Disk discovered around extragalactic star

Artist’s impression of the young star in the Large Magellanic Cloud drawing material from a dusty disk and ejecting some of it in jets. © ESO/M. grain knife

A rotating disk of gas and dust from which growing stars feed: Astronomers have now discovered a so-called circumstellar accretion disk for the first time in a neighboring galaxy of the Milky Way. Despite the great distance, the view of the system in the Large Magellanic Cloud is surprisingly clear. The scientists hope that they will now be able to observe star and disk formation in other galactic environments in order to expand our knowledge of these fundamental processes in the cosmos.

Birth, development and death also shape the history of the celestial bodies in the universe. At the beginning of the formation of stars there are accumulations of gas and dust, which ultimately condense into a central protostar. Due to the further influx of material from the environment, it then grows more and more. The “food” of the stellar baby comes from a kind of rotating plate: the matter arranges itself into a so-called circumstellar accretion disk, from which the growing star feeds. This structure can also later lead to the formation of planets around a star. Astronomers have already identified circumstellar disks around young stars in our Milky Way. These are often noticeable through a special feature: As the material accumulates, matter can be ejected into space in the form of so-called jets.

Such a structure was now the beginning of the first evidence of a circumstellar disk outside our galaxy: astronomers working with Anna McLeod from Durham University had discovered a jet in the star system HH 1177 in a galaxy 160,000 light-years away from us: in the Large Magellanic Cloud. “The jet emanating from this young massive star was an indication that ongoing disk accretion is occurring there,” says McLeod. But to confirm that such a disk actually exists, the team had to detect the movement of the dense gas around the star. Specifically, the typical dynamics should emerge: Near the center, the disk rotates faster and this difference in speed can then serve as evidence that an accretion disk is present.

Characteristic rotation emerges

To more accurately capture the orientation and dynamics of the HH 1177 system, the team leveraged the combined capabilities of ESO's Very Large Telescope (VLT) and the Atacama Large Millimeter/submillimeter Array (ALMA). The VLT's MUSE instrument was initially able to show that the upper part of the jet emanating from the star appears to be slightly aimed at us and therefore blueshifted - the lower part is moving away from us and therefore redshifted. ALMA's observations then provided information about the dynamics of the disk, whose sides also move towards and away from us.

“The frequency of the light changes depending on how quickly the glowing gas is moving towards or away from us,” explains co-author Jonathan Henshaw from Liverpool John Moores University. “This is exactly the same phenomenon that occurs when the pitch of an ambulance siren changes as it passes you. The frequency of the sound shifts from higher to lower,” said the astronomer. In this way, the scientists were able to document the characteristic rotation of the disk using the detailed frequency measurements with ALMA. This means: They have discovered the first accretion disk around an extragalactic young star.

Looking into the distance can be worthwhile

“When I first saw evidence of a rotating structure in the ALMA data, I couldn't believe that we had discovered the first extragalactic accretion disk. “This was a special moment,” says McLeod. “We know that the disks are crucial for the formation of stars and planets in our galaxy, and this is the first time we see direct evidence of this in another galaxy,” sums up the scientist.

As the team points out, an extragalactic view of star formation processes can actually offer an advantage because the view may be clearer there. Massive stars in the Milky Way are notoriously difficult to observe because they are often particularly obscured by the dusty material from which they form. However, in the Large Magellanic Cloud, the material from which new stars are born is different from that in the Milky Way, astronomers explain. Because of the lower dust content, HH 1177 was no longer wrapped in its birth cocoon and thus provided quite clear insights despite the great distance.

The successful discovery of this extragalactic circumstellar disk gives hope that astronomers will be able to detect other such systems using existing and planned telescope systems. Studying star and disk formation in different galactic environments could then help to complete our understanding of star formation processes. “We are in an era of rapid technological advances in astronomical facilities,” says McLeod. “It is very exciting to be able to study the formation of stars at such incredible distances and in another galaxy,” said the astronomer.

Source: Durham University, ESO, specialist article: Nature, doi: 10.1038/s41586-023-06790-2

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