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RE: Hypervelocity Stars
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Title: Hypervelocity Stars: Predicting the Spectrum of Ejection Velocities
Authors: Benjamin C. Bromley (1), Scott J. Kenyon (2), Margaret J. Geller (2), Elliott Barcikowski (1), Warren R. Brown (2), Michael J. Kurtz (2) ((1) University of Utah, (2) Smithsonian Astrophysical Observatory)

The disruption of binary stars by the tidal field of the black hole in the Galactic Centre can produce the hypervelocity stars observed in the halo. Researchers use numerical models to simulate the full spectrum of observable velocities of stars ejected into the halo by this binary disruption process. their model includes a range of parameters for binaries with 3-4 Solar mass primaries, consideration of radial orbits of the ejected stars through an approximate mass distribution for the Galaxy, and the impact of stellar lifetimes. The researchers calculate the spectrum of ejection velocities and reproduce previous results for the mean ejection velocity at the Galactic centre. The model predicts that the full population of ejected stars includes both the hypervelocity stars with velocities large enough to escape from the Galaxy and a comparable number of ejected, but bound, stars of the same stellar type. The predicted median speeds of the population of ejected stars as a function of distance in the halo are consistent with current observations. Combining the model with the data also shows that interesting constraints on the properties of binaries in the Galactic Centre and on the mass distribution in the Galaxy can be obtained even with modest samples of ejected stars.

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Hyper-Velocity Stars
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Title: Ejection of Hyper-Velocity Stars from the Galactic Centre by Intermediate-Mass Black Holes
Authors: Holger Baumgardt, Alessia Gualandris, Simon Portegies Zwart

Researchers have performed N-body simulations of the formation of hyper-velocity stars (HVS) in the centre of the Milky Way due to inspiralling intermediate-mass black holes (IMBHs). They considered IMBHs of different masses, all starting from circular orbits at an initial distance of 0.1 pc.
The researchers find that the IMBHs sink to the centre of the Galaxy due to dynamical friction, where they deplete the central cusp of stars. Some of these stars become HVS and are ejected with velocities sufficiently high to escape the Galaxy. Since the HVS carry with them information about their origin, in particular in the moment of ejection, the velocity distribution and the direction in which they escape the Galaxy, detecting a population of HVS will provide insight in the ejection processes and could therefore provide indirect evidence for the existence of IMBHs.
Their simulations show that HVS are generated in short bursts which last only a few Million yars until the IMBH is swallowed by the supermassive black hole (SMBH). HVS are ejected almost isotropically, which makes IMBH induced ejections hard to distinguish from ejections due to encounters of stellar binaries with a SMBH. After the HVS have reached the galactic halo, their escape velocities correlate with the distance from the Galactic centre in the sense that the fastest HVS can be found furthest away from the centre. The velocity distribution of HVS generated by inspiralling IMBHs is also nearly independent of the mass of the IMBH and can be quite distinct from one generated by binary encounters.
Finally, their simulations show that the presence of an IMBH in the Galactic centre changes the stellar density distribution inside r<0.02 pc into a core profile, which takes at least 100 million years to replenish.

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RE: Hypervelocity Stars
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Hypervelocity stars also offer a unique probe of galactic structure.

"During their lifetime, these stars travel across most of the Galaxy. If we could measure their motions across the sky, we could learn about the shape of the Milky Way and about the way the mysterious dark matter is distributed" - Margaret Geller.

The first newfound exile, in the direction of the constellation Ursa Major, is designated SDSS J091301.0+305120. It is travelling out of the galaxy at a speed of about 1.25 million miles per hour and currently is located at a distance of about 240,000 light-years from the earth. The second exile, in the direction of the constellation Cancer, is designated SDSS J091759.5+672238. It is moving outward at 1.43 million miles per hour and currently is located about 180,000 light-years from the earth.


This photograph from the Sloan Digital Sky Survey shows one of two newly discovered hypervelocity stars (marked with an arrow). SDSS J091301.0+305120 is travelling out of the galaxy at a speed of about 1.25 million miles per hour and currently is located at a distance of about 240,000 light-years from the earth. This image is about 7 arcminutes on a side, showing an area of the sky about 1/15 the size of the Full Moon.
Credit: SDSS Collaboration



This photograph from the Sloan Digital Sky Survey shows the second of two newly discovered hypervelocity stars (marked with an arrow). SDSS J091759.5+672238 is moving outward at 1.43 million miles per hour and currently is located about 180,000 light-years from the earth. This image is about 7 arcminutes on a side, showing an area of the sky about 1/15 the size of the Full Moon.
Credit: SDSS Collaboration



A Successful Targeted Search for Hypervelocity Stars
Authors: Warren R. Brown, Margaret J. Geller, Scott J.Kenyon, Michael J. Kurtz (Smithsonian Astrophysical Observatory)

Hypervelocity stars (HVSs) travel with velocities so extreme that dynamical ejection from a massive black hole is their only suggested origin. Following our discovery of the first HVS, we have undertaken a dedicated survey for more HVSs in the Galactic halo and present here the resulting discovery of two new HVSs: SDSS J091301.0+305120 and SDSS J091759.5+672238, travelling with Galactic rest-frame velocities at least +558+-12 and +638+-12 km/s, respectively.
Assuming the HVSs are B8 main sequence stars, they are at distances ~75 and ~55 kpc, respectively, and have travel times from the Galactic Centre consistent with their lifetimes. The existence of two B8 HVSs in our 1900 deg2 survey, combined with the Yu & Tremaine HVS rate estimates, is consistent with HVSs drawn from a standard initial mass function but inconsistent with HVS drawn from a truncated mass function like the one in the top-heavy Arches cluster.
The travel times of the five currently known HVSs provide no evidence for a burst of HVSs from a major in-fall event at the Galactic Center in the last \~160 Myr.



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Astronomers last year spotted three stars escaping the Milky Way galaxy. On Thursday, researchers announced the discovery of yet two more outbound stars.

The two newfound stars are racing out of the galaxy at more than a million miles an hour, fast enough to escape the galaxy’s gravity.
The escape velocity of the galaxy is 500km/ sec.
Five exiled stars now are known, making them a new class of objects called hypervelocity stars.
However, one of the stars previously discovered may have come from the Large Magellanic Cloud.
The researchers calculate that a star is flung from the galaxy every 100,000 years or so.

"These stars literally are castaways. They have been thrown out of their home galaxy and set adrift in an ocean of intergalactic space" - Warren Brown of the Harvard-Smithsonian Centre for Astrophysics.



Both of the new stars are outside the galaxy's main plane but have yet to leave the halo.
One is moving in the direction of the constellation Ursa Major at about 2 million kilometres per hour with respect to the galaxy. It is 240,000 light-years away.
The other is headed toward the constellation Cancer, outbound at 2.3 million kilometres per hour and 180,000 light-years away.

"Discovering these two new exiled stars was neither lucky nor random. We made a targeted search for them. By understanding their origin, we knew where to find them" - Margaret Geller, Smithsonian Astrophysical Observatory.

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