Fast Mover from the Large Magellanic Cloud

by Paul Gilster on January 28, 2008

Hypervelocity star

Stars being kicked out of the Milky Way — so-called ‘hypervelocity stars’ — follow a mechanism that seems understood. We know there is a supermassive black hole at galactic center, and it is likely the cause of the ejection of such stars from our galaxy. Nine stars have been found that fit this description, all of them over 50,000 parsecs from Earth. But the tenth is an anomaly, a young star ejected not from the Milky Way but from the Large Magellanic Cloud. A black hole is assumed to be the cause here as well, although the culprit has yet to be identified.

Image: A ‘hypervelocity star,’ shown flung from the Milky Way’s center. Now a similar star has been found exiting the Large Magellanic Cloud. Credit: Ruth Bazinet/Harvard-Smithsonian Center for Astrophysics.

One thing that assists researchers in identifying stellar origins is the fact that stars in the Large Magellanic Cloud (LMC) have their own particular characteristics. Alceste Bonanos (Carnegie Institution) was on the team that noted differences between this star — HE 0437-5439 — and the other galactic refugees. It couldn’t, for example, have come from the center of the Milky Way because getting it to its present location would have taken 100 million years, and the star is only 35 million years old. And then there is the matter of stellar elements. Says Bonanos:

“We were intrigued by the conundrum and decided to take up the challenge to solve this. Stars in the LMC are known to have lower elemental abundances than most stars in our galaxy, so we could determine if its chemistry was more like that galaxy’s or our own.”

The Carnegie team was able to verify the star’s age, and the fact that HE 0437-5439 is nine times the mass of the Sun. Its velocity was pegged at a scorching 722 kilometers per second. That’s fast indeed (ordinary stars in the Milky Way average out at around 100 km/s), but consistent with the velocities of other hypervelocity stars. And as for the relative abundance of various elements, the star shows a concentration about half that of the Sun, consistent with an origin in the LMC.

The ‘launch’ mechanism? Assume that the star was originally part of a binary system and you can set up a close pass by a massive black hole, with one star captured by the exotic object, the other flung out of the galaxy. This scenario was first proposed by Jack Hills (Los Alamos National Laboratory) in 1988, in a paper proposing that hypervelocity stars might exist and provide evidence of the black hole at galactic center. The first observational evidence for such stars came in 2005, and the assumption is that many more such stars are waiting to be found.

What we have here is the first observational evidence of a black hole in the Large Magellanic Cloud, surely a priority target for future Magellanic watchers. This is also the first time the abundance of key elements has been measured in a hypervelocity star. Thus does a rare find (only ten known hypervelocity stars) become rarer yet, and we measure the motions of a star whose origins lie in a satellite galaxy to our own.

The paper is Bonanos et al., “Low Metallicity Indicates that the Hypervelocity Star HE 0437-5439 was Ejected from the LMC,” scheduled to run in the Astrophysical Journal Letters and available online. And for you completists out there, Jack Hills’ paper is “Hyper-velocity and tidal stars from binaries disrupted by a massive Galactic black hole,” Nature 331 (25 February 1988), pp. 687-689 (abstract).

{ 16 comments }

david January 28, 2008 at 14:44

Could be Wan-To at play.

If the planets of such a star could survive the forces that accelerate their parent star to such velocities it would be an interesting spot for a civilization to develop. If it were a long lived civilization they might pass close by hundreds of stars within a few million years.

Hans Bausewein January 28, 2008 at 16:47

I guess such a civilization doesn’t have much time before its star goes supernova. (if I have the lifetimes of that massive stars correctly)
I even doubt that multicellular live can develop before the supernova time.

It would be a hard job to escape such a star, at least to find something that you can orbit. You cannot arrive at a relative speed of 700 km/s. (that’s more than 0.2 % of the speed of light)

ljk January 30, 2008 at 13:33

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

(Submitted on 29 Jan 2008)

Abstract: 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).

Comments: 4 pages, 3 figures; accepted for publication by Astronomy & Astrophysics

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0801.4456v1 [astro-ph]

Submission history

From: Norbert Przybilla [view email]

[v1] Tue, 29 Jan 2008 10:20:26 GMT (163kb)

http://arxiv.org/abs/0801.4456

Phil February 1, 2008 at 10:17

Sooooo…… 0.24% of the speed of light…. Does that mean any potential Aliens or Bacteria around that star (or I guess even the star itself) would appear to age more slowly to us??

Ron S February 1, 2008 at 13:41

Depends whether you consider ~90 seconds per year as being slow.

ljk February 4, 2008 at 12:31

Ejection of Supermassive Black Holes from Galaxy Cores

Authors: Alessia Gualandris, David Merritt

(Submitted on 6 Aug 2007 (v1), last revised 31 Jan 2008 (this version, v2))

Abstract: [Abridged] Recent numerical relativity simulations have shown that the emission of gravitational waves during the merger of two supermassive black holes (SMBHs) delivers a kick to the final hole, with a magnitude as large as 4000 km/s. We study the motion of SMBHs ejected from galaxy cores by such kicks and the effects on the stellar distribution using high-accuracy direct N-body simulations.

Following the kick, the motion of the SMBH exhibits three distinct phases. (1) The SMBH oscillates with decreasing amplitude, losing energy via dynamical friction each time it passes through the core. Chandrasekhar’s theory accurately reproduces the motion of the SMBH in this regime if 2 less than ln Lambda less than 3 and if the changing core density is taken into account. (2) When the amplitude of the motion has fallen to roughly the core radius, the SMBH and core begin to exhibit oscillations about their common center of mass. These oscillations decay with a time constant that is at least 10 times longer than would be predicted by naive application of the dynamical friction formula. (3) Eventually, the SMBH reaches thermal equilibrium with the stars. We estimate the time for the SMBH’s oscillations to damp to the Brownian level in real galaxies and infer times as long as 1 Gyr in the brightest galaxies. Ejection of SMBHs also results in a lowered density of stars near the galaxy center; mass deficits as large as five times the SMBH mass are produced for kick velocities near the escape velocity. We compare the N-body density profiles with luminosity profiles of early-type galaxies in Virgo and show that even the largest observed cores can be reproduced by the kicks, without the need to postulate hypermassive binary SMBHs. Implications for displaced AGNs and helical radio structures are discussed.

Comments: 18 pages, The Astrophysical Journal, in press. Replaced with revised version

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0708.0771v2 [astro-ph]

Submission history

From: Alessia Gualandris [view email]

[v1] Mon, 6 Aug 2007 14:46:28 GMT (496kb)

[v2] Thu, 31 Jan 2008 21:11:13 GMT (770kb)

http://arxiv.org/abs/0708.0771

ljk February 22, 2008 at 0:15

‘Slow’ and Fast Rotators among Hypervelocity Stars

Authors: Mercedes Lopez-Morales, Alceste Z. Bonanos

(Submitted on 20 Feb 2008)

Abstract: We measure the 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 consistent with an origin in a binary system, however, the fast rotation of HVS8 is more common of single B-type stars. Our results suggest that HVS8 could have 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.

Comments: 13 pages, 3 figures. Submitted to ApJL

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0802.2945v1 [astro-ph]

Submission history

From: Mercedes Lopez-Morales [view email]

[v1] Wed, 20 Feb 2008 21:05:21 GMT (52kb)

http://arxiv.org/abs/0802.2945

phil February 23, 2008 at 22:17

haveing had a facination with everything astronomicl all my life but little understanding of any of it, i find all your comments great..
I have a question..I,m sure it,s asked all the time…
is it at all possible for mankind to really travel faster than the speed of light.
I know it’s not possible right now but is anyone physicly working on it..
I think it’s crazy that the human race is more ingrossed in self destruction than looking at the fasination of the universe

Administrator February 24, 2008 at 8:18

phil, the question of faster than light travel has not yet been answered. It may be that nothing can travel faster than light, but possibly there are other options, such as wormholes, or actually changing the nature of spacetime so that someone could go to a distant place seemingly faster than light without violating Special Relativity. These are huge issues, and in general the research on them has been done by small groups of scientists working in their spare time. But do be aware that Marc Millis, who headed NASA’s Breakthrough Propulsion Physics program, is actively developing the Tau Zero Foundation to take over from where BPP left off when it lost NASA funding. We support Tau Zero here and will be providing news of current developments germane to its work and to the broader issues behind this research. Tau Zero itself is being developed behind the scenes and has already enlisted a number of scientist practitioners whose research is active. The Foundation intends to use philanthropy to fund research projects that its practitioners are engaged in. The public side of Tau Zero will take form this year as we work toward getting its Web site ready. More on this here on Centauri Dreams as things develop.

James M. Essig February 25, 2008 at 0:57

Hi Paul;

Great comments. I am greatly looking forward th the public side of Tau zero taking form this year. Come to think of it, it would be real cool if Tau Zero became so successful at marshaling public support such that the U.S Government would decide to step in and produce a seperate agency to look into and develope manned interstellar travel, in short a much much larger effort economically than the BPP was that lost NASA funding.

Even if only sub-C space craft are possible and no forms of wormholes or spacetime manipulation ever work out, C is still the limit. At C-e, where e can be made arbitrarilly tiny, we can in theory cover cosmic distance in one year ship time that are vastly greater than the confines of the currently observable universe. Who knows, if we figure out somehow to achieve inertial travel at C, we might find our selves entering some type of entirely new physical or cosmic realm at which we would be physically eternal or somehow timeless relative to the rest of the cosmos or realm we would have originated from.

To start off, I think we need a massive effort to develope efficient fusion rockets with Isp of as high as 3,000,000. We could use magsail or other electrodynamic means for breaking. This would atleast allow us to reach 1/3 C or perhaps even 2/5 C with reasonable but practical efforts thus putting all stars within 10 to 20 lightyears within reach within the healthy lifetimes of robust, viril, young ambitious astronauts just waiting to explore. Life time lengthening medical treatments migh make such all the more practical.

Even if we can never break the light barrier and produce wormholes or spactime distortions, we can come up with meaningful methods or travel and new vistas to explore that we have not even dreamed of yet. So in the Spirit of Tau Zero, I must say “To The Stars” and then to God know what! Let the fun begin! I get so enthused about the possibilities that I just have to borrow the phrase “What Dreams May Come!” from the movie by that name. I find myself like a child in an infinitely vast candy shop.

Thanks;

Your Friend Jim

ljk April 9, 2008 at 9:24

Searching for Hyper-Velocity Stars

Authors: A. Tillich, U. Heber, H. Hirsch

(Submitted on 7 Apr 2008)

Abstract: We present our survey for subluminous hyper-velocity candidates, which has been successfully initiated at the ESO NTT and the Calar Alto 3.5 m telescope.

Comments: to appear in “Hot Subdwarf Stars and Related Objects”, ASPC Volume Nr. 392 p.175

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0804.1003v1 [astro-ph]

Submission history

From: Alfred Tillich [view email]

[v1] Mon, 7 Apr 2008 10:03:33 GMT (17kb)

http://arxiv.org/abs/0804.1003

ljk May 7, 2008 at 22:49

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

(Submitted on 7 May 2008)

Abstract: 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.

Comments: 8 pages; Proceedings of the 3rd Meeting on Hot Subdwarf Stars and Related Objects, U. Heber, S. Jeffery, Napiwotzki eds. 2008, ASP Conference Series, 392, 167

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0805.1050v1 [astro-ph]

Submission history

From: Ulrich Heber [view email]

[v1] Wed, 7 May 2008 19:24:57 GMT (92kb)

http://arxiv.org/abs/0805.1050

ljk August 21, 2008 at 0:19

MMT Hypervelocity Star Survey

Authors: Warren R. Brown, Margaret J. Geller, Scott J. Kenyon (Smithsonian Astrophysical Observatory)

(Submitted on 19 Aug 2008)

Abstract: We describe a new survey for unbound hypervelocity stars (HVSs), stars traveling 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 colors 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 center 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 center.

Comments: 10 pages, submitted to ApJ

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0808.2469v1 [astro-ph]

Submission history

From: Warren R. Brown [view email]

[v1] Tue, 19 Aug 2008 13:13:51 GMT (118kb)

http://arxiv.org/abs/0808.2469

ljk June 10, 2009 at 13:55
ljk August 7, 2009 at 15:02

Runaway Stars, Hypervelocity Stars, and Radial Velocity Surveys

Authors: Benjamin C. Bromley (1), Scott J. Kenyon (2), Warren R. Brown (2), Margaret J. Geller (2) ((1) University of Utah (2) Smithsonian Astrophysical Observatory)

(Submitted on 31 Jul 2009)

Abstract: Runaway stars ejected from the Galactic disk populate the halo of the Milky Way. To predict the spatial and kinematic properties of runaways, we inject stars into a Galactic potential, compute their trajectories through the Galaxy, and derive simulated catalogs for comparison with observations.

Runaways have a flattened spatial distribution, with higher velocity stars at Galactic latitudes less than 30 degrees. Due to their shorter stellar lifetimes, massive runaway stars are more concentrated towards the disk than low mass runaways.

Bound (unbound) runaways that reach the halo probably originate from distances of 6–12 kpc (10–15 kpc) from the Galactic center, close to the estimated origin of the unbound runaway star HD 271791. Because runaways are brighter and have smaller velocities than hypervelocity stars (HVSs), radial velocity surveys are unlikely to confuse runaway stars with HVSs.

We estimate that at most 1 runaway star contaminates the current sample. We place an upper limit of 2% on the fraction of A-type main sequence stars ejected as runaways.

Comments: Astrophysical Journal, submitted; 45 pages of text and 10 figures

Subjects: Galaxy Astrophysics (astro-ph.GA); Solar and Stellar Astrophysics (astro-ph.SR)

Cite as: arXiv:0907.5567v1 [astro-ph.GA]

Submission history

From: Ben Bromley [view email]

[v1] Fri, 31 Jul 2009 16:20:24 GMT (266kb)

http://arxiv.org/abs/0907.5567

ljk September 30, 2009 at 0:00

On the origin of the hypervelocity runaway star HD271791

Authors: V.V.Gvaramadze

(Submitted on 27 Sep 2009)

Abstract: 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.

Comments: 6 pages, 1 figure, to appear in Star Clusters — Basic Galactic Building Blocks throughout Time and Space, Proceed. of the IAU Symp. 266, eds. R. de Grijs and J. Lepine

Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

Cite as: arXiv:0909.4928v1 [astro-ph.SR]

Submission history

From: Vasilii Gvaramadze [view email]

[v1] Sun, 27 Sep 2009 11:26:13 GMT (81kb)

http://arxiv.org/abs/0909.4928

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