What Are Hypervelocity Stars?
Hypervelocity stars seem too fantastic an object to exist in reality, yet they do. That something could be strong enough to send a star shooting out of a galaxy is hard to visualize, much less draw accurate predictions and forecasts for the phenomena. What causes stars to leave the galaxy in such a fashion?
The first work into this was published in 1988 by J.G Hills, where he showed that binary star system which wandered too close to a supermassive black hole could have one of the stars thrown out with speeds exceeding 1000 kilometers an hour and even going as fast as 4000! In 2003 Q. Yu and S. Tremaine further developed the idea by showing that single stars under the right gravitational conditions could eject one of them as a hypervelocity star or a single star passing by a binary black hole, though this is less likely. Some scenarios even show supernovas capable of ejecting a star at a fast enough velocity to qualify (Collins, Brown, Dormineg 24).
Hypervelocity stars should not be confused with high-velocity stars, another subcategory of fast moving objects. These stars move faster than 30 kilometers per second and are usually O/B type stars with a usually distance of about 15 kilo parsecs above the galactic plane. Most tend to top out at 200 kilometers per second, ensuring that they stay inside the galaxy. Hypervelocity stars exit the galaxy, making the distinction between them rather important (Brown).
Applications and Scientific Findings
These stars could reveal certain aspects of dark matter by noting how their escape paths deviate from expectations due to the gravitational effects of the unseen material. By comparing the actual path of the star to the predicted, it can help gain data that will eliminate some models of dark matter. And as more and more of these stars are found, certain characteristics begin to show. And we need these patterns, for according to the number crunching there are about 1000 hypervelocity stars in the Milky Way whose total population of stars exceeds 100 billion. And on top of that a star is expected to be launched once every 100,000 years. Clearly we need a bit of help here. Based on the trajectories of most of them, they arise from the center of our galaxy. Knowing where they came from can tell us about that place, especially if it came from the galactic center. Close encounters can give scientists mass measurements as well as star production models to compare with and see what works best. It may even show that Sagittarius A*, our supermassive black hole, could be a binary black hole system instead of a single one. And many of the elliptical orbits of stars around A* seem to point at an old binary companion lost to time – but which was really just shot out of our galaxy (Collins , Brown, Edelmann ,“Two Exiled”).
Notable Hypervelocity Stars
SDSS J090745.0+024507 was the first hypervelocity star to be found in 2005. It was discovered by Warren Brown (Harvard-Smithsonian Center for Astrophysics) and his team during a survey of “faint blue horizontal branch candidates” surrounding the center of our galaxy in an effort to better understand the mass distribution of the galaxy. They found SDSS to be about 3 solar masses in size, about 55 kilo parsecs away, and with a velocity of 853 ± 12 kilometers per second (well above the amount needed to leave our galaxy, which is 305 kilometers per second) and when compared to the motion of the galaxy it is moving at 709 kilometers per second away from it at 173.8 degrees from the center. Due to the huge velocity it is moving at, scientists suspect it was thrown out by A*. No supernova can send a star at that speed and no binary pair could also. Also, the angle of ejection hints at an A* encounter. Later observations proved the star to be a main-sequence B-type with slow pulsations (Brown, Edelmann, Dormineg 24-6).
HE 0437-5439 was another star found under a similar survey by Edelmann and team. Brighter than SDSS, it too seems to be a main sequence B-type star with a velocity of 723 ± 3 kilometers a second. It was thought to initially be a low-mass star whose spectrum mimicked the observed results but further analysis of the spectrum in terms of rotational speed (for a low-mass star would be fast) and lack of helium (something that a low-mass star would have present) proved it to be what it appear to be, which is very important if scientists are to find where it came from (Edelmann).
Another interesting puzzle arises with the identity of the star. The lifespan of such a star is about 25 million years yet according to its velocity and distance it has been travelling for over 100 million years. Uh-oh, somewhere something broke. No matter where they placed the origination point for 5439 it was still a longer flight time than life time. One possibility is that 5439 was actually a binary system that was ejected and then over the years merged into a single star. However, it would require nearly perfect interactions of a trinary star system with A*and even then the likelihood of survival is low. Another possible solution would be to have 5439 start its journey from the Large Magellanic Cloud, a satellite galaxy to us. 5439 is closer to the LMC at 11 ± 12 kilo parsecs than the center of our galaxy at 61 ± 12 kilo parsecs. If the star really did escape from there, 5439 left the LMC at over 600 kilometers a second and not too long after its formation. Eventually, additional observations pointed to 5439 having a Milky Way origination. When compared to our galaxy’s motion, 5439 is moving away at 563 kilometers per second at 16.3 degrees from the galactic center (Ibid).
Alright, so we have a few that were launched from our galactic center. What about one from a supernova? RX J0822-4300, found in 2012, was but its not a B-Type star. In fact, it is a neutron star moving away from the Puppis A supernova, whose light reached us 3700 years ago. The supernova was not symmetrical and thus released its energy of implosion more in one direction than the other, kicking out its neutron star companion with gusto. 4300 is currently moving at about 519 kilometers a second according to observations from Chandra (“Chandra Discovers,” Dormineg 26).
And not too long after that, some Sun-like hypervelocity stars were found. Unlike B-type stars, they are less massive (3-4 times smaller) and also older yet they too were found around A*. A survey of 130 yellow stars which were far from A* was conducted by Hawkins and Kraus while looking near the supermassive black hole, and from them trajectories and velocities were calculated to find a total of 6 hypervelocity stars similar in make to our Sun (Ghose).
Interestingly, a subclass of supernova may be hypervelocity stars. They are 20 times rarer than the main Ia variant and all seem to happen outside of galaxies, usually more than 100,000 light-years in distance from them. By looking at their redshifts we can indeed determine that these supernova are exceeding escape velocities for their galaxies. The catch is that the supernova seen are white dwarfs which means they should have a companion object yet models show binaries are not likely to be launched together. Some models show it is possible but only under the right conditions from a black hole binary system (Timmer).
A New Mystery
So far, scientists had only found single stars being propelled at these high velocities and most models indicate that something helped propel that star. So what can we make of PB3877, a binary star system found in SDSS data from 2011 that is 18,000 light years from us and is moving at speeds like other hypervelocity stars? Maybe a supermassive black hole helped it, but PB doesn't back track to our galactic center and is too far away now to be influenced by it. One of the stars is incredibly hot (5 times that of our sun) while the other is 1,000 degrees cooler than the sun, based off the weak absorptions lines seen in the spectrum of PB. Nothing unusual...but what if something unseen is helping the binary pair, like dark matter? it would give the star system the mass needed to ensure stability at such speeds (BEC, W.M. Keck Observatory).
BEC. "Astronomers have discovered a superfast star system that breaks current physics models." Sciencealert.com. Science Alert, 13 Apr. 2016. Web. 05 Aug. 2016.
Brown, Warren R. and Margaret J. Geller, Scott J. Kenyon, Michael J. Kurtz. “Discovery of an Unbound Hyper-Velocity Star in the Milky Way Halo.” The Astrophysical Journal 11 Jan. 2005. Web. 02 Nov. 2015.
“Chandra Discovers Cosmic Cannonball.” NewsWise.com. News Wise, Inc., 28 Nov. 2007. Web. 03 Nov. 2015.
Collins, Nathan. “Escape from the Milky Way.” Scientific American Dec. 2013: 20. Print.
Dormineg, Bruce. "How High Speed Stars Escape the Galaxy." Astronomy Mar. 2017: 24-6. Print.
Edelmann, H. and R. Napiwotzki, U. Heber, N. Christlieb, D. Reimers. “HE 0437-5439 – An Unbound Hyper-Velocity Main-Sequence B-type Star.” arXiv:astro-ph/0511321v1.
Ghose, Tia. “ Ultrafast Hypervelocity Stars Discovered.” Space.com. Purch, Inc., 12 Feb. 2013. Web. 03 Nov. 2015.
Timmer, John. "Black holes hurl stars out of galaxy, after which they explode." arstechnica.com. Conte Nast., 17 Aug. 2015. Web. 15 Aug. 2018.
“Two Exiled Stars Are Leaving Our Galaxy Forever.” SpaceDaily.com. Space Daily, 27 Jan. 2006. Web. 03 Nov. 2015.
W.M. Keck Observatory. "New hypervelocity binary star challenges dark matter, stellar acceleration models." Astronomy.com. Kalmbach Publishing Co., 13 Apr. 2016. Web. 05 Aug. 2016.
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© 2016 Leonard Kelley