Astronomers Discover Local Star’s Cool Companion

An international team, led by astronomers at the University of
Hertfordshire in the UK, have discovered one of the coolest
sub-stellar bodies ever found outside our own solar system, orbiting
the red dwarf star Wolf 940, some 40 light years from Earth.

“Although it has a temperature of 300 degrees Celsius, 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. In
fact, this is the first time we’ve been able to study an object as
cool as this in such detail”, says Dr Ben Burningham, of the
University of Hertfordshire, “the fact that it is orbiting a star
makes it extra special”.

The object is 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.

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
temperature these objects are very faint in visible light, and are
detected by their glow at infrared wavelengths.

Modeling the atmospheres of cool brown dwarfs is a complex task, but
it is key to understanding what we see when we look at 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”, explains Dr
Burningham. “You can think of it as a Rosetta Stone for decrypting
what the light from such cool objects is telling us”.

The object has been named Wolf 940B, after the red dwarf star that it
orbits, which was first catalogued by the pioneering German astronomer
Max Wolf ninety years ago.

“Red dwarfs are the most populous stars in the Galaxy, and systems
like this may be more common than we know” says Dr David Pinfield 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.”

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 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 team’s findings will soon be published in
the Monthly Notices of the Royal Astronomical Society.

Following its discovery with ground based telescopes, Wolf 940B, has
since been observed by the NASA’s Spitzer Space Telescope, and the
findings from those observations will be published later this year.

“This object is going to continue to provide insights into the
processes of cool brown dwarf, and warm planetary atmospheres for some
time to come”, says Dr Sandy Leggett, of the Gemini Observatory,
“finding it was just the first step”.

Dr Burningham will present this result at the European Week of
Astronomy and Space Science (NAM 2009) at the University of
Hertfordshire today.