Astronomers have discovered a disc around a young star in the Large Magellanic Cloud, a neighboring galaxy to our own. It’s the first time such a disc, similar to those that form planets in our own Milky Way, has been discovered outside of our galaxy. The new findings show a massive young star growing and accreting matter from its surroundings to form a rotating disc. The detection was made with the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, a collaboration with the European Southern Observatory (ESO).
“When I first saw evidence for a rotating structure in the ALMA data, I couldn’t believe we had discovered the first extragalactic accretion disc; it was a special moment,” says Anna McLeod, an associate professor at Durham University in the United Kingdom and lead author of the study published today in Nature. “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.”
Most exoplanets are discovered using methods such as the transit method or radial velocity measurements. These methods are less effective at detecting exomoons. One method proposed is to look for changes in the timing of the planet’s transits caused by the gravitational influence of a moon.
The frequency of light changes depending on how fast the gas emitting the light is moving towards or away from us. 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.
Jonathan Henshaw
This research builds on observations made with the Multi Unit Spectroscopic Explorer (MUSE) instrument on ESO’s Very Large Telescope (VLT), which discovered a jet from a forming star – the system was named HH 1177 – 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,” said McLeod. To confirm the presence of such a disc, the team needed to measure the movement of the dense gas surrounding the star.
When matter is drawn 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. “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.
Massive stars, like the one seen here, form much faster and live much shorter lives than low-mass stars like our Sun. These massive stars are notoriously difficult to observe in our galaxy, and they are frequently obscured from view by the dusty material from which they form as a disc forms around them. However, the material from which new stars are born in the Large Magellanic Cloud, a galaxy 160 000 light-years away, is fundamentally different from that in the Milky Way. Because of the lower dust content, HH 1177 is no longer encased in its natal cocoon, providing astronomers with an unobstructed, albeit distant, view of star and planet formation.
