While observing a massive protostar called G11.92-0.61 MM 1, a research team led by University of Leeds astronomers discovered it was not in fact one stellar object, but two. The main object, referred to as MM 1a, is a protostar surrounded by a rotating disk of gas and dust that was the focus of the scientists’ original investigation. A faint object, named MM 1b, was detected just beyond the disk in orbit around MM 1a. The scientists believe this is one of the first observed examples of the formation of a binary star via disk fragmentation around a massive young star.
An artist’s impression of the disk of dust and gas surrounding the massive protostar MM 1a, with its companion MM 1b forming in the outer regions. Image credit: J.D. Ilee, University of Leeds.
MM 1a — the main, central star of the G11.92-0.61 MM 1 system, which is located approximately 11,000 light-years from Earth — is truly colossal, a full 40 times more massive than the Sun.
The other star, MM 1b, which was newly detected by the Atacama Large Millimetre/submillimetre Array (ALMA) just outside the MM 1a protoplanetary disk, is a relatively puny one-eightieth (1/80) that mass.
Their striking difference in size suggests that they formed by following two very different paths.
The more massive star took the more traditional route by collapsing under gravity out of a dense ‘core’ of gas. The smaller one likely followed the road less traveled by — at least for stars — by accumulating mass from a portion of the disk that ‘fragmented’ away as it matured, a process that may have more in common with the birth of gas-giant planets.
“Astronomers have known for a long time that most massive stars orbit one or more other stars as partners in a compact system, but how they got there has been a topic of conjecture,” said team member Dr. Crystal Brogan, an astronomer with the National Radio Astronomy Observatory (NRAO).
“With ALMA, we now have evidence that the disk of gas and dust that encompasses and feeds a growing massive star also produces fragments at early stages that can form a secondary star.”
“This ALMA observation opens new questions, such as ‘does the secondary star also have a disk?’ and ‘how fast can the secondary star grow?’ The amazing thing about ALMA is that we have not yet used its full capabilities in this area, which will someday allow us to answer these new questions,” said team member Dr. Todd Hunter, also from the NRAO.
Observation of the dust emission (green) and the cool gas around MM1a (red is receding gas, blue is approaching gas), indicating that the outflow cavity rotates in the same sense as the central accretion disk. MM1b is seen orbiting in the lower left. Image credit: ALMA / ESO / NAOJ / NRAO / J.D. Ilee, University of Leeds.
Stars form within large clouds of gas and dust in interstellar space. When these clouds collapse under gravity, they begin to rotate faster, forming a disk around them.
“In low-mass stars like our Sun, it is in these disks that planets can form,” said team leader Dr. John Ilee, an astronomer at the University of Leeds.
“In this case, the star and disk we have observed are so massive that, rather than witnessing a planet forming in the disk, we are seeing another star being born.”
By observing the millimeter wavelength light naturally emitted by the dust, and subtle shifts in the frequency of light emitted by the gas, the researchers were able to calculate the mass of MM 1a and MM 1b.
“Many older massive stars are found with nearby companions. But binary stars are often very equal in mass, and so likely formed together as siblings,” Dr. Ilee said.
“Finding a young binary system with a mass ratio of 80-to-1 is very unusual, and suggests an entirely different formation process for both objects.”
The newly-discovered young star MM 1b could also be surrounded by its own circumstellar disk, which may have the potential to form planets of its own — but it will need to be quick.
“Stars as massive as MM 1a only live for around a million years before exploding as powerful supernovae, so while MM 1b may have the potential to form its own planetary system in the future, it won’t be around for long,” Dr. Ilee said.
The discovery will be reported in the Astrophysical Journal Letters.
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J.D. Ilee et al. 2018. G11.92-0.61 MM 1: A fragmented Keplerian disk surrounding a proto-O star. ApJL, in press; arXiv: 1811.05267
