Young star caught forming around another star

A small star has been observed forming out of the dust surrounding a larger star, in a similar way to how planets are born.

Astronomers were observing the formation of a massive young star, called MM 1a, when they discovered an unexpected object nearby.

MM 1a is surrounded by rotating disc of gas and dust. But orbiting just beyond this disc, they discovered a faint object they called MM 1b, which they discovered was a smaller star. MM 1b is believed to have formed out of the gas and dust surrounding the larger MM 1a.

Seeing the disc itself in so much detail is exciting, but detecting a second star forming within the disc, perhaps in a similar way to how planets form, was a huge unexpected bonus. Dr Thomas Haworth

The team of astronomers, led by the University of Leeds and including an Imperial College London researcher, have published their discovery today in the journal Astrophysical Journal Letters.

Co-author Dr Thomas Haworth, from the Department of Physics at Imperial, helped predict what might be observed around MM 1a, and then to interpret what they actually found. He said: “It’s great when the new data surprises you, which was definitely the case here.

“Seeing the disc itself in so much detail is exciting, but detecting a second star forming within the disc, perhaps in a similar way to how planets form, was a huge unexpected bonus. There is a lot of work ahead of us to fully understand the consequences of this new discovery.”

An entirely different formation process

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 disc around them. It is in these discs that planets can form around low mass stars like our Sun.

Lead author Dr John Ilee, from the School of Physics and Astronomy at the University of Leeds, said: “In this case, the star and disc we have observed is so massive that, rather than witnessing a planet forming in the disc, we are seeing another star being born.”

By measuring the amount of radiation 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.

They found that MM 1a weighs 40 times the mass of our Sun. The smaller orbiting star MM 1b was calculated to weigh less than half the mass of our Sun.

Dr Ilee said: “Many older massive stars are found with nearby companions. But these ‘binary’ stars are often very equal in mass, and so likely formed together as siblings. Finding a young binary system with a mass ratio of 80:1 is very unusual, and suggests an entirely different formation process for both objects.”

The team believe stars like MM 1b could form in the outer regions of cold, massive discs. These discs are unable to hold themselves up against the pull of their own gravity, collapsing into one or more fragments.

The team believe their discovery is one of the first examples of a ‘fragmented’ disc to be detected around a massive young star.

Only a million years to live

Dr Duncan Forgan, a co-author from the Centre for Exoplanet Science at the University of St Andrews, added: "I've spent most of my career simulating this process to form giant planets around stars like our Sun. To actually see it forming something as large as a star is really exciting.”

The researchers note that newly discovered young star MM 1b could also be surrounded by its own disc, which may have the potential to form planets of its own – but it will need to be quick. 

Dr Ilee added: “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.”

The astronomers made this surprising discovery by using a unique new instrument situated high in the Chilean desert – the Atacama Large Millimetre/submillimetre Array (ALMA). 

Using the 66 individual dishes of ALMA together in a process called interferometry, the astronomers were able to simulate the power of a single telescope nearly 4km across, allowing them to image the material surrounding the young stars for the first time.

Funders for this research include the Science and Technologies Facilities Council (UK) and the European Research Council.

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‘G11.92-0.61 MM1: A Fragmented Keplerian Disk Surrounding a Proto-O Star’ by J Ilee, C Cyganowski, C Brogan, T Hunter, D Forgan, T Haworth, C Clarke and T Harries is published in Astrophysical Journal Letters. Read the Open Access version.

Top image caption: Artist's impression of the disc of dust and gas surrounding the massive protostar MM 1a, with its companion MM 1b forming in the outer regions. Credit: J. D. Ilee / University of Leeds

Based on a press released by the University of Leeds.

See the press release of this article