Brown Dwarfs: A New Class of Stellar Lighthouse (Forwarded)

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EMBARGOED for Release 7 p.m., EDT, April 17, 2007

Brown Dwarfs: A New Class of Stellar Lighthouse

Brown dwarfs, thought just a few years ago to be incapable of emitting any
significant amounts of radio waves, have been discovered putting out
extremely bright "lighthouse beams" of radio waves, much like pulsars. A
team of astronomers made the discovery using the National Science
Foundation's Very Large Array (VLA) radio telescope.

"These beams rotate with the brown dwarf, and we see them when the beam
passes over the Earth. This is the same way we see pulses from pulsars,"
said Gregg Hallinan of the National University of Ireland Galway. "We now
think brown dwarfs may be a missing link between pulsars and planets in our
own Solar System, which also emit, but more weakly," he added.

Brown dwarfs are enigmatic objects that are too small to be stars but too
large to be planets. They are sometimes called "failed stars" because they
have too little mass to trigger hydrogen fusion reactions at their cores,
the source of the energy output in larger stars. With roughly 15 to 80 times
the mass of Jupiter, the largest planet in our Solar System, brown dwarfs
were long thought to exist. However, it was not until 1995 that astronomers
were able to actually find one. A few dozen now are known.

In 2001, a group of summer students at the National Radio Astronomy
Observatory used the VLA to observe a brown dwarf, even though they had been
told by seasoned astronomers that brown dwarfs are not observable at radio
wavelengths. Their discovery of a strong flare of radio emission from the
object surprised astronomers and the students' scientific paper on the
discovery was published in the prestigous scientific journal Nature.

Hallinan and his team observed a set of brown dwarfs with the VLA last year,
and found that three of the objects emit extremely strong, repeating pulses
of radio waves. They concluded that the pulses come from beams emitted from
the magnetic poles of the brown dwarfs. This is similar to the beamed
emission from pulsars, which are superdense neutron stars, and much more
massive than brown dwarfs.

The characteristics of the beamed radio emission from the brown dwarfs
suggest to the scientists that it is produced by a mechanism also seen at
work in planets, including Jupiter and Earth. This process involves
electrons interacting with the planet's magnetic field to produce radio
waves that then are amplified, or strengthened, by natural masers that
amplify radio waves the same way a laser amplifies light waves.

"The brown dwarfs we observed are between planets and pulsars in the
strength of their radio emissions," said Aaron Golden, also of the National
University of Ireland Galway. "While we don't think the mechanism that's
producing the radio waves in brown dwarfs is exactly the same as that
producing pulsar radio emissions, we think there may be enough similarities
that further study of brown dwarfs may help unlock some of the mysteries
about how pulsars work," he said.

While pulsars were discovered 40 years ago, scientists still do not
understand the details of how their strong radio emissions are produced.

The brown dwarfs rotate at a much more leisurely pace than pulsars. While
pulsars rotate -- and produce observed pulses -- typically several times a
second to hundreds of times a second, the brown dwarfs observed with the VLA
are showing pulses roughly once every two to three hours.

Hallinan and Golden worked with Stephen Bourke and Caoilfhionn Lane, also of
the National University of Ireland Galway; Tony Antonova and Gerry Doyle of
Armagh Observatory in Northern Ireland; Robert Zavala and Fred Vrba of the
U.S.Naval Observatory in Flagstaff, Arizona; Walter Brisken of the National
Radio Astronomy Observatory in Socorro, New Mexico; and Richard Boyle of the
Vatican Observatory Research Group at Steward Observatory in Arizona.

The scientists presented their results to the Royal Astronomical Society's
National Astronomy Meeting at the University of Central Lancashire in the
United Kingdom.

The National Radio Astronomy Observatory is a facility of the National
Science Foundation, operated under cooperative agreement by Associated
Universities, Inc. This work was supported by Science Foundation Ireland
under its Research Frontiers Programme, the Higher Education Authority's
Programme for Research in Third Level Institutions, and the Irish Research
Council for Science, Engineering and Technology.

Graphics and Movie,
http://www.nrao.edu/pr/2007/browndwa...graphics.shtml