A Black Hole Boost into Intergalactic Space

by Paul Gilster on January 28, 2006

What would fling a star out of the galaxy at over 1 million miles per hour? Warren Brown (Harvard-Smithsonian Center for Astrophysics) and colleagues have some thoughts on that, based on their own and other studies in Europe that have so far identified five stellar exiles, a group now called ‘hypervelocity stars.’ “These stars literally are castaways,” says Brown. “They have been thrown out of their home galaxy and set adrift in an ocean of intergalactic space.”

Brown’s team went after galactic escapees in a targeted manner, using computer models that showed such stars would be forced into their current trajectories by interactions in the galactic core. The idea is this: a binary star swings too close to the black hole at the galaxy’s center. Its gravity tears the duo apart, in the process capturing one of the stars and ejecting the other one at high velocity.

Evidence exists not only in the exiled stars themselves but in the other half of the binary pairs that once contained them; the stars the exiles leave behind orbit the central black hole in just the kind of elongated, elliptical orbit that would be expected from the computer models. Brown’s team figures a star is ejected from the galaxy about once every 100,000 years, although our knowledge of the extreme conditions at the core still needs a great deal of refinement to understand the process.

And in a unique way, these curious objects are interstellar probes of their own. By studying them, we learn something about the structure of the Milky Way. Says Margaret Geller, co-author of the paper on this work, “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.”

The team’s study has been submitted to the Astrophysical Journal Letters as “A Successful Targeted Search for Hypervelocity Stars,” now available online at the arXiv site.

ljk July 16, 2007 at 15:27

Hypervelocity binary stars: smoking gun of massive binary black holes

Authors: Youjun Lu, Qingjuan Yu, D.N.C. Lin

(Submitted on 12 Jul 2007)

Abstract: The hypervelocity stars recently found in the Galactic halo are expelled from the Galactic center 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 center with velocities up to 10^3 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 center.

Comments: 5 pages, 3 figures

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0707.1872v1 [astro-ph]

Submission history

From: Qingjuan Yu [view email]

[v1] Thu, 12 Jul 2007 20:24:22 GMT (41kb)

http://arxiv.org/abs/0707.1872

ljk December 13, 2007 at 10:20

Dynamical and evolutionary constraints on the nature and origin of hypervelocity stars

Authors: Hagai B. Perets

(Submitted on 12 Dec 2007)

Abstract: In recent years several hypervelocity stars (HVSs) have been observed in the halo of our Galaxy. Such stars are thought to be ejected through dynamical interactions near the massive black hole (MBH) in the Galactic center. Three scenarios have been suggested for their ejection; binary disruption by a MBH, scattering by inspiraling IMBH and scattering by stellar BHs close to MBH. These scenarios involve different stellar populations in the Galactic center. Here we use observations of the Galactic center stellar population, dynamical and evolutionary arguments to constrain the nature and origin of HVSs. We show that the IMBH inspiral scenario requires too many, O(10^3), main sequence B stars to exist close to the MBH (<0.01 pc) at the time of inspiral, where current observations show O(10) such stars. Scattering by SBHs are also not likely to be consistent with the observed population of B stars in the Galactic center, although this scenario can still be compatible with observations under extreme conditions. The binary disruption scenario is still consistent with current observations. In addition we show that due to the conditions close to the MBH most binary star systems are not expected to survive for long in this region. Consequently, unique stellar populations that require long evolution in binaries are not expected to be ejected as HVSs in the BHs scattering mechanisms (this may also be related to to the recently observed asymmetry in the velocity distribution of HVSs). Furthermore, it is shown that recently suggested signatures for HVSs origin such as hypervelocity binaries and slow rotating HVSs may be much weaker than suggested and require large statistics.

Comments: 6 pages, 2 figures. Comments are most welcome

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0712.1888v1 [astro-ph]

Submission history

From: Hagai B. Perets [view email]

[v1] Wed, 12 Dec 2007 09:39:04 GMT (88kb)

http://arxiv.org/abs/0712.1888

Low Metallicity Indicates that the Hypervelocity Star HE 0437-5439 was Ejected from the LMC

Authors: Alceste Z. Bonanos, Mercedes Lopez-Morales (Carnegie-DTM)

(Submitted on 11 Dec 2007)

Abstract: We measure the metallicity of the unusual hypervelocity star HE 0437-5439 from high resolution spectroscopy to be half-solar. We determine a spectral type of B2 III-IV for the star and derive an effective temperature Teff = 22,000 +/- 1,000 K and a surface gravity log(g) = 4.00 +/- 0.25 (cgs). We also present BV time series photometry and find the star to be non-variable at the 0.02 mag level. We refine the magnitude of the hypervelocity star to V = 16.36 +/- 0.04 mag, with a color B-V = -0.23 +/- 0.03 mag, confirming its early-type nature. Our metallicity result establishes the origin of HE 0437-5439 in the Large Magellanic Cloud and implies the existence of a massive black hole somewhere in this galaxy.

Comments: 12 pages, 4 figures. Submitted to ApJL

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0712.1825v1 [astro-ph]

Submission history

From: Mercedes Lopez-Morales [view email]

[v1] Tue, 11 Dec 2007 23:07:21 GMT (55kb)

http://arxiv.org/abs/0712.1825

ljk January 24, 2008 at 0:04

Hypervelocity Stars: From the Galactic Center to the Halo

Authors: S. J. Kenyon, B. C. Bromley, M. J. Geller, W. R. Brown

(Submitted on 22 Jan 2008)

Abstract: 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 Center, properties of binaries in the Galactic Center, 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 Center 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 Center); 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.

Comments: 32 pages of text, 5 tables, 12 figures, submitted to ApJ

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0801.3479v1 [astro-ph]

Submission history

From: Scott J. Kenyon [view email]

[v1] Tue, 22 Jan 2008 22:58:45 GMT (90kb)

http://arxiv.org/abs/0801.3479

ljk January 28, 2008 at 9:34

Hyperfast star proven to be alien

Washington, D.C. – 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
traveled too far to have come from our galaxy. Now by
analyzing its velocity, light intensity, and for the first time
its tell-tale elemental composition, Carnegie astronomers
Alceste Bonanos and Mercedes Lopez-Morales, and
collaborators Ian Hunter and Robert Ryans from Queen’s
University Belfast have determined that it came from our
neighboring 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.

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