Astronomers have spotted what they believe is a white dwarf star in very close proximity to a black hole in the globular cluster 47 Tucanae.
The binary star system known as X9 isn’t a newly discovered white dwarf-black hole system, but astronomers have managed to study the system through NASA’s space-based telescopes, the Chandra X-ray Observatory and NuSTAR, and the Australia Telescope Compact Array located in New South Wales, Australia, in 2015 to determine that the white dwarf is likely very close to a black hole and orbiting it at whopping speeds that take it around the black hole twice every hour.
“This white dwarf is so close to the black hole that material is being pulled away from the star and dumped onto a disk of matter around the black hole before falling in,” said first author Dr. Arash Bahramian, from the University of Alberta in Canada and Michigan State University in the United States. “Luckily for this star, we don’t think it will follow this path into oblivion — it should stay in orbit.”
The white dwarf isn’t falling into the black hole, but there is all the possibility that the black hole is feeding on the material of the white dwarf and will eventually empty out all the stellar material and just leave a rocky celestial object behind.
Associate Professor James Miller-Jones, from Curtin University and ICRAR, said, “We think the star may have been losing gas to the black hole for tens of millions of years and by now has now lost the majority of its mass.”
“Over time, we think that the star’s orbit will get wider and wider as even more mass is lost, eventually turning into an exotic object similar to the famous diamond planet discovered a few years ago,” he said.
Astronomers are still looking for an answer to what got the white dwarf so close to the black hole. One possibility is that the black hole smashed into a red giant star and as gas from the outer regions of the star were ejected a binary was formed containing a black hole and a white dwarf. The orbit of the binary would then have shrunk as gravitational waves were emitted until the black hole started pulling material from the white dwarf.
The gravitational waves being produced by the binary system have a frequency too low to be detected by the ground-based facilities that confirmed the existence of gravity waves last year but it is possible that space-based gravity wave observatories in the future could be sensitive enough to detect them.