Astronomers at Armagh Observatory Find a Solitary Superstar

Tarantula Nebula
(TRAPPIST national telescope at La Silla)

Combined YJKs image from the
VISTA Magellanic Clouds survey
(Cioni et al. 2011).

An international team of astronomers, led by staff from the Armagh Observatory, has detected a monster star in one of the Milky Way's nearest neighbour galaxies, the Large Magellanic Cloud (LMC). The LMC is located about 160,000 light years away in the southern constellations of Dorado and Mensa The star is extremely luminous, perhaps 3 million times that of the Sun, is about 150 times the mass of the Sun, and is unusual in that it lies in relative isolation.

All previous such superstars have been found along with many companion stars in the centres of crowded star clusters. The star is known as VFTS 682 from the VLT-FLAMES Tarantula Survey catalogue of the European Southern Observatory's Very Large Telescope (VLT) located at Paranal in Chile by means of the attached FLAMES instrument. Being found in isolation makes this star mysterious: was it formed in isolation or was it ejected from a cluster? Either way it is a challenge to current star-formation theories.

The team, led by Jorick Vink, Götz Gräfener and Joachim Bestenlehner of Armagh Observatory, used FLAMES to analyze the star's light and found that it is about three million times brighter than the Sun. "We were very surprised to find such a massive star on its own, and not in a rich star cluster. Its origins are mysterious", notes Joachim Bestenlehner, PhD student and lead author of the new study.

The star was spotted earlier in a survey of the most brilliant stars in and around the Tarantula Nebula in the LMC. This huge region of gas, dust and young stars is the most active star-forming region in the Local Group of galaxies. At first glance this star 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 the shorter wavelength blue and green light is scattered more and lost. As a result the star appears to us to be red, rather like the view of our Sun through the Earth's dusty atmosphere at sunset, although if the view were unobstructed it would shine brilliant blue white.

As well as being very bright VFTS 682 is also very hot with a surface temperature of 52,000 degrees Celsius. Stars with these unusual properties may end their short lives not just as a supernova, as is normal for high-mass stars, but possibly as an even more dramatic long-duration Gamma Ray Burst, the brightest explosions in the Universe. Such an event would make this star one of the brightest objects in the sky for a short time.

Although VFTS 682 seems to be alone now it is not very far away from a very rich star cluster known as RMC 136 that 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," adds Paco Najarro (Centre of Astrobiology, Madrid), another member of the team.

As for the ejection theory, such "runaway stars" are known, but all are much smaller than VFTS 682 and it is hard 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," adds Jorick Vink. "And although it may be possible, it is harder to understand how these brilliant beacons could form on their own. However one looks at it, VFTS 682 is a really fascinating object."

Astronomical research at the Armagh Observatory is funded by the Northern Ireland Department of Culture, Arts and Leisure. This project used the European Southern Observatory VLT and was supported by the UK Science and Technology Facility Council.

FOR FURTHER INFORMATION PLEASE CONTACT: Jorick Vink or Joachim Bestenlehner at the Armagh Observatory, College Hill, Armagh, BT61 9DG. Tel.: 028-3752-2928; FAX: 028-3752-7174; jsv at sign arm.ac.uk; jbl at sign @arm.ac.uk.

Research paper - PDF

Last Revised: 2011 May 25th