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Galactic hypervelocity stars
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Milky Way's fast-moving galactic hypervelocity stars may have come from another galaxy

These so-called "galactic hypervelocity stars" are large and short-lived but travel up to 1,000 kilometres per second.
Strangely, most of them appear to be in an unusual cluster in the northern hemisphere sky, and the origin of these huge speedsters has been a bit of a puzzle.
But now, researchers from the University of Cambridge argue these stars may have been flung off the front of the Large Magellanic Cloud, a dwarf galaxy travelling at high speed past the edge of the Milky Way galaxy.

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RE: Hypervelocity Stars
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Artificial brain helps Gaia catch speeding stars

With the help of software that mimics a human brain, ESA's Gaia satellite spotted six stars zipping at high speed from the centre of our Galaxy to its outskirts. This could provide key information about some of the most obscure regions of the Milky Way.
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Runaway Stars
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Runaway Stars Leave Infrared Waves in Space

When some speedy, massive stars plough through space, they can cause material to stack up in front of them in the same way that water piles up ahead of a ship. Called bow shocks, these dramatic, arc-shaped features in space are leading researchers to uncover massive, so-called runaway stars.
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RE: Hypervelocity Stars
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Title: Nearby Low-Mass Hypervelocity Stars
Author: Yanqiong Zhang, Martin C. Smith, Jeffrey L. Carlin

Hypervelocity stars are those that have speeds exceeding the escape speed and are hence unbound from the Milky Way. We investigate a sample of low-mass hypervelocity candidates obtained using data from the high-precision SDSS Stripe 82 catalogue, which we have combined with spectroscopy from the 200-inch Hale Telescope at Palomar Observatory. We find four good candidates, but without metallicities it is difficult to pin-down their distances and therefore total velocities. Our best candidate has a significant likelihood that it is escaping the Milky Way for a wide-range of metallicities.

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Fastest Unbound Stars
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Title: The Fastest Unbound Stars in the Universe
Author: James Guillochon (1), Abraham Loeb (1) ((1) Harvard ITC)

The discovery of hypervelocity stars (HVS) leaving our galaxy with speeds of nearly 10^3 km s^-1 has provided strong evidence towards the existence of a massive compact object at the galaxy's center. HVS ejected via the disruption of stellar binaries can occasionally yield a star with v_{\infty} \lesssim 10^4 km s^-1, here we show that this mechanism can be extended to massive black hole (MBH) mergers, where the secondary star is replaced by a MBH with mass M2 \gtrsim 10^5 solar masses. We find that stars that are originally bound to the secondary MBH are frequently ejected with v_{\infty}>10^4 km s^-1, and occasionally with velocities ~10^5 km s^-1 (one third the speed of light), for this reason we refer to stars ejected from these systems as "semi-relativistic" hypervelocity stars (SHS). Bound to no galaxy, the velocities of these stars are so great that they can cross a significant fraction of the observable universe in the time since their ejection (several Gpc). We demonstrate that if a significant fraction of MBH mergers undergo a phase in which their orbital eccentricity is \gtrsim 0.5 and their periapse distance is tens of the primary's Schwarzschild radius, the space density of fast-moving (v_{\infty}>10^4 km s^-1) SHS may be as large as 10^3 Mpc^-3. Hundreds of the SHS will be giant stars that could be detected by future all-sky infrared surveys such as WFIRST or Euclid and proper motion surveys such as LSST, with spectroscopic follow-up being possible with JWST.

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RE: Hypervelocity Stars
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Title: The Nature of Hypervelocity Stars and the Time Between Their Formation and Ejection
Authors: Warren R. Brown (1), Judith G. Cohen (2), Margaret J. Geller (1), Scott J. Kenyon (1) ((1) Smithsonian, (2) Caltech)

We obtain Keck HIRES spectroscopy of HVS5, one of the fastest unbound stars in the Milky Way halo. We show that HVS5 is a 3.62 ± 0.11 solar mass main sequence B star at a distance of 50 ±5 kpc. The difference between its age and its flight time from the Galactic center is 105 ±18(stat)±30(sys) Myr; flight times from locations elsewhere in the Galactic disk are similar. This 10^8 yr `arrival time' between formation and ejection is difficult to reconcile with any ejection scenario involving massive stars that live for only 10^7 yr. For comparison, we derive arrival times of 10^7 yr for two unbound runaway B stars, consistent with their disk origin where ejection results from a supernova in a binary system or dynamical interactions between massive stars in a dense star cluster. For HVS5, ejection during the first 10^7 yr of its lifetime is ruled out at the 3-sigma level. Together with the 10^8 yr arrival times inferred for three other well-studied hypervelocity stars (HVSs), these results are consistent with a Galactic center origin for the HVSs. If the HVSs were indeed ejected by the central black hole, then the Galactic center was forming stars ~200 Myr ago, and the progenitors of the HVSs took ~100 Myr to enter the black hole's loss cone.

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Title: Binary Disruption by Massive Black Holes: Hypervelocity Stars, S Stars, and Tidal Disruption Events
Authors: Benjamin C. Bromley (1), Scott J. Kenyon (2), Margaret J. Geller (2), Warren R. Brown (2) ((1) University of Utah (2) Smithsonian Astrophysical Observatory)

We examine whether disrupted binary stars can fuel black hole growth. In this mechanism, tidal disruption produces a single hypervelocity star (HVS) ejected at high velocity and a former companion star bound to the black hole. After a cluster of bound stars forms, orbital diffusion allows the black hole to accrete stars by tidal disruption at a rate comparable to the capture rate. In the Milky Way, HVSs and the S star cluster imply similar rates of 10^{-5}--10^{-3} yr^{-1} for binary disruption. These rates are consistent with estimates for the tidal disruption rate in nearby galaxies and imply significant black hole growth from disrupted binaries on 10 Gyr time scales.

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Title: Metal-poor hypervelocity star candidates from the Sloan Digital Sky Survey
Authors: Yinbi Li, Ali Luo, Gang Zhao, Juanjuan Ren, Fang Zuo

Hypervelocity stars are believed to be ejected out from the Galactic center through dynamical interactions of (binary) stars with the central massive black hole(s). In this letter, we report 13 metal-poor F-type hypervelocity star candidates selected from 370,000 stars of the data release 7 of the Sloan Digital Sky Survey. With a detailed analysis of the kinematics of these stars, we find that seven of them were likely ejected from the Galactic center (GC) or the Galactic disk, four neither originated from the GC nor the Galactic disk, and the other two were possibly ejected from either the Galactic disk or other regions. Those candidates which unlikely originated from the GC or the Galactic disk, may be explained by other mechanisms, like the tidal disruption of the Milky Way's dwarf galaxies in the Galactic potential, or the gravitational interactions with a massive black hole at the center of M31 or M32.

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Title: Runaway and hypervelocity stars. The supernova connection
Authors: Ralf Napiwotzki, Manuel D. V. Silva

We present an investigation of the known sample of runaway stars. The orbits of these stars are traced back to their origin in the Galactic disc. The velocity distribution of these stars is compared to theoretical predictions. We conclude that the majority of stars is well explained by the standard binary ejection mechanism (BEM) and the dynamical ejection mechanism (DEM). However, we find a sample of ten stars which has ejection velocities in excess of those predicted by standard scenarios. We discuss how these can be explained by a variant of the BEM. This mechanism can create runaway stars exceeding the Galactic escape velocity (known as hypervelocity stars). The number of runaway stars in our Galaxy is estimated and compared to the known sample of high mass X-ray binaries, whose formation is linked to the BEM channel.
 
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Google earth file: Hypervelocity stars.kmz (2kb, kmz)



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