to have formed elsewhere, either in the Galactic disk or perhaps the Galactic centre, and subsequently ejected at high velocities to their current position. However, some of these stars have apparent lifetimes shorter the required flight time from the Galactic disk/centre. We suggest that such stars have evolved in close runaway or hypervelocity binaries. Stellar evolution of such binaries can drive them into mass transfer configurations and even mergers. Such evolution could then rejuvenate them (e.g. blue stragglers) and extend their lifetime after their ejection. The extended lifetimes of such stars could then be reconciled with their flight times to the Galactic halo. We study the possibilities of binary runaway and hypervelocity stars and show that such binaries could have been ejected in triple disruptions and other dynamical interactions with stars or with massive black holes. We show that currently observed ``too young'' star in the halo could have been ejected from the Galactic disk or the Galactic centre and be observable in their current position if they were ejected as binaries (including the hypervelocity star HE 0437-5439; whereas other suggestions for its ejection from the LMC are shown to be highly unlikely). In addition, we suggest that triple disruptions by the massive black hole in the Galactic centre could also capture binaries in close orbits near the MBH, some of which may later evolve to become more massive rejuvenated stars.

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Title: Hypervelocity Stars: From the Galactic Center to the Halo
Authors: S. J. Kenyon, B. C. Bromley, M. J. Geller, W. R. Brown

Hypervelocity stars (HVS) traverse the Galaxy from the central black hole to the outer halo. We show that the Galactic potential within 200 pc acts as a high pass filter preventing low velocity HVS from reaching the halo. To trace the orbits of HVS throughout the Galaxy, we construct two forms of the potential which reasonably represent the observations in the range 5--100,000 pc, a simple spherically symmetric model and a bulge-disk-halo model. We use the Hills mechanism (disruption of binaries by the tidal field of the central black hole) to inject HVS into the Galaxy and compute the observable spatial and velocity distributions of HVS with masses in the range 0.6--4 Msun. These distributions reflect the mass function in the Galactic Centre, properties of binaries in the Galactic Centre, and aspects of stellar evolution and the injection mechanism. For 0.6--4 Msun main sequence stars, the fraction of unbound HVS and the asymmetry of the velocity distribution for their bound counterparts increases with stellar mass. The density profiles for unbound HVS decline with distance from the Galactic Centre approximately as r^{-2} (but are steeper for the most massive stars which evolve off the main sequence during their travel time from the Galactic Centre); the density profiles for the bound ejecta decline with distance approximately as r^{-3}. In a survey with a limiting visual magnitude V of 23, the detectability of HVS (unbound or bound) increases with stellar mass.

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Title: Hypervelocity A & B Stars should be slow rotators
Authors: Brad Hansen (UCLA)

The most commonly accepted explanation for the origin of hypervelocity stars in the halo of the Milky Way is that they are the result of tidal disruption of binaries by the massive black hole at the centre of the Galaxy. We show that, if this scenario is correct, and if the original binary properties are similar to those in the local stellar neighbourhood, then the hypervelocity stars should rotate with velocities measurably lower than those for field stars of similar spectral type. This may prove to be a more direct test of the model than trying to predict the position and velocity distributions.

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Title: Hypervelocity Stars III. The Space Density and Ejection History of Main Sequence Stars from the Galactic Centre
Authors: Warren R. Brown (1), Margaret J. Geller (1), Scott J. Kenyon (1), Michael J. Kurtz (1), Benjamin C. Bromley (2) ((1) Smithsonian Astrophysical Observatory (2) University of Utah)

We report the discovery of 3 new unbound hypervelocity stars (HVSs), stars travelling with such extreme velocities that dynamical ejection from a massive black hole (MBH) is their only suggested origin. We also detect a population of possibly bound HVSs. The significant asymmetry we observe in the velocity distribution -- we find 26 stars with v_rf > 275 km/s and 1 star with v_rf < -275 km/s -- shows that the HVSs must be short-lived, probably 3 - 4 Msun main sequence stars. Any population of hypervelocity post-main sequence stars should contain stars falling back onto the Galaxy, contrary to the observations. The spatial distribution of HVSs also supports the main sequence interpretation: longer-lived 3 Msun HVSs fill our survey volume; shorter-lived 4 Msun HVSs are missing at faint magnitudes. We infer that there are 96 ± 10 HVSs of mass 3 - 4 Msun within R < 100 kpc, possibly enough HVSs to constrain ejection mechanisms and potential models. Depending on the mass function of HVSs, we predict that SEGUE may find up to 5 - 15 new HVSs. The travel times of our HVSs favor a continuous ejection process, although a ~120 Myr-old burst of HVSs is also allowed.

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Title: Hypervelocity Stars from the Andromeda Galaxy
Authors: Blake D. Sherwin, Abraham Loeb, Ryan M. O'Leary

Hypervelocity stars (HVSs) discovered in the Milky Way (MW) halo are thought to be ejected from near the massive black hole (MBH) at the galactic centre. In this paper we investigate the spatial and velocity distributions of the HVSs which are expected to be similarly produced in the Andromeda galaxy (M31). We consider three different HVS production mechanisms: (i) the disruption of stellar binaries by the galactocentric MBH; (ii) the ejection of stars by an in-spiralling intermediate mass black hole; and (iii) the scattering of stars off a cluster of stellar-mass black holes around the MBH. While the first two mechanisms would produce large numbers of HVSs in M31, we show that the third mechanism would not be effective in M31. We numerically calculate 1.2*10^6 trajectories of HVSs from M31 within a simple model of the Local Group. Gravitational focusing of the HVSs by the MW and the diffuse Local Group medium leads to high densities of low mass (~ solar mass) M31 HVSs near the MW. For both relevant mechanisms, we expect there to be a few thousand solar mass M31 HVSs within the virialised MW halo, many of which should have distinctively large approach velocities (< -500 km/s). In addition, we predict ~5 hypervelocity RGB stars within the M31 halo which could be identified observationally. Future MW astrometric surveys or searches for distant giants could thus find HVSs from M31.

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Title: Hypervelocity binary stars: smoking gun of massive binary black holes
Authors: Youjun Lu, Qingjuan Yu, D.N.C. Lin

The hypervelocity stars recently found in the Galactic halo are expelled from the Galactic centre through interactions between binary stars and the central massive black hole or between single stars and a hypothetical massive binary black hole. In this paper, we demonstrate that binary stars can be ejected out of the Galactic centre with velocities up to 10ł km/s, while preserving their integrity, through interactions with a massive binary black hole. Binary stars are unlikely to attain such high velocities via scattering by a single massive black hole or through any other mechanisms. Based on the above theoretical prediction, we propose a search for binary systems among the hypervelocity stars. Discovery of hypervelocity binary stars, even one, is a definitive evidence of the existence of a massive binary black hole in the Galactic centre.

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Title: Hypervelocity stars and the environment of Sgr A*
Authors: A. Sesana, F. Haardt, P. Madau
(version v2)

Hypervelocity stars (HVSs) are a natural consequence of the presence of a massive nuclear black hole (Sgr A*) in the Galactic Centre. Here we use the Brown et al. sample of unbound and bound HVSs together with numerical simulations of the propagation of HVSs in the Milky Way halo to constrain three plausible ejection mechanisms: 1) the scattering of stars bound to Sgr A* by an inspiraling intermediate-mass black hole (IMBH); 2) the disruption of stellar binaries in the tidal field of Sgr A*; and 3) the two-body scattering of stars off a cluster of stellar-mass black holes orbiting Sgr A*. We compare the predicted radial and velocity distributions of HVSs with the limited-statistics dataset currently available, and show that the IMBH model appears to produce a spectrum of ejection velocities that is too flat. Future astrometric and deep wide-field surveys of HVSs should shed unambiguous light on the stellar ejection mechanism and probe the Milky Way potential on scales as large as 200 kpc.

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A speeding star may have been flung away by a black hole lurking unnoticed in a nearby galaxy, new research suggests. The finding is evidence for the existence of middleweight black holes, with a mass of around 1000 Suns.
There is abundant evidence that supermassive black holes with a mass of millions or billions of Suns dwell at the centres of most medium-to-large galaxies.
Some astronomers have suggested that they formed suddenly out of collapsing gas clouds, but most suspect that the supermassive black holes grew after their initial formation. If so, then somewhere out there could be a class of black holes that grew to medium size and then stopped, leaving them with just a few thousand solar masses.
Recent observations have hinted at the presence of such middleweights, but none of the evidence has been conclusive.

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Blog: Does Our Galaxy's Black Hole Have a Companion?
At the hub of the Milky Way, off in the constellation of Sagittarius, is a giant black hole weighing three million solar-masses. If a binary pair of stars approached the hole, the hole's gravitational tidal forces could rip apart the system and send one star careening out of the galaxy.
What's utterly strange, though, is that the hypervelocity stars aren't scattered randomly across the sky. They are aligned. Brown speculated that the alignment comes about because the galaxy's central black hole has a hitherto unnoticed companion black hole. The orbit of the companion would introduce a bias in the direction of ejected stars.

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