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Astronomer's First Direct Evidence: Young Low-Mass Objects are Twice as Heavy as Predicted



 
 
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Old January 19th 05, 07:19 PM
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Default Astronomer's First Direct Evidence: Young Low-Mass Objects are Twice as Heavy as Predicted

ASTRONOMERS' FIRST DIRECT EVIDENCE: YOUNG LOW-MASS OBJECTS ARE TWICE AS
HEAVY AS PREDICTED

Wednesday, Jan. 19, 2005

Lori Stiles

Although mass is the most important property of stars, it has proved
very
hard to measure for the lowest mass objects in the universe. Thanks to
a
powerful new camera, a very rare, low-mass companion has finally been
photographed.

The discovery suggests that, due to errors in the models, astronomers
have
overestimated the number of young "brown dwarfs" and "free floating"
extrasolar planets. An international team of astronomers lead by
University
of Arizona Associate Professor Laird Close reports the discovery in
today's
(Jan 20.) issue of Nature.

The image has allowed the team to directly measure the mass of a young,
very
low mass object for the first time. The object, more than 100 times
fainter
than its close primary star, is 93 times as massive as Jupiter --
almost
twice as heavy as theory predicts it should be. Their findings
challenge
current ideas about the astronomical brown dwarf population and the
existence of widely publicized free-floating extrasolar planets.

----------------------------------------------------------------------------

Contact Information
Laird M. Close
520.626.5992

Related Web sites

NACO_SDI camera -
http://exoplanet.as.arizona.edu/~lcl.../SDI_NACO.html

Laird Close homepage - http://athene.as.arizona.edu/~lclose/

----------------------------------------------------------------------------


Brown dwarfs are objects 75 times more massive than Jupiter but not
massive
enough to burn as stars. If young objects identified as brown dwarfs
are
twice as massive as has been thought, many actually are low mass stars.
Objects recently identified as 'free-floating' planets are in turn
likely
just low mass brown dwarfs.

Close of the UA's Steward Observatory and his international colleagues
detected the faint, very-low-mass companion, named AB Dor C, which
orbits
the very young star AB Doradus A (AB Dor A) at only 2.3 times the
distance
between the Earth and the sun, or about the distance between the sun
and the
asteroids beyond Mars.

Astronomers searching for very low mass objects look at young nearby
stars
because low mass companion objects will be brightest when young, before
they
contract and cool. Astronomers had suspected since the early 1990s that
well-known AB Dor A -- a star 48 light years (14.9 parsecs) from Earth
and
only 50 million years old -- has a low-mass companion because its
position
'wobbles' as it is pulled by an unseen companion. But even the Hubble
Space
Telescope tried and failed to detect the companion because it was too
faint
and too close to the glare of the primary star.

Close and his colleagues from Germany (Rainer Lenzen, Wolfgang
Brandner),
Spain (Jose C. Guirado), Chile (Markus Hartung, Chris Lidman), and the
United States (Eric Nielsen, Eric Mamajek, and Beth Biller) succeeded
in
photographing the elusive companion. They used Close and Lenzen's novel
high-contrast camera on the European Southern Observatory's 8.2-meter
Very
Large Telescope in Chile in February 2004.

Close and Lenzen developed the new high-contrast adaptive optics
camera, the
NACO Simultaneous Differential Imager, or NACO SDI, for hunting
extrasolar
planets. The SDI camera enhances the ability of the powerful 8.2-meter
VLT
telescope and its existing adaptive optics system to detect faint
companions
that normally would be lost in the glare of the primary star.

Close and his team are the first to image a companion so faint 120
times
fainter than its star -- and so near its star. The tiny distance
between the
star and the faint companion (0.156 arcseconds) is the same as the
width of
a dime (1.5 centimeters) seen 8 miles (13 kilometers) away. Once they
located the companion, they observed it at near infrared wavelengths to
measure its temperature and luminosity.

"We were surprised to find that the companion was 400 degrees Celsius
cooler
and 2.5 times fainter than the latest models predicted," Close said.

"We used our discovery of the companion's exact location, along with
the
star's known 'wobble', to accurately determine the companion's mass,"
team
member Jose Guirado said.

"Theory predicts that this low-mass, cool object would be about 50
Jupiter
masses," Close said. "But theory is incorrect: This object is between
88-98
Jupiter masses. This discovery will force astronomers to rethink what
masses
of the smallest objects produced in nature really are."

"Objects like AB Dor C are very rare," Wolfgang Brandner said. "Only
one
percent of stars have close very low mass companions -- and only about
one
percent of nearby stars are young. Hence, we are very lucky to be able
to
accurately measure the mass of even a single low mass companion that is
accurately known to be young."

The NACO SDI camera is a unique type of camera using adaptive optics,
which
removes the blurring effects of Earth's atmosphere to produce extremely
shape images. SDI splits light from a single star into four identical
images, then passes the resulting beams through four slightly different
methane-sensitive filters. When the filtered light beams hit the
camera's
detector array, astronomers can subtract the images so the bright star
disappears, revealing a fainter, lower-mass methane-rich object
otherwise
hidden in the star's scattered light halo.

The National Science Foundation awarded Close a prestigious 5-year,
$545,000
Faculty Early Career Development award that supports his search for
extrasolar planets using SDI cameras on the European Southern
Observatory's
8.2-meter VLT in Chile and on the UA/Smithsonian 6.5-meter MMT on Mount
Hopkins, Ariz. This research was also supported by NASA.

Authors of the Jan. 20 Nature letter, "A dynamical calibration of the
mass-luminosity relation at very low stellar masses and young ages,"
a
Laird Close of the UA Steward Observatory, Rainer Lenzen of the Max
Planck
Institute for Astronomy in Heidelberg, Jose C. Guirado of the
University of
Valencia (Spain), Eric L. Nielsen of UA Steward Observatory, Eric E.
Mamajek
of the Harvard-Smithsonian Center for Astrophysics, Wolfgang Brandner
of the
Max Planck Institute for Astronomy in Heidelberg, Markus Hartung and
Chris
Lindman of the European Southern Observatory (Chile), and Beth Biller
of the
UA Steward Observatory.

More information and images are available on the Web sites
http://exoplanet.as.arizona.edu/~lcl.../SDI_NACO.html and
http://athene.as.arizona.edu/~lclose/

 




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