Veiled in gas and clouds, the Milky Way’s center does not easily give up secrets. The initial detection of its supermassive black hole (SMBH) decades ago posed as many riddles 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 binaries in the galaxy’s far-flung spiral arms. But at the galactic center, they appeared to all be loners.
Called S stars, they orbit at hypervelocities around the Milky Way’s center, and their only neighbors at the galactic core are the apparent dust and gas clouds called G objects that travel at similar trajectories and speeds. But as announced today in Nature Communications, it turns out G objects are far more than clouds. One of them has yielded the first compelling evidence of a binary star system — called D9 — orbiting the galactic center. This pair of stars orbit each other about once a year. Based on the study, there are likely many more hiding in plain sight.
“The D9 system is actually a missing link,” says first author Florian Peissker at the University of Cologne in Germany. “It explains the presence of G objects, but also the presence — or non-presence — of binary S 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 tracked D9’s behavior over 15 years, analyzing data for each night and noting recurring variations in the object’s velocity. In the same way that an exoplanet can be detected by looking at the pull on its parent star, the wobble in D9’s orbit — what astronomers call its radial velocity — indicated there were two bodies orbiting one another.
Peissker’s team estimates that the two stars are youngsters, 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 one million years, which helps explain the apparent lack of binary stars in the center of the galaxy. By the time the “dust” clears around G objects, Peissker says, they are merged S stars.
This may also explain another paradox, the fact that, if S stars were captured from the outer reaches of the galaxy by the SMBH and dragged inward, they would have to be on the order of 1,000 times older to have completed the journey. If, however, they reformed from merging binary systems hidden behind veils of dust and gas circling the SMBH, their “born again” status would make them appear much younger.
The nature of G objects has long been a mystery, but there have been clues along the way. In 2014, a G object called G2 whipped around the Milky Way’s center at a large fraction of the speed of light a mere 36 light hours from the SMBH. Astronomers 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 inside holding it together. The discovery of D9 appears to confirm this, but success didn’t come easily.
The stars aligning
Dealing with objects 26,000 light years away that are obscured by gas and dust poses tremendous challenges. Accuracy in measurements, what scientists call “signal to noise” ratio, is paramount. One common method is to stack months of observations while slightly changing fields of view to create a mosaic of a larger field of view that cancels out instrumental anomalies.
Another approach — one Peissker dreamt up while riding his bicycle home from work one evening — is to comb through data for every 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 tight enough pas-de-deux 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 its periodic pattern, I did this for all 15 years.”
In particular, spectral readings showing ionized hydrogen emissions (called Brackett-gamma lines), helped Peissker track the Doppler effect — the familiar phenomenon of wavelengths becoming stretched or compressed due to fluctuations in their direction and velocity. This gave him the pattern of cyclical radial velocities, the telltale sign of two bodies tugging and pushing one another as they danced their tango through space.
Brackett-gamma lines are also signs of stellar winds and young stellar objects, indicating electron temperatures of at least 10,000 Kelvin. While some astronomers contend that G objects are “coreless clouds,” Peissker has calculated that dust and gas clouds subjected to such ferocious stellar winds could not survive more than a few seconds or years in the absence of a hidden stellar core holding them together.
In the end, the team’s discovery was a matter of stars aligning.
“We were super lucky because D9 is on the descending part of its orbit,” says Peissker, referring to the binary’s two-hundred-year period, only half of which is spent traveling away from the SMBH.
“If it were on the ascending part, it would be much faster due to the upcoming periapsis. Because it’s slowed down, we were able to see this really nice spectroscopic pattern, and then everything made sense.”
