Failed star triumphs as dwarf role model

BY KEITH COOPER

ASTRONOMY NOW

Posted: 20 April, 2009

The Wolf 940 system. Image: UKIRT.

One of the coolest brown dwarfs yet discovered, with a surface temperature of 300 degrees Celsius, could turn out to be an invaluable 'Rosetta Stone' for decoding the spectra of other brown dwarfs. This is because judging the age of brown dwarfs is typically quite difficult, and age plays a large role in various assumptions made in theories of brown dwarf formation. However, this newly discovered brown dwarf, Wolf 940B, is orbiting a red dwarf for which we do know the age.

Brown dwarfs are commonly termed 'failed stars'. Invariably only a few dozen times more massive than the planet Jupiter, they are unable to maintain nuclear fusion reactions of hydrogen into helium in their cores like fully-fledged stars. A red dwarf, on the other hand, is the smallest type of star that can generate energy through fusion. Red dwarfs are also the most common stars in the Universe.

A comparison between the spectrum of Jupiter, and the only slightly warmer and more massive brown dwarf Wolf 940B.

The brown dwarf/red dwarf binary system, 40 light years from Earth, is being announced today at the RAS' National Astronomy Meeting at the University of Hertfordshire. Indeed, astronomers from the university led the discovery that was stumbled across in a major deep sky survey utilising the United Kingdom Infrared Telescope (UKIRT). Brown dwarfs are so cool and faint that they can only be picked up at infrared wavelengths.

Wolf 940B orbits the red dwarf, Wolf 940A, at a distance of 65 billion kilometres; 440 times the distance between Earth and the Sun. It takes 18,000 years to complete a single orbit of the red dwarf, and is between 20 and 30 times the mass of Jupiter.

"Although its surface is hot enough to melt lead, temperature is relative when you study this sort of thing, and this object is cool by stellar standards," says Dr Ben Burningham of the University of Hertfordshire.

"What's so exciting," he continues, "is that we can use what we know about the primary [red dwarf] star to find out about the properties of the brown dwarf." This will enable astronomers to match Wolf 940B to various models, see which fits best, and then apply these to other brown dwarfs.

"This object is going to continue to provide insights into the processes of cool brown dwarfs for some time to come," adds Dr Sandy Leggett of the Gemini Observatory, which chipped in to help confirm Wolf 940B's temperature. "Finding it was just the first step."

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