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HD 271791
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Mystery of hypervelocity star unveiled
A Russian astronomer from Moscow State University has explained the origin of the hypervelocity runaway star HD 271791 and the nature of its extremely high velocity.

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Title: HD 271791: an extreme supernova run-away B star escaping from the Galaxy
Authors: Norbert Przybilla, M. Fernanda Nieva, Ulrich Heber and Keith Butler

Hyper-velocity stars (HVSs) were first predicted by theory to be the result of the tidal disruption of a binary system by a super-massive black hole (SMBH) that accelerates one component to beyond the Galactic escape velocity (the Hills mechanism). Because the Galactic centre hosts such a SMBH it is the suggested place of origin for HVSs. However, the SMBH paradigm has been challenged recently by the young HVS HD 271791 because its kinematics point to a birthplace in the metal-poor rim of the Galactic disc. Here we report the atmosphere of HD 271791 to indeed show a sub-solar iron abundance along with an enhancement of the alpha -elements, indicating capture of nucleosynthesis products from a supernova or a more energetic hypernova. This implies that HD 271791 is the surviving secondary of a massive binary system disrupted in a supernova explosion. No such run-away star has ever been found to exceed the Galactic escape velocity, hence HD 271791 is the first hyper-runaway star. Such a run-away scenario is an alternative to the Hills mechanism for the acceleration of some HVSs with moderate velocities. The observed chemical composition of HD 271791 puts invaluable observational constraints on nucleosynthesis in a supernova from the core-collapse of a very massive star (M_ZAMS > 55 M_o), which may be observed as a gamma-ray burst of the long-duration/soft-spectrum type.

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HD271791
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Title: On the origin of the hypervelocity runaway star HD271791
Authors: V.V.Gvaramadze

We discuss the origin of the runaway early B-type star HD271791 and show that its extremely high velocity (\simeq 530-920 km/s) cannot be explained within the framework of the binary-supernova ejection scenario. Instead, we suggest that HD271791 attained its peculiar velocity in the course of a strong dynamical encounter between two hard massive binaries or via an exchange encounter between a hard massive binary and a very massive star, formed through runaway mergers of ordinary massive stars in the dense core of a young massive star cluster.

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Runaway stars
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Ripples in cosmic gas that resemble speedboat wakes have revealed a new population of young, renegade stars barrelling through the universe at more than 180,250 kilometres an hour.
The stars appear to be just a few million years old and a few times larger than the sun.
As they careen through the cosmos, the stars' winds slam against nearby gas, creating enormous bow shocks billions or even a trillion miles wide.
So far astronomers have found 14 of these rogue stars using images from the Hubble Space Telescope.

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Hubble Finds Stars That 'Go Ballistic'
Resembling comets streaking across the sky, these four speedy stars are plowing through regions of dense interstellar gas and creating brilliant arrowhead structures and trailing tails of glowing gas. The stars in these NASA Hubble Space Telescope images are among 14 young runaway stars spotted by the Advanced Camera for Surveys between October 2005 and July 2006. The images will be presented today at the American Astronomical Society meeting in Long Beach, Calif.

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RE: Hypervelocity Stars
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Some of the Milky Way's fastest stars may be immigrants from an orbiting dwarf galaxy that merged with our own, new research suggests.
"Hypervelocity" stars are racing at breakneck speeds out of our galaxy. The prevailing view holds that they are natives of the Milky Way's crowded centre, and that they were catapulted to such extreme speeds by the colossal black hole thought to lurk there.
In that scheme, a pair of stars wanders too close to the supermassive black hole, and one star gets captured while the other gets flung outwards at up to 4000 km/sec.

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Title: MMT Hypervelocity Star Survey
Authors: Warren R. Brown, Margaret J. Geller, Scott J. Kenyon (Smithsonian Astrophysical Observatory)
(Version v2)

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Title: 'Slow' and Fast Rotators among Hypervelocity Stars
Authors: Mercedes Lopez-Morales, Alceste Z. Bonanos
(Version v2)

We measure the projected 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 in principle consistent with having originated in a binary system, assuming a high inclination angle of the stellar rotation axis. However, the fast rotation of HVS8 is more typical of single B-type stars. HVS8 could have therefore 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: MMT Hypervelocity Star Survey
Authors: Warren R. Brown, Margaret J. Geller, Scott J. Kenyon (Smithsonian Astrophysical Observatory)

We describe a new survey for unbound hypervelocity stars (HVSs), stars travelling with such extreme velocities that dynamical ejection from a massive black hole (MBH) is their most likely origin. We investigate the possible contribution of unbound runaway stars, and show that the physical properties of binaries constrain low mass runaways to bound velocities. We measure radial velocities for HVS candidates with the colours of early A-type and late B-type stars. We report the discovery of 6 unbound HVSs with velocities and distances exceeding the conservative escape velocity estimate of Kenyon and collaborators. We additionally report 4 possibly unbound HVSs with velocities and distances exceeding the lower escape velocity estimate of Xue and collaborators. These discoveries increase the number of known HVSs by 60%-100%. Other survey objects include 19 newly identified z~2.4 quasars. One of the HVSs may be a horizontal branch star, consistent with the number of evolved HVSs predicted by Galactic centre ejection models. Finding more evolved HVSs will one day allow a probe of the low-mass regime of HVSs and will constrain the mass function of stars in the Galactic centre.

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Title: Hypervelocity Stars: Young and Heavy or Old and Light?
Authors: Uli Heber, Heiko Hirsch, Heinz Edelmann, Ralf Napiwotzki, Simon O`Toole, Warren Brown, Martin Altmann

The first three hyper-velocity stars (HVS) unbound to the Galaxy were serendipitously discovered in 2005. The only suggested origin of hyper-velocity stars is the Galactic Centre as it hosts a super-massive black hole capable of accelerating stars to such high velocities. Only one HVS, the sdO star US 708, is known to be an old low mass star, while HE 0437- 5439 is an apparently normal early-type B-star, too short-lived to originate from the Galactic Centre, but could possibly come from the LMC. A systematic survey has led to the discovery of seven new HVS of late B-type (similar to the prototype HVS1), which can either be massive stars 3 M(sun) or horizontal branch stars, sufficiently long-lived to have travelled from the Galactic Centre. We present new spectral analyses of five known HVS as well as of a newly discovered candidate. It is possible that the late B-type HVS are a mix of main sequence and evolved BHB stars. In view of the time scale problem we revisit HE 0437 -5439 and discuss a possible subluminous nature of this star.

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