Discovery of rotating disc around nascent star 'a special moment'

Astronomers are getting one of the best views ever of the birth of a massive star, after spotting the tell-tale signs of its formation – a rapidly spinning disc of matter.

The young star, named HH 1177, is located in the Large Magellanic Cloud, a galaxy neighbouring ours 160,000 light years away.

The disc that led to its discovery is called an accretion disc and is the first ever found outside our galaxy.

“When I first saw evidence for a rotating structure, I could not believe that we had detected the first extragalactic accretion disc, it was a special moment,” says Anna McLeod, an associate professor at Durham University who worked with LJMU’s Astrophysics Research Institute.

“We know discs are vital to forming stars and planets in our galaxy, and here, for the first time, we’re seeing direct evidence for this in another galaxy.” 

Massive stars like HH 1177 are notoriously challenging to observe in our home galaxy as they are often obscured from view by dusty material at the time a disc is shaping around them.

In the Large Magellanic Cloud, the material from which new stars are being born is fundamentally different from that in the Milky Way. Thanks to the lower dust content, HH 1177 is no longer cloaked in its natal cocoon, offering astronomers an unobstructed, if far away, view of star and planet formation.

The detection was made using the Atacama Large Millimeter/Submillimeter Array (ALMA) in Chile, in which the European Southern Observatory (ESO) is a partner and follows up observations on ESO’s Very Large Telescope (VLT), which spotted a jet from a forming star deep inside a gas cloud in the Large Magellanic Cloud. 

“We discovered a jet being launched from this young massive star, and its presence is a signpost for ongoing disc accretion,” McLeod says. But to confirm that such a disc was indeed present, the team needed to measure the movement of the dense gas around the star.

As matter is pulled towards a growing star, it cannot fall directly onto it; instead, it flattens into a spinning disc around the star. Closer to the centre, the disc rotates faster, and this difference in speed is the smoking gun that shows astronomers an accretion disc is present.

“The frequency of light changes depending on how fast the gas emitting the light is moving towards or away from us,” explains Jonathan Henshaw, a research fellow at Liverpool John Moores University in the UK, and co-author of the study published today in Nature. “This is precisely the same phenomenon that occurs when the pitch of an ambulance siren changes as it passes you and the frequency of the sound goes from higher to lower.” 

The detailed frequency measurements from ALMA allowed the authors to distinguish the characteristic spin of a disc, confirming the detection of the first disc around an extragalactic young star.

“We are in an era of rapid technological advancement when it comes to astronomical facilities,” McLeod says. “Being able to study how stars form at such incredible distances and in a different galaxy is very exciting.”