Was a Star Ejected from Our Central Black Hole?

Was a Star Ejected from Our Central Black Hole?

Generally thought to be the point of no return, our very own black hole seems to have ejected a star at hyper velocity.

In something known as the Hills mechanism – which occurs in binary star systems when they are disrupted by a super massive black hole – the stars are pulled apart and left to continue on their separate journeys. The closest star is pulled into an orbit around the black hole while the other is ejected at extremely high velocity. However, although this was proposed in 1988 by astronomer Jack Hills, it has never been confirmed.

Now, a worldwide team of scientists led by Ting Li have observed what they believe to be the first example of such a mechanism.

The team utilised data from the 3.9 metre Anglo-Australian Telescope as part of the Southern Stellar Stream Spectroscopic Survey – a survey that aims to map the kinematics and chemistry of long, dense regions of stars, known as stellar streams. Looking through the data for any stars with velocities greater than 800km/s, the team came across a star with a radial velocity of ~1020 km/s – that’s more than 2 million miles per hour. Further analysis revealed the star, known as S5-HVS, is a hot dwarf star more than twice the mass of our Sun and located 9 kpc (kila parsecs) – approximately 30 thousand light years – from the galactic centre in the Jhelum stellar stream system. Given the measured distance, the proper motion and the radial velocity, the total velocity of the star in the Galactic rest frame is a whopping 1755 km/s – almost 4 million miles per hour – making it one of the fastest known stars in the Galaxy.

To infer the origin of the star, the team studied the kinematics and traced the orbit backwards in time in the gravitational potential of the Milky Way. Remarkably, they found that the star can unambiguously be traced back to the Galactic Centre where it was ejected at a speed of 1800km/s 4.8 million years ago, making S5-HVS the first clear demonstration of the Hill Mechanism.

A runaway star ejected from the galactic heart of darkness

A runaway star ejected from the galactic heart of darkness

Astronomers have spotted an ultrafast star, traveling at a blistering 6 million km/h, that was ejected by the supermassive black hole at the heart at the Milky Way five million years ago.

The discovery of the star, known as S5-HVS1, was made by Carnegie Mellon University Assistant Professor of Physics Sergey Koposov as part of the Southern Stellar Stream Spectroscopic Survey (S5). Located in the constellation of Grus -- the Crane -- S5-HVS1 was found to be moving ten times faster than most stars in the Milky Way.

"The velocity of the discovered star is so high that it will inevitably leave the galaxy and never return," said Douglas Boubert from the University of Oxford, a co-author on the study.

Astronomers have wondered about high velocity stars since their discovery only two decades ago. S5-HVS1 is unprecedented due to its high speed and close passage to the Earth, "only" 29 thousand light years away. With this information, astronomers could track its journey back into the center of the Milky Way, where a four million solar mass black hole, known as Sagittarius A*, lurks.

"This is super exciting, as we have long suspected that black holes can eject stars with very high velocities. However, we never had an unambiguous association of such a fast star with the galactic center," said Koposov, the lead author of this work and member of Carnegie Mellon's McWilliams Center for Cosmology. "We think the black hole ejected the star with a speed of thousands of kilometers per second about five million years ago. This ejection happened at the time when humanity's ancestors were just learning to walk on two feet."

Superfast stars can be ejected by black holes via the Hills Mechanism, proposed by astronomer Jack Hills thirty years ago. Originally, S5-HSV1 lived with a companion in a binary system, but they strayed too close to Sagittarius A*. In the gravitational tussle, the companion star was captured by the black hole, while S5-HVS1 was thrown out at extremely high speed.

"This is the first clear demonstration of the Hills Mechanism in action," said Ting Li from Carnegie Observatories and Princeton University, and leader of the S5 Collaboration. "Seeing this star is really amazing as we know it must have formed in the galactic center, a place very different to our local environment. It is a visitor from a strange land."

The discovery of S5-HVS1 was made with the 3.9-meter Anglo-Australian Telescope (AAT) near Coonabarabran, NSW, Australia, coupled with superb observations from the European Space Agency's Gaia satellite, that allowed the astronomers to reveal the full speed of the star and its journey from the center of the Milky Way.

"The observations would not be possible without the unique capabilities of the 2dF instrument on the AAT," said Daniel Zucker, an astronomer at Macquarie University in Sydney, Australia, and a member of the S5 executive committee. "It's been conducting cutting-edge research for over two decades and still is the best facility in the world for our project."

These results were published on November 4 online in the Monthly Notices of the Royal Astronomical Society, and the S5 collaboration unites astronomers from the United States, United Kingdom, Australia and Chile.

"I am so excited this fast-moving star was discovered by S5," says Kyler Kuehn, at Lowell Observatory and a member of the S5 executive committee. "While the main science goal of S5 is to probe the stellar streams -- disrupting dwarf galaxies and globular clusters -- we dedicated spare resources of the instrument to searching for interesting targets in the Milky Way, and voila, we found something amazing for 'free.' With our future observations, hopefully we will find even more!"

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Materials provided by Carnegie Mellon University. Note: Content may be edited for style and length.