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.
EDITORS: A full-color image and animations are online at
http://cfa-www.harvard.edu/cfa/ep/pressrel/monnier_images.html
For more information, contact:
David A. Aguilar, Public Affairs
Harvard-Smithsonian Center for Astrophysics
Phone: 617-495-7462 Fax: 617-495-7468
daguilar@cfa.harvard.edu
Dr. John D. Monnier
Harvard-Smithsonian Center for Astrophysics
Phone: 617-496-7898
jmonnier@cfa.harvard.edu
