‘Gravitational slingshot’ in star clusters may explain stellar streams

5 Jul 2021

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After finding three times more black holes than expected in the Palomar 5 cluster, scientists believe they may have unlocked an understanding of how stellar streams are formed.

In the darkness of space, there are voyages of stars travelling in bright streams.

The origins of these stellar ‘tidal streams’ aren’t fully known, but they are thought to be ejected from disrupted dwarf galaxies or from star clusters.

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A new paper published in Nature Astronomy addresses star clusters, black holes and their relation with star streams by examining Palomar 5, a globular cluster.

“We do not know how these streams form, but one idea is that they are disrupted star clusters,” explained Prof Mark Gieles from the Institute of Cosmos Sciences of the University of Barcelona, who is lead author of the paper.

“However, none of the recently discovered streams have a star cluster associated with them, hence we can not be sure. So, to understand how these streams formed, we need to study one with a stellar system associated with it. Palomar 5 is the only case, making it a Rosetta Stone for understanding stream formation.”

The 10bn-year-old cluster Palomar 5 is located about 80,000 light-years away in the Serpens constellation, and is one of roughly 150 globular clusters that orbit around the Milky Way.

Born during the earliest stages of galaxy formation, it is now in its final stages of dissolution. In Palomar 5, the researchers believe that they might have unlocked an understanding of these stellar streams.

‘Gravitational slingshot’

Researchers simulated the orbits and the evolution of each star from the formation of the cluster until its final dissolution. They varied the initial properties of the cluster until a good match with observations of the stream and the cluster was found.

While Palomar 5 formed with a lower black hole fraction, they found that stars escaped more efficiently than black holes.

“The number of black holes is roughly three times larger than expected from the number of stars in the cluster, and it means that more than 20pc of the total cluster mass is made up of black holes,” said Gieles.

“They each have a mass of about 20 times the mass of the sun and they formed in supernova explosions at the end of the lives of massive stars, when the cluster was still very young.”

Over time, the black hole fraction began to increase, puffing up the cluster in what researchers called a “gravitational slingshot” interaction. This launched even more stars into the void, creating the star stream.

As more and more stars fly out of the cluster, in approximately a billion years’ time, Palomar 5 will disappear and only black holes will be there at its demise.

“This work has helped us understand that even though the fluffy Palomar 5 cluster has the brightest and longest tails of any cluster in the Milky Way, it is not unique,” added Dr Denis Erkal, a co-author of the paper from the University of Surrey.

“Instead, we believe that many similarly puffed up, black hole-dominated clusters have already disintegrated in the Milky Way tides to form the recently discovered thin stellar streams.”

The researchers highlighted the importance of the work for understanding globular cluster formation, the initial masses of stars and for the evolution of massive stars. What’s more, it has important implications for the study of gravitational waves.

“It is believed that a large fraction of binary black hole mergers form in star clusters,” said Dr Fabio Antonini from Cardiff University, another co-author on the paper.

“A big unknown in this scenario is how many black holes there are in clusters, which is hard to constrain observationally because we cannot see black holes. Our method gives us a way to learn how many [black holes] there are in a star cluster by looking at the stars they eject.”

Sam Cox was a journalist at Silicon Republic covering sci-tech news