* Astronomy

Blobrana · L

RE: HE 0437-5439

Blobrana · L

A hundred million years ago, a triple-star system was travelling through the bustling centre of our Milky Way galaxy when it made a life-changing misstep. The trio wandered too close to the galaxy's giant black hole, which captured one of the stars and hurled the other two out of the Milky Way. Adding to the stellar game of musical chairs, the two outbound stars merged to form a super-hot, blue star.
This story may seem like science fiction, but astronomers using NASA's Hubble Space Telescope say it is the most likely scenario for a so-called hypervelocity star, known as HE 0437-5439, one of the fastest ever detected. It is blazing across space at a speed of 2.5 million kilometres an hour, three times faster than our Sun's orbital velocity in the Milky Way. Hubble observations confirm that the stellar speedster hails from our galaxy's core.
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Blobrana · L

Speeding star traced back to Milky Way's heart

Black holes are picky eaters, swallowing some stars and spitting out others. That is what is suggested by the recent mapping of the path taken by a hypervelocity star.
Models show that when a binary star approaches a supermassive black hole, the gravitational interaction between the three objects can cause one star to fall in while the other is flung out at up to 4000 kilometres per second. About 20 such hypervelocity stars have so far been detected, but their precise origin was unclear.
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Blobrana · L

NASA'S Hubble Shows Hyperfast Star Was Booted From Milky Way

NASA's Hubble Space Telescope has detected a hypervelocity star, a rare phenomenon moving three times faster than our sun.
The star may have been created in a cosmic misstep. A hundred million years ago, a triple-star system was travelling through the bustling centre of our Milky Way galaxy when it wandered too close to the galaxy's giant black hole. The black hole captured one of the stars and hurled the other two out of the Milky Way. The two outbound stars merged to form a super-hot blue star travelling at incredible speeds.
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Blobrana · L

Title: A Galactic Center Origin for HE 0437-5439, the Hypervelocity Star near the Large Magellanic Cloud
Authors: Warren R. Brown (1), Jay Anderson (2), Oleg Y. Gnedin (3), Howard E. Bond (2), Margaret J. Geller (1), Scott J. Kenyon (1), Mario Livio (2) ((1) Smithsonian Astrophysical Observatory, (2) Space Telescope Science Institute, (3) University of Michigan)

We use Hubble Space Telescope imaging to measure the absolute proper motion of the hypervelocity star (HVS) HE 0437-5439, a short-lived B star located in the direction of the Large Magellanic Cloud (LMC). We observe (\mu_\alpha, \mu_\delta)=(+0.53±0.25(stat)±0.33(sys), +0.09±0.21(stat)±0.48(sys)) mas/yr. The velocity vector points directly away from the center of the Milky Way; an origin from the center of the LMC is ruled out at the 3-sigma level. The flight time of the HVS from the Milky Way exceeds its main-sequence lifetime, thus its stellar nature requires it to be a blue straggler. The large space velocity rules out a Galactic-disk ejection. Combining the HVS's observed trajectory, stellar nature, and required initial velocity, we conclude that HE 0437-5439 was most likely a compact binary ejected by the Milky Way's central black hole.

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Blobrana · L

Title: Runaway and hypervelocity stars in the Galactic halo: Binary rejuvenation and triple disruption
Authors: Hagai B. Perets
(Version v3)

Young stars observed in the distant Galactic halo are usually thought to have formed elsewhere, either in the Galactic disk or perhaps the Galactic center, 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/center. 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 center and be observable in their current position if they were ejected as binaries. Specifically it is shown that the hypervelocity star HE 0437-5439 could be such a rejuvenated star. Other suggestions for its ejection from the LMC are found to be highly unlikely. Moreover, it is shown that its observed metallicity is most consistent with a Galactic origin and a Galactic center origin can not currently rule out. In addition, we suggest that triple disruptions by the massive black hole in the Galactic center 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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Blobrana · L

A young star is speeding away from the Milky Way so fast that astronomers have been puzzled by where it came from; based on its young age it has travelled too far to have come from our galaxy.

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Blobrana · L

Title: LMC origin of the hyper-velocity star HE 0437-5439. Beyond the supermassive black hole paradigm
Authors: N. Przybilla, M. F. Nieva, U. Heber, M. Firnstein, K. Butler, R. Napiwotzki, H. Edelmann

Context: Hyper-velocity stars move so fast that only a supermassive black hole (SMBH) seems to be capable to accelerate them. Hence the Galactic centre (GC) is their only suggested place of origin. Edelmann et al. (2005) found the early B-star HE0437-5439 to be too short-lived to have reached its current position in the Galactic halo if ejected from the GC, except if being a blue straggler. Its proximity to the LMC suggested an origin from this galaxy. Aims: The chemical signatures of stars at the GC are significantly different from those in the LMC. Hence, an accurate measurement of the abundance pattern of HE0437-5439 will yield a new tight constraint on the place of birth of this star. Methods: High-resolution spectra obtained with UVES on the VLT are analysed using state-of-the-art non-LTE modelling techniques. Results: We measured abundances of individual elements to very high accuracy in HE0437-5439 as well as in two reference stars, from the LMC and the solar neighbourhood. The abundance pattern is not consistent at all with that observed in stars near the GC, ruling our an origin from the GC. However, there is a high degree of consistency with the LMC abundance pattern. Our abundance results cannot rule out an origin in the outskirts of the Galactic disk. However, we find the life time of HE0437-5439 to be more than 3 times shorter than the time of flight to the edge of the disk, rendering a Galactic origin unlikely. Conclusions: Only one SMBH is known to be present in Galaxy and none in the LMC. Hence the exclusion of an GC origin challenges the SMBH paradigm. We conclude that there must be other mechanism(s) to accelerate stars to hyper-velocity speed than the SMBH. We draw attention to dynamical ejection from dense massive clusters, that has recently been proposed by Gvaramadze et al. (2008).

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Blobrana · L

Title: HE 0437-5439 -- an unbound hyper-velocity main-sequence B-type star
Authors: Heinz Edelmann, Ralf Napiwotzki, Uli Heber, Norbert Christlieb, Dieter Reimers

We report the discovery of a 16th magnitude star, HE0437-5439, with a heliocentric radial velocity of +723 ±3km/s. A quantitative spectral analysis of high-resolution optical spectra obtained with the VLT and the UVES spectrograph shows that HE0437-5439 is a main sequence B-type star with Teff=20350K, log g=3.77, solar within a factor of a few helium abundance and metal content, rotating at v sin i=54km/s. Using appropriate evolutionary tracks we derive a mass of 8 Msun and a corresponding distance of 61 kpc. Its galactic rest frame velocity is at least 563km/s, almost twice the local Galactic escape velocity, indicating that the star is unbound to the Galaxy. Numerical kinematical experiments are carried out to constrain its place of birth. It has been suggested that such hyper-velocity stars can be formed by the tidal disruption of a binary through interaction with the super-massive black hole at the Galactic centre (GC). HE0437-5439 needs about 100Myrs to travel from the GC to its presentposition, much longer than its main sequence lifetime of 25Myrs. This can only be reconciled if HE0437-5439 is a blue straggler star. In this case, the predicted proper motion is so small that it can only be measured by future space missions. Since the star is much closer to the Large Magellanic Cloud (LMC, 18kpc) than to the GC, it can reach its position from the centre of the LMC. The proper motion predicted in this case is about 2mas/y (relative to the LMC), large enough to be measurable with conventional techniques from the ground. The LMC origin could also be tested by a high-precision abundance analysis.

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Blobrana · L

A young star is speeding away from the Milky Way so fast that astronomers have been puzzled by where it came from; based on its young age it has travelled too far to have come from our galaxy. Now by analysing its velocity, light intensity, and for the first time its tell-tale elemental composition, Carnegie astronomers Alceste Bonanos and Mercedes López-Morales, and collaborators Ian Hunter and Robert Ryans from Queens University Belfast have determined that it came from our neighbouring galaxy, the Large Magellanic Cloud (LMC). The result suggests that it was ejected from that galaxy by a yet-to-be-observed massive black hole. The research will be published in an upcoming issue of the Astrophysical Journal Letters.
The star, dubbed HE 0437-5439, is an early-type star and one of ten so-called hypervelocity stars so far found speeding away from the Milky Way.

But this one is different from the other nine. Their type, speed, and age make them consistent with having been ejected from the centre of our galaxy, where we know there is a super-massive black hole. This star, discovered in 2005, initially appeared to have an elemental makeup like our Suns, suggesting that it, too, came from the centre of our galaxy. But that didnt make sense because it would have taken 100 million years to get to its location, and HE 0437-5439 is only 35 million years old - López-Morales.

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