Astronomers Weigh the Coldest Brown Dwarfs with Astronomy's Sharpest Eyes (Forwarded)

Public Relations
University of Hawaii at Manoa

Contacts:
Dr. Michael Liu
Institute for Astronomy
University of Hawaii at Manoa
(808) 956-6666

Dr. Michael Ireland
University of Sydney
+61 2 6795 9270

Mr. Trent Dupuy
Institute for Astronomy
University of Hawaii at Manoa
tdupuy @ ifa.hawaii.edu

Embargoed until 11:30 a.m. Central Daylight Time (6:30 a.m. HST), June 2,
2008

Astronomers Weigh the Coldest Brown Dwarfs with Astronomy's Sharpest Eyes

Astronomers have used ultrasharp images obtained with the Keck Telescope
and Hubble Space Telescope to determine for the first time the masses of
the coldest class of "failed stars," a.k.a. brown dwarfs. With masses as
light as 3 percent the mass of the sun, these are the lowest mass
free-floating objects ever weighed outside the solar system. The
observations are a major step in testing the theoretical predictions of
objects that cannot generate their own internal energy, both brown dwarfs
and gas-giant planets. The new findings, which are being presented in a
press conference today at the American Astronomical Society meeting in St.
Louis, show that the predictions may have some problems.

"Mass is the fundamental parameter that governs the life-history of a
free-floating object, and thus after many years of patient measurements,
we are delighted to report the first masses of the very faintest, coldest
brown dwarfs," said Dr. Michael Liu of the Institute for Astronomy at the
University of Hawaii (IfA/UH). "After weighing these tiny, dim, cold
objects, we have confirmed that the theoretical predictions are mostly
correct, but not entirely so." The team announcing the results is composed
of Dr. Liu, Mr. Trent J. Dupuy (IfA/UH), and Dr. Michael J. Ireland
(University of Sydney).

Brown dwarfs are a class of objects that represent the missing link
between the lowest-mass stars and the gas-giant planets, such as Jupiter
and Saturn. Brown dwarfs are the faintest and coolest objects that can be
directly observed outside the solar system. They emit as little as about
1/300,000 the energy of the sun and have surface temperatures comparable
to the inside of a pizza oven (800 F), more than 9,000 F cooler than the
surface of the sun.

"Astronomers have measured the energy output and temperatures for a myriad
of brown dwarfs. However, the most important property of all is the
hardest one to measure -- the mass," said Dr. Ireland.

To determine the masses, the team has spent the last several years
studying brown dwarfs that occur in binaries, that is two brown dwarfs
that are mutually bound together by gravity and orbit each other, in a
fashion similar to how Earth orbits the sun. As first shown by Johannes
Kepler in the 17th century, the total mass of any binary system can be
determined by precisely measuring the orbit's size and how long it takes
for the two objects to complete one orbital cycle.

"These are very challenging measurements, because brown dwarf binaries
have tiny separations on the sky and orbit each other very slowly. We
needed to obtain the sharpest measurements that are possible with current
telescopes to precisely monitor their motion," said Mr. Dupuy.

The astronomers obtained images using the 10-meter (400-inch) Keck II
Telescope on Mauna Kea, Hawaii. Keck II is equipped with a powerful
adaptive optics system that corrects for the blurring of astronomical
images caused by turbulence in Earth's atmosphere. The Keck system can
also employ a low-power laser to create an "artificial" star to enable
such correction for almost anywhere in the sky.

The resulting images have an angular resolution as good as 1/20 of an arc
second, about 1/40,000 the diameter of the full moon. A person with vision
as sharp as the Keck adaptive optics system would be able to read a
magazine that was about a mile away. In fact, the positional accuracy
achieved with such sharp images is equivalent to hitting a bull's-eye on a
dartboard that is 8,000 miles away.

By regularly monitoring binaries with Keck adaptive optics and analyzing
previous data obtained by the Hubble Space Telescope, the team was able to
precisely measure the size and duration of the binaries' orbits, and
thereby determine the masses.

The team measured the masses of two brown dwarf binaries. One, known as
2MASS 1534-2952AB, is composed of two "methane" brown dwarfs, the coolest
type of brown dwarf, which is characterized by the presence of methane gas
in their atmospheres. This is the first mass measurement for this type of
brown dwarf. The team found that the total mass of 2MASS 1534-2952AB is
only 6 percent of the sun's mass, and each brown dwarf in it has a mass of
about 3 percent of the sun's (about 30 times the mass of Jupiter). The
other binary system, HD 130948BC, is a pair of slightly warmer "dusty"
brown dwarfs with a total mass of only 11 percent of the sun's mass and
individual masses of about 5.5 percent of the sun's.

Theoretical models predict the masses of brown dwarfs based on their
energy output and temperature. But when the team compared their mass
measurements to the theoretical predictions, they did not agree. For
example, the surface temperature of 2MASS 1534-2952AB was much cooler than
expected given its current level of energy output, while HD 130948BC was
much warmer.

"While there is general agreement between our data and the predictions,
something is not quite right with the theoretical studies of brown dwarfs,
either in determining their temperatures or in predicting their energy
output. Or perhaps both," said Dr. Liu. "These findings will be a
challenge for the theorists, and we are inspired to measure the masses of
more brown dwarfs in the coming years to better understand the problem."

The two binaries, located in the constellations of Libra (the Scales) and
Bootes (the Herdsman), are about 45-60 light-years from Earth. The two
components of each binary have a typical separation of about 2
astronomical units (AU), where 1 AU is the distance from Earth to the sun
(93 million miles). This is somewhat larger than the 1.5 AU distance
between Mars and the sun. Their orbital periods are about 10-15 years,
compared with 2 years for Mars around the sun.

The team's results are described in two upcoming papers submitted to the
Astrophysical Journal. This research has been supported by the National
Science Foundation and the Alfred P. Sloan Foundation.

First discovered in 1995, brown dwarfs represent a class of objects with
masses less than 7 percent the mass of the sun (about 70 times Jupiter's
mass). While ordinary stars become hot and dense enough in their interiors
to generate their own energy via nuclear fusion, brown dwarfs have
insufficient mass to do this, so instead they steadily fade and cool over
their lifetime. In many ways, brown dwarfs are very similar to gas-giant
planets like Jupiter and Saturn, since both types of objects are unable to
steadily generate their own energy and have very low surface temperatures.

Scientists have discovered hundreds of brown dwarfs within 100 light-years
of Earth. About 15 percent of them are binary systems. Dr. Adam Burgasser
(then at the University of California, Los Angeles, now at MIT) and Dr.
Daniel Potter (then at IfA/UH) used the Hubble Space Telescope and the
Gemini-North Observatory, respectively, to identify 2MASS 1534-2952AB and
HD 130948BC as binaries around 2001.

Founded in 1967, the Institute for Astronomy at the University of Hawaii
at Manoa conducts research into galaxies, cosmology, stars, planets, and
the sun. Its faculty and staff are also involved in astronomy education,
deep space missions, and in the development and management of the
observatories on Haleakala and Mauna Kea.

Established in 1907 and fully accredited by the Western Association of
Schools and Colleges, the University of Hawaii is the state's sole public
system of higher education. The UH System provides an array of
undergraduate, graduate, and professional degrees and community programs
on 10 campuses and through educational, training, and research centers
across the state. UH enrolls more than 50,000 students from Hawaii, the
U.S. mainland, and around the world.

The W. M. Keck Observatory operates twin 10-meter telescopes located on
the summit of Mauna Kea on the island of Hawaii and is managed by the
California Association for Research in Astronomy, a non-profit corporation
whose board of directors includes representatives from Caltech, the
University of California and NASA. For more information, please visit:
http://www.keckobservatory.org

The Hubble Space Telescope is operated by the Space Telescope Science
Institute with funding from NASA.

IMAGE CAPTIONS:

[Figure 1:
http://www.ifa.hawaii.edu/info/press-releases/Liu_AAS_June08/2mass1534-29.jpg
(52KB)]
Infrared image of the very low-temperature binary 2MASS 1534-2952AB,
composed of two methane brown dwarfs. This was obtained with the laser
guide star adaptive optics system on the Keck II Telescope, located on
Mauna Kea, Hawaii. The image is 1.5 arc seconds across (about 1/1,000 of
the size of the moon), and the binary's separation is about 0.2 arc
seconds. Each component of the binary has a mass of about 3 percent the
mass of the sun and emits about 1/100,000 the energy of the sun. These are
the coolest free-floating objects ever directly weighed outside the solar
system. Credit: Dr. Michael Liu (Institute for Astronomy, University of
Hawaii).

[Figure 2:
http://www.ifa.hawaii.edu/info/press-releases/Liu_AAS_June08/HD130948.jpg
(41KB)]
Infrared image of the dusty brown dwarf binary HD 130948BC. The binary is
seen in the upper left and has a total mass about 11 percent the mass of
the sun. The binary is in orbit around a young sun-like star, seen to the
lower right. This image was obtained with the adaptive optics system on
the Keck II Telescope, located on Mauna Kea, Hawaii. The image is 3.75 arc
seconds on a side (about 1/500 the size of the moon), and the binary's
separation is about 0.1 arc seconds. Credit: Mr. Trent Dupuy and Dr.
Michael Liu (Institute for Astronomy, University of Hawaii).


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