Hypervelocity stars ultrafast stars with speeds up to a few hundred miles per second above the average were likely ejected from the Large Magellanic Cloud, a neighboring dwarf galaxy some 160,000 light-years away, say astronomers at the University of Cambridge, UK.

A hypervelocity star leaving the Large Magellanic Cloud. Image credit: NASA / CXC / M.Weiss / Ruth Bazinet, CfA / Sci.News.

Astronomers first thought that the hypervelocity stars, which are large blue stars, may have been ejected from the giant black hole at the Milky Ways heart.

Other scenarios involving disintegrating dwarf galaxies or chaotic star clusters can also account for the speeds of these stars but all three mechanisms fail to explain why they are only found in a certain part of the sky.

To date, over 20 hypervelocity stars have been spotted, mostly in the northern hemisphere, although its possible that there are many more that can only be observed in the southern hemisphere.

The hypervelocity stars are mostly found in the Leo and Sextans constellations we wondered why that is the case, said team member Douglas Boubert, a PhD student at the Institute of Astronomy at the University of Cambridge.

An alternative explanation to the origin of hypervelocity stars is that they are runaways from a binary system.

In binary star systems, the closer the two stars are, the faster they orbit one another. If one star explodes as a supernova, it can break up the binary and the remaining star flies off at the speed it was orbiting. The escaping star is known as a runaway.

Runaway stars originating in the Milky Way are not fast enough to be hypervelocity because blue stars cant orbit close enough without the two stars merging. But a fast-moving galaxy could give rise to these speedy stars.

The Large Magellanic Cloud (LMC) is the largest and fastest of the dozens of dwarf galaxies in orbit around the Milky Way. It only has 10% of the mass of the Milky Way, and so the fastest runaways born in this dwarf galaxy can easily escape its gravity.

The LMC flies around our Galaxy at 250 miles per second and the speed of runaway stars is the velocity they were ejected at plus the velocity of their host galaxy. This is fast enough for them to be the hypervelocity stars.

This also explains their position in the sky, because the fastest runaways are ejected along the orbit of the LMC towards the constellations of Leo and Sextans, said team member Dr. Rob Izzard, also from the Institute of Astronomy at the University of Cambridge.

The researchers used a combination of data from the Sloan Digital Sky Survey and computer simulations to model how hypervelocity stars might escape the LMC and end up in the Milky Way.

They simulated the birth and death of stars in the LMC over the past two billion years, and noted down every runaway star.

The orbit of the runaway stars after they were kicked out of the LMC was then followed in a second simulation that included the gravity of the LMC and the Milky Way.

These simulations allow the authors to predict where on the sky we would expect to find runaway stars from the LMC.

We are the first to simulate the ejection of runaway stars from the LMC we predict that there are 10,000 runaways spread across the sky, Boubert said.

Half of the simulated stars which escape the LMC are fast enough to escape the gravity of the Milky Way, making them hypervelocity stars.

If the previously known hypervelocity stars are runaway stars it would also explain their position in the sky.

The results are published in the Monthly Notices of the Royal Astronomical Society (arXiv.org preprint).

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D. Boubert et al. 2017. Hypervelocity runaways from the Large Magellanic Cloud. Mon Not R Astron Soc 469 (2): 2151-2162; doi: 10.1093/mnras/stx848

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Hypervelocity Stars are 'Runaways' from Large Magellanic Cloud, Astronomers Say - Sci-News.com