The centre of the Milky Way in the constellation Sagittarius. The image was taken in the visible portion of the spectrum, where the centre of the Galaxy is obscured by dust (dark clouds in the foreground). The smaller image was taken with the SINFONI spectrograph on the Very Large Telescope in the infrared domain. Brighter regions are due to hydrogen emission (Brγ emission line) around 2.2μm. The binary system D9 is also the source of this emission due to its circumstellar envelope. The components of the binary cannot be resolved even with large eight-meter telescopes. They can only be identified by periodic shifts of the Brγ hydrogen line into the red and blue regions of the spectrum, reflecting their orbit around a common centre of mass. Credit: ESO/ F Peißker et al., S Guisard.
More than 15 years of spectroscopic analysis of near-infrared data has identified a binary star in the close vicinity of Sagittarius A*, the supermassive black hole at the centre of our Galaxy, The Milky Way. This has significant implications both for the formation of stars in the Galactic centre, as well as their future evolution, as Masaryk University’s Michal Zajaček and Emma Bordier and Florian Peißker from the University of Cologne explain.
A stellar pair has been found in the grasp of Sagittarius A* (Sgr A*), the supermassive black hole at the centre of our Galaxy, the Milky Way. Stellar binary systems are quite frequent in the Milky Way, and some of them are popular among astrophotographers since the components of the pair appear like universe gems with different colours. In fact, in stellar nurseries associated with dense molecular clouds at least half of the stars are associated with binaries. But are binary systems evenly distributed across the Milky Way? Are there regions in the Galaxy where it is difficult for them to form and exist?
One such intriguing region is the centre of the Milky Way. The presence of the supermassive black hole and one of the largest stellar densities in the Galaxy was long thought to cause hard times for the existence of binaries in this environment.
However, recently the team led by Florian Peißker from the University of Cologne, together with the scientists from the Masaryk University and the Czech Academy of Sciences, identified a pair of stars, which orbits Sgr A* once every 400 years. The components of the pair complete one orbit in about one year.
The binary system also known as D9 can thus successfully withstand the strong gravitational forces of the central massive black hole, and this has significant implications for the formation of the system and also its future. But can it also potentially tell us something more profound about the heart of our Galaxy?
D9 is the first star pair ever found near Sagittarius A*, the supermassive black hole at the centre of the Milky Way. This image shows an emission line of hydrogen mapped by the SINFONI instrument on ESO’s Very Large Telescope. The instrument provides a spectrum for every single pixel; over the years, the emission around D9 was found to oscillate periodically towards red and blue wavelengths, which revealed that D9 is actually two stars orbiting each other. D9 is the orange patch near the top right of the image, to the left of a brighter and slightly bigger patch. Sagittarius A* is located almost in the middle of the image. Credit: ESO/ F Peißker et al.
Mysterious dusty objects
Before the binary identification, D9 was known as the representative of mysterious dusty objects orbiting the supermassive black hole. The most famous dusty source, G2, was detected in 2011 as a ‘cloud’ approaching the central black hole really fast. It was predicted that it would disrupt and cause an enhanced activity of the Galactic centre. However, around its closest approach to the black hole and after it, no disruption was observed and the source continued quite intact on its orbit.
Michal Zajaček a co-author of the study from Masaryk University, said: “It has been a big discussion about what G2 stands for – is it just a cloud or some kind of a strange star? D9 has now potentially provided the final answer.”
While the cloud would get completely tidally disrupted, the preserved structure of G2 implied that it is a star embedded in a compact gaseous-dusty envelope. Since D9 shows similar characteristics to G2, it is considered to be another dust-enshrouded star orbiting the supermassive black hole.
Lead author of the study, Florian Peißker, a researcher at the University of Cologne, said: “Although we agreed on the nature of G objects, still there was the question of their origin. They could be very young stellar objects that are surrounded by primordial envelopes and accretion discs. Or they could be stars that just appear to be young since they formed via the merger of two already mature stars.”
Zajaček clarified: “Each scenario had its difficulties. For young stars, you need the dense molecular material like proper ‘dough’, from which stars can form. And it is extremely challenging to keep dense material so close to the supermassive black hole, where tidal forces are very strong and shred any approaching cloud rather fast.”
Emma Bordier, a researcher from the University of Cologne, added: “On the other hand, the merger scenario requires a substantial population of binary stars, which is not so difficult in standard stellar populations, but the Galactic centre environment is again a bit special – binaries at a given distance need to be sufficiently compact, otherwise they would disrupt.”
The Milky Way arches across this rare 360-degree panorama of the night sky above the Paranal platform, home of ESO’s Very Large Telescope. The image was made from 37 individual frames with a total exposure time of about 30 minutes, taken in the early morning hours. The Moon is just rising and the zodiacal light shines above it, while the Milky Way stretches across the sky opposite the observatory. The open telescope domes of the world’s most advanced ground-based astronomical observatory are all visible in the image: the four smaller 1.8-metre Auxiliary Telescopes that can be used together in the interferometric mode, and the four giant 8.2-metre Unit Telescopes. To the right in the image and below the arc of the Milky Way, two of our Galactic neighbours, the Small and Large Magellanic Clouds, can be seen. Credit: ESO/ HH Heyer.
Binary solves the puzzle
Since D9 is clearly a star and it shows the presence of a gaseous-dusty envelope, it has helped to confirm that the nature of G objects is stellar. When the spectroscopic analysis revealed periodic variations of the radial velocity of the gas associated with one of the hydrogen emission lines, it was clear that this effect can best be explained by the setup where two stars orbit a common centre of mass.
Peißker added: “Binary stars are to astronomers the same as laboratory scales – they can reveal the mass of the involved stars as well as the conditions in which they were formed.”
Zajaček, who performed additional analysis, explained: “The comparison of D9 with the evolutionary tracks of pre-main-sequence stars showed that the object is only a few million years old.”
The D9 binary, therefore, likely formed in a dense gaseous-dusty disc that was rotating around Sgr A* a few million years ago. Due to its large density, it started to fragment into clumps, from which stars formed. Among those, some were binaries. Stars orbiting each other on wider orbits got disrupted due to tidal forces while tighter binaries remained. However, these ‘harder’ systems do not last long either – coexistence with the supermassive black hole brings its toll, and the components will eventually likely collide.
Evolution of the radial velocity of the D9 system over time. The top three graphs (A, B and C) represent spectra in the vicinity of the Brγ emission line (rest wavelength 2.165μm), which correspond to the values of the radial velocities in the lower plot (three squares in plot D). The lower plot (D) shows the radial velocity values over time from 2006 to 2022. Green values are the measured values derived from the Brγ line detected by the SINFONI spectrograph on the Very Large Telescope (VLT), while the two most recent black points are measured by the ERIS spectrograph also on the VLT. The red curve represents the best fit of the binary’s orbital motion. The axis of the plot on the left represents the heliocentric radial velocity including the motion of the centre of mass of the D9 system relative to the Sun, while the axis on the right represents the radial velocity relative to the binary’s centre of mass. Credit: Nature Communications (http://creativecommons.org/licenses/by/4.0/)
The collision of stars – a Universe face-lifting procedure
As the D9 components orbit each other, the binary orbit is perturbed by the gravitational pull of the supermassive black hole. Zajaček, who also performed detailed dynamical calculations, explained: “On a longer timescale, the eccentricity of the system will increase, while the inclination decreases, and vice versa. This effect is also known as Kozai-Lidov resonance and is quite ubiquitous in the Universe – it operates anywhere where the bound two-body system is perturbed by a third distant massive body.”
By coincidence, in the case of D9, the time for the eccentricity-inclination changes to take place is about one million years, which is comparable to its age. Especially during high-eccentricity phases, the components approach each other on grazing orbits, which will eventually result in their collision and subsequent merger.
Zajaček added: “As the stars merge, a large fraction of the stellar material is ejected into their surroundings, causing profound absorption of the stellar emission due to dust formation. Such a merger product looks and behaves as a young star. Therefore stellar collisions work as a face-lifting procedure, after which the merger product appears younger than the original stars.”
D9 can thus be in a unique pre-stage of the G-object formation. The hunt for more binaries in this extraordinary region is under way. Future 30- and 40-metre class telescopes will definitely help in the search due to their significant increase in sensitivity that will enable them to look for fainter objects, potentially to the brown-dwarf level.
Peißker concluded: “The Galactic centre has been full of unknowns and surprises. Our study has shown that the binaries can form and exist in the region previously considered to be an unlikely place for them. Let us see what the future instruments bring us – there is a good chance that one day we will study exoplanets in this exciting region full of extremes.”
The Laser Guide Star (LGS) is launched from the VLT’s 8.2-metre Yepun Telescope and aims at the centre of our Galaxy, in the heart of the brightest part of the Milky Way. The laser beam is part of the VLT’s adaptive optics system. It creates an artificial star at 90km altitude in the Earth´s mesosphere. This star is used as reference to correct images and spectra for the blurring effect of the atmosphere. The plane of the Milky Way is crossed by prominent dark lanes, huge clouds of interstellar dust that block the visible light. Thanks to the infrared instruments mounted in the Yepun Telescope, astronomers can ‘see through’ and study the complex and turbulent core of our Galaxy, where a supermassive black hole is lurking. The ESO’s Very Large Telescope is composed by four 8.2-metre Unit Telescopes (UTs, where Yepun is UT4) plus four 1.8-metre movable Auxiliary Telescopes (ATs). Credit: G Hüdepohl (atacamaphoto.com)/ ESO.
The link to the original paper can be found here.
