When massive stars are born, they are always paired with a twin, and the two stars usually orbit each other.
But astronomers at UCLA’s Galactic Center Group and Keck have analyzed more than a decade’s worth of data on 16 supermassive stars orbiting the supermassive black hole at the center of the Milky Way. Their findings, published today in Astrophysical Journalrevealing a surprising result: They’re all parties.
but why? Do stars, about 10 times more massive than our Sun, form on their own in the hostile environment around a black hole? Did the black hole expel their “twins”? Or pairs of stars merged to form single stars?
The results support a scenario in which a central supermassive black hole drives nearby stellar binaries to merge or disrupt, with one of the pair being ejected from the system.
The stars the scientists have observed are known as S stars, and most of them are young — they formed within the past 6 million years — and they’re massive. Most of them lie within a light-month, or just under 500 billion miles, of the black hole.
“The stars this young shouldn’t even be near the black hole in the first place,” said UCLA postdoctoral researcher Devin Chu, first author of the study. “They could not have migrated to this region in just 6 million years. But for them to have a star shape in such a hostile environment is amazing.”
Zhu and colleagues used data taken with Keck’s adaptive optics tools to conduct the first-ever search for spectral S interstellar binary stars. Through optical telescopes, spectral binary stars appear as single stars, but when the emitted light is analyzed by scientists, it is revealed Its actually pairs of stars.
All the stars who seemed to be single were actually all alone.
Even more surprising, the researchers found that the number of pairs of S stars that could exist near the black hole was much lower than the number of similar stars in the section of space surrounding Earth’s sun, known as the solar neighborhood.
They did this by calculating a measure called the binary fraction, which determines how many stars in a given area can come in pairs; The higher the binary fraction, the more stars that can exist in pairs. Previous studies have shown that the binary fraction of S-like stars in Earth’s solar neighborhood is about 70%. In the new study, the researchers found that near the Milky Way’s black hole, the upper limit is only 47% — indicating that the black hole’s extreme environment limits the survival of stellar binaries.
“This difference speaks to the incredibly interesting environment of our galaxy’s center; we’re not dealing with a normal environment here,” Chu said. “This also indicates that the black hole is driving these nearby binary stars to merge or disrupt, which has important implications for the production of gravitational waves and hypervelocity stars ejected from the galactic center.”
The UCLA researchers now plan to explore how the limit on binary fraction they calculated compares with the binary fraction for similar stars that are farther away from the black hole, but are still within the influence of gravity.