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Posted by baalke on July 24, 2006, 4:49 pm
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MEDIA RELATIONS OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIF. 91109 TELEPHONE (818) 354-5011
http://www.jpl.nasa.gov
Whitney Clavin (818) 354-4673
Jet Propulsion Laboratory, Pasadena, Calif.
News Release: 2006-094 July 24, 2006
Planet-Forming Disks Might Put the Brakes on Stars
Astronomers using NASA's Spitzer Space Telescope have
found evidence that dusty disks of planet-forming material
tug on and slow down the young, whirling stars they
surround.
Young stars are full of energy, spinning around like tops
in half a day or less. They would spin even faster, but
something puts on the brakes. While scientists had
theorized that planet-forming disks might be at least part
of the answer, demonstrating this had been hard to do until
now.
"We knew that something must be keeping the stars' speed in
check," said Dr. Luisa Rebull of NASA's Spitzer Science Center,
Pasadena, Calif. "Disks were the most logical answer, but we
had to wait for Spitzer to see the disks."
Rebull, who has been working on the problem for nearly a
decade, is lead author of a new paper in the July 20 issue
of the Astrophysical Journal. The findings are part of a
quest to understand the complex relationship between young
stars and their burgeoning planetary systems.
Stars begin life as collapsing balls of gas that spin faster
and faster as they shrink, like twirling ice skaters pulling in
their arms. As the stars whip around, excess gas and dust
flatten into surrounding pancake-like disks. The dust and gas
in the disks are believed to eventually clump together to form
planets.
Developing stars spin so fast that, left unchecked, they would
never fully contract and become stars. Prior to the new study,
astronomers had theorized that disks might be slowing the super
speedy stars by yanking on their magnetic fields. When a star's
fields pass through a disk, they are thought to get bogged down
like a spoon in molasses. This locks a star's rotation to the
slower-turning disk, so the shrinking star can't spin faster.
To prove this principle, Rebull and her team turned to Spitzer
for help. Launched in August of 2003, the infrared observatory
is an expert at finding the swirling disks around stars, because
dust in the disks is heated by starlight and glows at infrared
wavelengths.
The team used Spitzer to observe nearly 500 young stars in the
Orion nebula. They divided the stars into slow spinners and
fast spinners, and determined that the slow spinners are five
times more likely to have disks than the fast ones.
"We can now say that disks play some kind of role in slowing
down stars in at least one region, but there could be a host of
other factors operating in tandem. And stars might behave
differently in different environments," Rebull said.
Other factors that contribute to a star's winding down over
longer periods of time include stellar winds and possibly
full-grown planets.
If planet-forming disks slow down stars, does that mean stars
with planets spin more slowly than stars without planets? Not
necessarily, according to Rebull, who said slowly spinning stars
might simply take more time than other stars to clear their
disks and develop planets. Such late-blooming stars would, in
effect, give their disks more time to put on the brakes and slow
them down.
Ultimately, the question of how a star's rotation rate is
related to its ability to support planets will fall to planet
hunters. So far, all known planets in the universe circle stars
that turn around lazily. Our sun is considered a slowpoke,
currently plodding along at a rate of one revolution every 28 days.
And, due to limits in technology, planet hunters have not been
able to find any extrasolar planets around zippy stars.
"We'll have to use different tools for detecting planets around
rapidly spinning stars, such as next-generation ground and space
telescopes," said Dr. Steve Strom, an astronomer at the National
Optical Astronomy Observatory, Tucson, Ariz.
Other members of Rebull's team include Drs. John Stauffer of the
Spitzer Science Center; S. Thomas Megeath at the University of Toledo,
Ohio; and Joseph Hora and Lee Hartmann of the Harvard-Smithsonian
Center for Astrophysics, Cambridge, Mass. Hartmann is also affiliated
with the University of Michigan, Ann Arbor.
NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the
Spitzer Space Telescope mission for NASA's Science Mission
Directorate, Washington. Science operations are conducted at the
Spitzer Science Center at the California Institute of Technology.
Caltech manages JPL for NASA.
For an animation depicting how disks slow stars and more information
about Spitzer, visit www.spitzer.caltech.edu/spitzer .
-end-
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