David Aguilar
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Christine Pulliam
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CfA Press Release
 Release No.: 02-01
For Release: 9:30 am EST, January 7, 2002

High-Resolution Images of Binary Star System Support Colliding Winds Theory of Dust Formation

Cambridge, MA - On the Earth, dust is everywhere - under beds, on bookshelves, even floating in the air. We take it for granted. In space, dust is also common in cold, dark molecular clouds and in cool, dense winds from long-period variable stars. But, astronomers wouldn't generally expect to find dust in the hot region surrounding a massive, luminous star because the fierce heat of the star burns it away.

Now, high-resolution near-infrared images of an evolved binary star system, located about 4,500 light-years away in the constellation Cygnus, have confirmed that the system is periodically forming and ejecting large arcs of dust. The images and their interpretation are being presented today to the American Astronomical Society meeting in Washington, D.C. by Drs. John Monnier of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts; Peter Tuthill of the University of Sydney in Australia; and William Danchi of NASA Goddard Space Flight Center in Greenbelt, Maryland. These images support the theory that dust can form in otherwise hostile hot binary systems within regions where stellar winds collide.

"In the new images, we can see the dust forming in the boundary region between the colliding stellar winds and being blown away at 6 million miles per hour," said Monnier. Tuthill agreed, saying, "We were thrilled to get such detailed observations of this system, which is the real Rosetta Stone of colliding-wind systems."

Monnier's team studies Wolf-Rayet (WR) binary systems. These systems consist of two hot, bright, massive stars swinging around each other, one of which is a Wolf-Rayet star - a luminous star with little hydrogen that will soon explode as a supernova. Observations of WR systems with close circular orbits have found a handful where dust forms constantly as the two stars orbit each other. High-resolution near-infrared images of such systems have shown that, when we view these systems face on, we see a pinwheel-shaped nebula of dust surrounding the two stars.

The binary system being discussed today, dubbed WR 140, has a highly elliptical orbit. These stars orbit each other every eight years, approaching as closely as 2.5 astronomical units. (An astronomical unit is the average distance between the Earth and Sun.) Dust is formed only when the stars are at or near their closest approach.

The team's observations of WR 140 indicate that we are viewing it nearly edge on, rather than face on. Combined with the fact that dust is only generated for a brief time at closest approach, the view is very different from that of face-on systems. Models of the WR 140 system show that edge-on viewers see strange structures like arcs, filaments, and other complex shapes.

"Without knowing the orbital parameters independent of our observations, we can't tell how much of the structure is due to geometry and how much is from intrinsic clumpiness of the dusty region," said Monnier. "There are still a lot of questions to be answered."

Physics predicts that a system like WR 140 would not normally be able to produce dust because the environment near these stars is so hot. Dust grains are generally found in cold, dark clouds where they are protected from heat and light that can destroy them.

In 1991, astronomer Vladimir Usov proposed that dust could form at the boundary between colliding stellar winds. At this boundary, the atoms streaming from the stars are compacted together enough that the outer atoms shield the inner ones, allowing dust to form. Only when the WR 140 stars are at their closest do the winds compact enough to allow dust formation. Thus, every eight years at their closest passage, an arc of dust is swept out into space as the energetic Wolf-Rayet wind blows away the newly formed dust.

The team studied WR 140 by using the Keck I telescope on Mauna Kea, Hawaii as an interferometer. Computer processing produced images with a resolution of about 20 milliarcseconds - sufficient to distinguish the two headlights of a car in Rome from 6,000 miles away in Los Angeles.

"The full resolution of the world's biggest telescope, the Keck, was necessary to get the close-up frames needed to observe the outflow of dust in detail," said Tuthill. Danchi agreed and said, "The interferometric technique we used allowed us to compensate for the distortion from the atmosphere, giving us twenty times more resolution than we could have obtained otherwise."

Future observations will aid astronomers in disentangling effects of our viewing angle from real features of dust distribution. The latest arc of dust, produced during a close passage in 2001, is cooling to the point that soon it won't show up in near-infrared images. High-resolution mid-infrared images can follow the evolution of the dust arc over a longer period of time.

"Once other observations have determined the geometry of this system and its orbital parameters, we can use our images to map the dust distribution," said Monnier. "We urge the astronomical community to take advantage of the unique opportunity to observe this rapidly evolving system over the next few years."

"Infrared and optical interferometry is revolutionizing astronomy in the same way new types of microscopes have revolutionized biology. The exciting results shown today are just the beginning," said Danchi.

Headquartered in Cambridge, Massachusetts, the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists organized into seven research divisions study the origin, evolution, and ultimate fate of the universe.

For more information, contact:

David A. Aguilar
Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics

EDITORS: A full-color image and animations are online at http://www.cfa.harvard.edu/news/archive/monnier_images.html

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