ESO’s VLT Finds a Brilliant but Solitary Superstar

Using the European Southern Observatory’s Very Large Telescope (VLT), astronomers have found that the star VFTS 682 is about 150 times the mass of the sun. But unlike other massive stars that have thus far been found only in the crowded centers of star clusters, VFTS 682 lies on its own within the Large Magellanic Cloud.

"We were very surprised to find such a massive star on its own, and not in a rich star cluster," said Joachim Bestenlehner, the lead author of a study revealing details on VFTS 682. "Its origin is mysterious."

This star was spotted earlier in a survey of the most brilliant stars in and around the Tarantula Nebula in the Large Magellanic Cloud. It lies in a stellar nursery: a huge region of gas, dust and young stars that is the most active star-forming region in the Local Group of galaxies, which includes the Milky Way and Andromeda galaxies, as well as the Magellanic Clouds and many smaller galaxies. VFTS is short for VLT-FLAMES Tarantula Survey, a European Southern Observatory Large Program led by Christopher Evans of the UK Astronomy Technology Center in Edinburgh, Scotland.

At the exact center of the very active star-forming region around the Tarantula Nebula in the Large Magellanic Cloud lies the brilliant but isolated star VFTS 682, and to its lower right the very rich star cluster R 136. The origins of VFTS are unclear — was it ejected from R 136, or did it form on its own? (Image: European Southern Observatory/M.-R. Cioni/VISTA Magellanic Cloud survey. Acknowledgment: Cambridge Astronomical Survey Unit)

At first glance, VFTS 682 was thought to be hot, young and bright, but unremarkable. But the new study using the VLT has found that much of the star's energy is being absorbed and scattered by dust clouds before it gets to Earth. It is actually more luminous than previously thought and among the brightest stars known.

Red and infrared light emitted by the star can get through the dust, but shorter-wavelength blue and green light is scattered more. As a result, the star appears reddish, although if the view were unobstructed, it would shine a brilliant blue-white.

As well as being very bright, VFTS 682 is also very hot, with a surface temperature of about 50,000 °C, compared with our sun’s 5500 °C. Stars with these unusual properties may end their short lives not just as a supernova, as is normal for high-mass stars, but just possibly as an even more dramatic long-duration gamma-ray burst, one of the brightest explosions in the universe.

Although VFTS 682 seems to be alone, it is not very far away from the very rich star cluster RMC 136 (often called just R 136), which contains several similar "superstars."

"The new results show that VFTS 682 is a near-identical twin of one of the brightest superstars at the heart of the R 136 star cluster," said team member Paco Najarro.

Is it possible that VFTS 682 formed there and was ejected? Such "runaway stars" are known, but all are much smaller than VFTS 682, and it would be interesting to see how such a heavy star could be thrown from the cluster by gravitational interactions.

"It seems to be easier to form the biggest and brightest stars in rich star clusters," added Jorick Vink, another member of the team. "And although it may be possible, it is harder to understand how these brilliant beacons could form on their own. This makes VFTS 682 a really fascinating object."

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