
A binary star system at the center of the Milky Way provides new clues as to how stars form around the central black hole
An international research team led by Florian Peißker has for the first time found a binary star in the immediate vicinity of the supermassive black hole Sagittarus A* at the center 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 in such close proximity. The researchers assume that the binary star system 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 center of our galaxy and the conditions around the supermassive black hole.
For about thirty years it has been possible to observe individual stars in the vicinity of the black hole using infrared telescopy. Until now, there has been a question mark over these observations. 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 the high density of stars, there should in theory be many double stars there. In fact, however, the five known double stars are located in other, more distant areas, while none have yet been detected in this star cluster.
Researchers attributed this to gravitational forces. This is because 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 sometimes reach speeds of several thousand kilometers per second - not good conditions 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 than was previously used. Normally, several individual observations from one year are superimposed and added together to amplify the signal of the objects. "Nobody has to look closely at individual observations of the dust sources each night," says Florian Peißker from the Institute of Astrophysics at the University of Cologne. "That was the crux of our study: examining and analyzing every single night. The data from the images are noisier, but still good enough. This allowed us to identify the double 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, forming a new, somewhat 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 in the publication consider it plausible that some of the young stars formed from binary star systems that had previously migrated from the vicinity of the "inner parsec" to the supermassive black hole.
Co-author Dr. Michael Zajacek from Masaryk University in Brno, Czech Republic, says: "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, which is necessary for star formation. The discovery of this binary system will significantly expand our knowledge in this direction." Dr. Emma Bordier, co-author and postdoc in the Collaborative Research Center 1601 "The Cosmic Evolution of the Habitats of Massive Stars" at the University of Cologne, says: "Different generations of Very Large Telescope instruments were used in this work. The new findings impressively show how the combination of archival data and recent observations can complement each other to enable innovative studies and lead to exciting discoveries."