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University of Florida
News and Public Affairs
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E-mail: ahoover{at}ufl.edu

EMBARGOED FOR RELEASE: 12:30 p.m. CDT, Monday, May 26, 2003

RELEASE NO: NOAO 03-05

Astronomers Find Extremely Large Planet-Forming Disks Around Seven
Young Stars

An international team of astronomers has discovered seven extremely 
large circumstellar disks silhouetted against the forming stars that 
they surround. These new disks are 10 to 100 times larger than both
our solar system and other planet-forming disks that have been imaged
previously, suggesting that it may be possible for planets to form at 
much larger distances from their stars than previously thought.

The team that made this discovery is led by Richard Elston and
Elizabeth Lada of the University of Florida in Gainesville. Team
members include University of Florida students and research
associates Bruno Ferreira, Joanna Levine, Eric McKenzie, Nick Raines,
Noah Rashkind and Carlos Roman-Zuniga, as well as Charles Lada of the
Harvard-Smithsonian Center for Astrophysics, Cambridge, MA; Joao
Alves of the European Southern Observatory in Garching, Germany; and,
August Muench of the SIRTF Science Center, Pasadena, CA. The results
were presented today in Nashville at the 202nd meeting of the
American Astronomical Society. 

As a cloud of molecular gas collapses under the pull of gravity to
form a star, it rotates, and the dust, gas and debris gradually
coalesce and come together in the shape of a two-dimensional disk.
The material in these disks is not only fed to the forming star, but
steadily coagulates into bigger and bigger chunks, and finally forms
planets. The remnant of this process can be viewed in our own solar
system, where the planets all line up, more or less, along the same
two-dimensional plane, with all the planets orbiting in the same
direction. 

Due to the blinding glare of the forming stars, such disks are not 
normally seen from Earth. However, when the disk lies between Earth
and the star -- effectively hiding the light from the star -- it
becomes visible in light scattered by dust. Such nearly edge-on disks
have the appearance of a pair of butterfly wings with a dark lane
down the middle and are often called silhouette disks, since the
dense planet-forming disk is silhouetted against the scattered light
from the star. 

Elston and Lada found these large disks as part of a major survey for 
newborn celestial objects in mammoth "molecular clouds" in the 
constellations Orion and Perseus. These clouds, which contain the raw 
material for stars and planets, are the largest features of our Milky 
Way galaxy, stretching hundreds of light-years across. The clouds in 
this study are located approximately 1,000 light-years from Earth.

The astronomers worked at the National Science Foundation's 2.1-meter 
(84-inch) telescope at Kitt Peak National Observatory near Tucson,
AZ. They used a near-infrared spectrometer and imager instrument
called the Florida Multi-object Imaging Grism Spectrometer
(FLAMINGOS), developed by the University of Florida in partnership
with Kitt Peak, to take near-infrared images of tens of thousands of
stars each night -- many more than could have been examined without
this new instrument. Such observations must be made in the
near-infrared because visible light from young stars is nearly
completely absorbed by dust in the molecular clouds, rendering the
forming stars invisible optically. 

Although the survey relies on invisible infrared light, computers can 
encode the light in optical wavelengths to create visible images.
Most of these "snapshots" revealed only mature stars, and the
resulting images appear similar to the sky on a clear night. However,
the survey also revealed in several positions, in Elston's words,
"wild places" -- startling clusters of infant stars in varied stages
of formation. These stars appear in the image as colorful balls of
light, with the seven disks resembling dark swaths surrounding each
star. 

When Lada and Elston measured the disks, they realized they ranged in 
size from 10 to 100 times larger than any of the handful of similar 
disks yet seen and imaged, with each disk's diameter stretching 
thousands of Astronomical Units (1 AU = 93 million miles, the
distance from Earth to the Sun.). The diameter of our solar system,
as defined by the most distant planets, is approximately 60 AU. But
the disk of our solar system extends beyond the planets to several
hundred or even one-thousand Astronomical Units, as traced by the
comet-like bodies found in the Kuiper Belt. The fact that these disks
extend many times farther than this local comparison suggests that
planets, too, could extend well beyond the relatively close proximity
observed in our solar system and others.

"That would be good news for astronomers, because planets are 
notoriously difficult to detect near stars, which swamp their visible 
light, infrared emissions and other telltale indicators that make the 
planets detectable," Elston notes. "So if the more distant reaches of 
the disk are conducive to planet formation, they will be easier to
find once astronmers start looking there," he said.

Interestingly, these newly discovered silhouette disks are located in 
several dense clusters composed of hundreds of young forming stars.
The fact that the FLAMINGOS survey only found silhouette disks in
clusters and not peppered thoughout the immense volume of the
molecular clouds supports the point of view that such clusters of
stars are the birthplace for most of the stars in our galaxy.

"It's surprising that these new disks are so large, given that they
are found in dense clusters of young stars," Lada says. "Naively, you
would think that the gravitational interactions between such stars
would tend to disrupt extremely large disks, since the average
distance between the forming stars is only 20,000 Astronomical Units
and our largest disks are nearly 10,000 Astronomical Units in
diameter." Silhouette disks that have been found previously by
astronomers have generally been located in nearby regions, where a
few stars form in relative isolation. 

These large disks will require further observations to better define 
their properties. Imaging with NASA's Space Infrared Telescope and 
ground based millimeter-wave radio telescopes will help determine the 
masses of the disks. High-resolution imaging with the Hubble Space 
Telescope will help to confirm that these candidate disks are indeed 
true scattered-light disks, and will better define their geometries.

The FLAMINGOS instrument and the Giant Molecular Cloud Survey are
funded by research grants from the National Science Foundation (NSF)
to the University of Florida. Kitt Peak National Observatory is part
of the National Optical Astronomy Observatory (NOAO) in Tucson, which
is operated by the Association of Universities for Research in
Astronomy (AURA) Inc., under a cooperative agreement with the NSF.

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