Astronomers searching for dusty disks around nearby stars instead have discovered relatives of the Seven Sisters.

In a paper presented today (Tuesday, Jan. 8) at the American
Astronomical Society meeting in Washington, D.C., a team headed
by assistant research astronomer Paul Kalas at the University of
California, Berkeley, reports an unexpected result from its search
for dusty disks. Hoping to image for the first time extrasolar analogs
to the Kuiper Belt, a ring of comets encircling the sun beyond the
orbit of Neptune, the scientists found that the dust surrounding five
stars out of 100 surveyed is actually a cloud of interstellar dust on
a collision course with each star.

These cosmic collisions produce giant reflection nebulosities, a
brilliant phenomenon best known around the Seven Sisters, the brightest
stars in the Pleiades cluster in the constellation Taurus.

The results show that solid matter detected around stars could be
foreign to the system rather than indigenous material related to the
formation of extrasolar planetary systems.

"We knew that each star in our sample was heating dust because of the
far-infrared emission observed, a phenomenon first associated with
the star Vega, and thought to indicate the existence of cometary and
asteroidal dust around other stars," Kalas explained. "But what we
discovered instead was five stars sweeping through the galactic dirt
and producing beautiful nebulosities like the ones you see in the
Pleiades."

In their paper, the astronomers call this effect the "Pleiades
Phenomenon." When a star and a cloud of galactic dust approach each
other, pressure from stellar radiation splits the cloud and dust
travels in streams around the star. The result is a reflection
nebulosity dominated by many linear features crossing the star in
roughly the same direction.

In the images presented today, the length of nebulous filaments
corresponds to several tens of thousands of astronomical units, or over
a trillion kilometers. An astronomical unit is the average distance
between the Earth and the sun — equal to 93 million miles.

The astronomers made their observations at the University of Hawaii
2.2-meter telescope on Mauna Kea, Hawaii, and with a powerful
University of California adaptive optics system on the 3-meter
telescope at Lick Observatory. Since most of the stars in their survey
are very bright, a coronagraph was used to artificially eclipse each
star, uncovering the much fainter dust surrounding it in optical and
near-infrared reflected light.

New distance measurements from the Hipparcos satellite reveal that
the stars are significantly farther than originally thought — over
400 light years from the Sun. The team concluded that the Pleiades
Phenomenon is more likely to explain the dust surrounding distant
stars because the stars lie outside the Local Bubble.

Astronomers have long known that the sun resides in a relatively
dust-free bubble approximately 300 light years in radius, possibly due
to an ancient supernova that cleared material from the solar vicinity.
Stars within the bubble are unlikely to encounter galactic dust clouds,
but may have circumstellar dust as a consequence of the formation and
erosion of asteroid and comet-like bodies orbiting the star. Beyond
the Local Bubble, the dust found in close proximity to stars is more
likely to have an interstellar origin.

The research team cautions that, in some cases, a star could have
a circumstellar debris disk and interact with interstellar dust
simultaneously.

"High-resolution observations are what you really need to see the
origin of far-infrared emission from these stars, to distinguish
between disks and other dusty structures," remarked UC Berkeley
astronomer Ray Jayawardhana, an expert on dust disks. "These pictures
show that, in some cases, much of the emission comes from wispy
clouds, but do not rule out disks very close to the stars."

The results will be published in the March 2002 issue of The
Astrophysical Journal. The authors are Paul Kalas and James Graham of
UC Berkeley; Steven Beckwith of the Space Telescope Science Institute;
David Jewitt of the University of Hawaii; and James Lloyd of UC
Berkeley. This work was supported by the National Science Foundation’s
Center for Adaptive Optics and the NASA Astronomical Search for
Origins program.

Images of the five stars embedded in giant dust complexes are on the
Web at
http://astron.berkeley.edu/~kalas/press/pleiades.html

IMAGE CAPTION:

Clouds of galactic dust break into thin filaments when they have close
encounters with fiery stars. These five stars are candidates for
having cometary and asteroidal dust in their system because they emit
thermal radiation in much the same way that dust in our solar system
radiates thermally. But these new optical images reveal the tangled
structure of a giant dust complex surrounding each star.

To detect the faint light reflected off of the dust clouds, astronomers
used a coronagraph to artificially eclipse each star before imaging
the region on a CCD. The black disk seen in each photo is supported
by thin wires and prevents stellar light from directly striking the
camera. With the brightest object in the field blocked in this manner,
objects and nebulosities that are much fainter that the central star
become detectable. Though the black disks are less than a millimeter
in diameter, the region they block corresponds to about 150 billion
kilometers in diameter. The nebulosities shown here are therefore
spread over huge distances.

The stars seen here have the following names:

Column 1, top to bottom: HD 4881, HD 23680, and HD 49662.

Column 2, top to bottom: HD 23362 and HD 26676

PHOTO CREDIT: Paul Kalas (University of California Berkeley)

EDITORS: This material was presented to the American Astronomical
Society meeting in Washinton, D.C., on January 8, 2002. This false-
color optical photograph can be obtained at
http://astron.berkeley.edu/~kalas/press/pleiades.html