Astrophysics: Mystery of the ‘missing’ binary stars solved

An international team of researchers led by PD Dr Florian Peißker has found the first binary star in the immediate vicinity of the supermassive black hole Sgr A* (Sagittarius A star) at the centre of our galaxy. Although it is known that most stars in the universe do not form alone, so far there are only five confirmed binary stars at a greater distance from the black hole. None of the systems are so close. The researchers assume that the binary star system they found, named D9, will merge into a single star in the near future. The discovery was published in Nature Communications under the title ‘A binary system in the S cluster close to the supermassive black hole Sagittarius A*’. The work contributes to a better understanding of the centre of our galaxy and the conditions around the supermassive black hole.

For approximately thirty years it has been possible to observe individual stars in the vicinity of the black hole using infrared telescopy. So far, many of the observations have raised more questions. The central region around the supermassive black hole Sgr A* contains millions of stars and is divided into various sub-regions. A particularly interesting region of this so-called ‘inner parsec’ is the S star cluster, which contains Sgr A*. Due to its high density of stars, there should in theory be many binary stars. However, the five known binary stars are actually located in other, more distant regions, while none have yet been detected in this star cluster.

Researchers attributed this to gravitational forces: The stars in the S star cluster move in stable orbits around the black hole, similar to the Earth orbiting the Sun. However, the conditions there are much more extreme, as Sgr A* is four million times heavier than our sun. The stars therefore can reach speeds of several thousand kilometres per second, making it an unfavourable environment for the formation of binary star systems.

The researchers discovered D9 by taking a different approach to observing certain dust sources in the S star cluster. Normally, several individual observations over the course of a year are superimposed and added together to amplify the signal of the objects. “Nobody has looked closely at individual observations of the dust sources each night,” said Florian Peißker from the University of Cologne’s Institute of Astrophysics. “That was the crux of our study: investigating and analysing every single night. The data from the recordings is noisier, but still good enough. This is how we identified the binary star.”

The discovery of D9 now opens up the possibility for researchers to investigate the processes of star formation in more detail, as the system is very likely to merge in the coming decades to millennia and thus form a new, slightly heavier star. This would solve another mystery. Because the stars in the S star cluster close to the supermassive black hole are younger than any star cluster theory has predicted. The presence of the binary star system could therefore provide new clues as to how the stars form around the central black hole. The researchers assume that some of the young stars formed from binary star systems that had previously migrated from the area of the ‘inner parsec’ to the supermassive black hole.

Co-author Dr Michael Zajaček from Masaryk University in Brno, Czech Republic, said: “Until now, it was a mystery how such young stars could form so close to Sgr A*, which in principle should prevent any gravitational collapse that is necessary for star formation. The discovery of this binary star system will significantly expand our knowledge regarding star formation.” Dr Emma Bordier, co-author and postdoc in the Collaborative Research Centre 1601 ‘Habitats of Massive Stars across Cosmic Time’ at the University of Cologne, added: “Different generations of Very Large Telescope instruments were used for these observations. The new findings clearly demonstrate how the combination of archival data and recent observations can complement each other to enable innovative studies and lead to exciting discoveries.”

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