UK, US Astronomers: That’s One Cool Star

An international team, led by astronomers at the University of Hertfordshire in the UK, has discovered one of the coolest sub-stellar bodies ever found outside our own solar system.

The new object — dubbed Wolf 940B — orbits the red dwarf star Wolf 940, 40 light years from Earth. It’s thought to have formed like a star, but has ended up looking more like Jupiter. It is roughly the same size, despite being between 20 and 30 times as heavy, and when the infrared spectral “fingerprints” of the two objects are compared, their resemblance is striking, say Wolf 940B’s discoverers.

Wolf 940B was initially discovered as part of a major infrared sky survey – the UKIRT Infrared Deep Sky Survey (UKIDSS) which is being carried out using the United Kingdom Infrared Telescope (UKIRT) on Mauna Kea in Hawaii. The telescope’s wide field camera is the long black tube in the image at left.

The object was found as part of a wider effort to find the coolest and least luminous bodies in our local Galactic neighborhood, but it was then found to be a companion to the nearby red dwarf Wolf 940 through its common motion across the sky. The data used to confirm the discovery were obtained using telescopes in Chile, the Canary Islands and Hawaii.

Its temperature was then confirmed using data from the Gemini-North telescope on Mauna Kea. The findings are being reported at the European Week of Astronomy and Space Science (NAM 2009) at the University of Hertfordshire, and will soon be published in the Monthly Notices of the Royal Astronomical Society.

The new object orbits its star at about 440 times the distance at which the Earth orbits the sun. At such a wide distance, it takes about 18,000 years to complete a single orbit.

Too small to be stars, so-called “brown dwarfs” have masses lower than stars but larger than gas giant planets like Jupiter. Due to their low temperatures, these objects are very faint in visible light, and are detected by their glow at infrared wavelengths.

“Although it has a temperature of 300 degrees Celsius [572 degrees F], which is almost hot enough to melt lead, temperature is relative when you study this sort of thing, and this object is very cool by stellar standards,” said Ben Burningham, of the University of Hertfordshire. “In fact, this is the first time we’ve been able to study an object as cool as this in such detail. The fact that it is orbiting a star makes it extra special.”

Modeling the atmospheres of cool brown dwarfs is a complex task, but it is key to understanding planets that orbit other stars. Models of emitted light from such objects, which are dominated by absorption due to water and methane gas, are sensitive to assumptions about their age and chemical make-up.

In most cases, astronomers don’t initially know much about the age and composition of brown dwarfs — and this can make it hard to tell where the models are right, and where they are going wrong.

“What’s so exciting in this case, is that we can use what we know about the primary star to find out about the properties of the brown dwarf, and that makes it an extremely useful find,” Burningham said. “You can think of it as a Rosetta Stone for decrypting what the light from such cool objects is telling us.”

Wolf 940A, the red dwarf star that is Wolf 940B’s namesake, was first catalogued by the pioneering German astronomer Max Wolf 90 years ago.

“Red dwarfs are the most populous stars in the Galaxy, and systems like this may be more common than we know” said David Pinfield, also of the University of Hertfordshire. “As the generation of ongoing large scale surveys continues, we may discover a pack of Wolf-940B-like objects in our solar back yard.”

Source: Joint Astronomy Centre. For more information, visit: 

The UK Infrared Telescope
NAM 2009
Gemini Observatory