Scientists Track Collision of Powerful Stellar Winds (Forwarded)

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April 11, 2005

Scientists Track Collision of Powerful Stellar Winds

Astronomers using the National Science Foundation's Very Long Baseline
Array (VLBA) radio telescope have tracked the motion of a violent region
where the powerful winds of two giant stars slam into each other. The
collision region moves as the stars, part of a binary pair, orbit each
other, and the precise measurement of its motion was the key to
unlocking vital new information about the stars and their winds.

Both stars are much more massive than the Sun -- one about 20 times the
mass of the Sun and the other about 50 times the Sun's mass. The
20-solar-mass star is a type called a Wolf-Rayet star, characterized by
a very strong wind of particles propelled outward from its surface. The
more massive star also has a strong outward wind, but one less intense
than that of the Wolf-Rayet star. The two stars, part of a system named
WR 140, circle each other in an elliptical orbit roughly the size of our
Solar System.

"The spectacular feature of this system is the region where the stars'
winds collide, producing bright radio emission. We have been able to
track this collision region as it moves with the orbits of the stars,"
said Sean Dougherty, an astronomer at the Herzberg Institute for
Astrophysics in Canada. Dougherty and his colleagues presented their
findings in the April 10 edition of the Astrophysical Journal.

The supersharp radio "vision" of the continent-wide VLBA allowed the
scientists to measure the motion of the wind collision region and then
to determine the details of the stars' orbits and an accurate distance
to the system.

"Our new calculations of the orbital details and the distance are
vitally important to understanding the nature of these Wolf-Rayet stars
and of the wind-collision region," Dougherty said.

The stars in WR 140 complete an orbital cycle in 7.9 years. The
astronomers tracked the system for a year and a half, noting dramatic
changes in the wind collision region.

"People have worked out theoretical models for these collision regions,
but the models don't seem to fit what our observations have shown," said
Mark Claussen, of the National Radio Astronomy Observatory in Socorro,
New Mexico. "The new data on this system should provide the theorists
with much better information for refining their models of how Wolf-Rayet
stars evolve and how wind-collision regions work," Claussen added.

The scientists watched the changes in the stellar system as the star's
orbits carried them in paths that bring them nearly as close to each
other as Mars is to the Sun and as far as Neptune is from the Sun. Their
detailed analysis gave them new information on the Wolf-Rayet star's
strong wind. At some points in the orbit, the wind collision region
strongly emitted radio waves, and at other points, the scientists could
not detect the collison region.

Wolf-Rayet stars are giant stars nearing the time when they will explode
as supernovae.

"No other telescope in the world can see the details revealed by the
VLBA," Claussen said. "This unmatched ability allowed us to determine
the masses and other properties of the stars, and will help us answer
some basic questions about the nature of Wolf-Rayet stars and how they
develop." he added.

The astronomers plan to continue observing WR 140 to follow the system's
changes as the two massive stars continue to circle each other.

Dougherty and Claussen worked with Anthony Beasley of the Atacama Large
Millimeter Array office, Ashley Zauderer of the University of Maryland
and Nick Bolingbroke of the University of Victoria, British Columbia.

The National Radio Astronomy Observatory is a facility of the National
Science Foundation, operated under cooperative agreement by Associated
Universities, Inc.

IMAGE CAPTION:
[http://www.nrao.edu/pr/2005/wr140/wr140.dates.big.jpg (412KB)]
Motion of Wind Collision Region

Graphic superimposes VLBA images of wind collision region on diagram of
orbit of Wolf-Rayet (WR) star and its giant (O) companion. CREDIT:
Dougherty et al., NRAO/AUI/NSF