This image looks towards the heart of the Milky Way, towards its nucleus. Credit: ESO/S. Guisard (www.eso.org/~sguisard)
Shrouded in gas and clouds, the center of the Milky Way does not easily reveal its secrets. The initial discovery of its supermassive black hole (SMBH) decades ago posed as many puzzles as it solved, including the puzzling absence of binary stars around it. The hot, massive stars that populate the region are almost always found as binary stars in the galaxy’s extended spiral arms. But at the galactic center they all seemed lonely.
Called S stars, they orbit at hypervelocity around the center of the Milky Way, and their only neighbors in the galactic core are apparent clouds of dust and gas called G objects that travel with similar trajectories and speeds. But as announced today in Nature communicationsit turns out that G objects are much more than clouds. One of these provided the first convincing evidence of a binary star system – called D9 – orbiting the galactic center. This pair of stars orbit each other about once a year. According to the study, there are probably many more hidden in plain sight.
“The D9 system is actually a missing link,” says first author Florian Peissker of the University of Cologne in Germany. “This explains the presence of G objects, but also the presence – or non-presence – of S binary stars, because S stars were initially G objects.”
Present at birth
Using two spectrometers called ERIS and SINFONI mounted on the Very Large Telescope in Chile, Peissker monitored D9’s behavior for 15 years, analyzing the data for each night and noticing recurring variations in the object’s speed. In the same way that an exoplanet can be detected by observing the pull on its parent star, the wobble in D9’s orbit – what astronomers call radial velocity – indicated that there were two bodies orbiting each other. ‘other.
Peissker’s team estimates that the two stars are young, just 2.7 million years old, with an orbital period around the SMBH of a couple of hundred years. They predict that the binary system will merge into a single star within a million years, which helps explain the apparent lack of binary stars at the galaxy’s center. When the “dust” clears out around G objects, Peissker says, they are molten S stars.
This could also explain another paradox, that if S stars were captured from the outer edges of the galaxy by the SMBH and pulled inward, they would have to be on the order of 1,000 times older to have completed the journey. If, however, they reformed by merging binary systems hidden behind veils of dust and gas surrounding the SMBH, their “born again” status would make them appear much younger.
The nature of the G objects has long been a mystery, but clues have emerged along the way. In 2014, a G object called G2 orbited the center of the Milky Way at a large fraction of the speed of light, just 36 light. hours from SMBH. Astronomers had predicted that the extreme gravity at periapsis (G2’s closest approach) would tear it apart. Instead it emerged intact. This suggested that a dense body, perhaps a protostar, was buried deep within it and holding it together. The discovery of D9 seems to confirm this, but success did not come easily.
The stars align
Tackling objects 26,000 light-years away that are obscured by gas and dust poses enormous challenges. Precision in measurements, what scientists call the “signal-to-noise” ratio, is critical. A common method is to stack months of observations by slightly modifying the fields of view to create a mosaic of a larger field of view that cancels out instrumental anomalies.
Another approach – one that Peissker devised one evening as he cycled home from work – is to examine the data for each night covering an extended period, filtering observations in a particular region based on their quality. This increases the chances of spotting a binary like D9, dancing a pas-de-deux tight enough to survive the strong forces they experience so close to the black hole.
“I wrote down all the values and all the Doppler-shifted radial velocities of these objects for every night for a year.” says Peissker: “I noticed that D9 was somehow strange or different. Once I saw the periodic pattern of it, I did it for the entire 15 years.”
In particular, spectral readings showing emissions of ionized hydrogen (called Brackett-gamma lines), helped Peissker track the Doppler effect – the familiar phenomenon of wavelengths stretching or compressing due to fluctuations in their direction and speed. This gave him the pattern of cyclic radial velocities, the telltale sign of two bodies pulling and pushing each other as they tango in space.
Gamma-bracket lines are also signs of stellar winds and young stellar objects, indicating electron temperatures of at least 10,000 Kelvin. While some astronomers argue that G objects are “coreless clouds,” Peissker calculated that clouds of dust and gas subjected to such ferocious stellar winds could not survive more than a few seconds or years in the absence of a hidden stellar core to hold them Together.
Ultimately, the team’s discovery was a matter of the stars aligning.
“We were super lucky because D9 is in the descending part of its orbit,” says Peissker, referring to the binary system’s two-hundred-year period, of which only half is spent moving away from the SMBH.
“If it was on the ascending side, it would be much faster due to the impending periapsis. Because it slowed down, we were able to see this really beautiful spectroscopic pattern, and then it all made sense.”
