A new substellar object, named SOri70, has been discovered
near the young star Sigma Orionis. Is it a young planet,
or a wandering old brown dwarf in the line of sight? This
is a question to be discussed by the astronomers attending
the International Astronomical Union Symposium on Brown
Dwarfs that opens today on the Big Island of Hawaii.

Deep sky images and follow-up spectroscopy obtained by an
international team of astronomers revealed this extremely
cool and dim object close to the multiple stellar system
Sigma Orionis. The astronomers made the observations with
large telescopes in Hawaii and the Canary Islands.

Since the acute visual observations of Sir William
Herschel in the eighteenth century, astronomers have noted
a clustering of stars in a region of the sky of about the
size of the full moon surrounding the hot star Sigma
Orionis. Many X-ray emitting low-mass stars in this
cluster were found by Scott Wolk and Fred Walter of SUNY
at Stony Brook. Several brown dwarfs in this region were
revealed by some of the members of the team that today
reports on the discovery of the coolest and faintest
object ever seen around Sigma Orionis.

The story of how SOri70 was found includes two of the
world’s most powerful telescopes separated by more than
8,000 miles and about 4 years of international
collaborative effort. It is an example of the complicated
work that is needed to hunt for the elusive brown dwarfs
and extrasolar planets.

In December 1998, team members Victor Bejar and Eduardo
Martin pointed one of the world’s largest optical
telescopes, the 10-meter Keck I on Mauna Kea (Hawaii),
at several fields around Sigma Orionis and obtained CCD
images of unprecedented sensitivity for this region of
the sky. They found several extremely faint red objects,
but they did not have enough information to determine
their basic properties. They had to wait patiently for a
chance to obtain additional data. It came when they used
an infrared camera at the William Herschel Telescope in
La Palma (Canary Islands) in November 2000.

One of the objects turned out to have blue infrared
colors despite being very red at optical wavelengths, a
unique signature of the coolest known dwarfs. The
unusual colors of these dwarfs are explained by the
presence of methane in their atmospheres, which is a
gas that can be present only at temperatures lower than
about 1,200 degrees Kelvin (about 900 degrees Celsius or
1650 degrees Fahrenheit). An object of this temperature
must have a mass smaller than a star.

The intriguing object was observed once more with the
Keck I telescope in December 2001 by team member Maria
Rosa Zapatero Osorio of the Laboratory for Fundamental
Astrophysics in Madrid, Spain. These observations
confirmed spectroscopically the presence of methane in
the object, which unambiguously classifies it as a brown
dwarf or planet. If the object is located at the same
distance as the Sigma Orionis system (1,150 light-years
from Earth), it should have an age between 1 and 8
million years and a mass close to that of Jupiter, the
largest planet in the Solar System.

However, the distance to the object is not known yet;
it will take the sharp imaging capabilities of the
Hubble Space Telescope to determine it. There is about
a 20% probability that SOri70 is a wandering old brown
dwarf that happens to be in the direction of the Sigma
Orionis, but is actually closer to Earth.

If the new ultracool dwarf is related to the Sigma
Orionis system, it would be the lowest mass extrasolar
object imaged to date. Because it would be located more
than 180,000 astronomical units from Sigma Orionis (more
than 36,000 times the Jupiter-Sun distance), it would
challenge our ideas about the formation of extrasolar
giant planets.

Four members of the science team are attending the
International Astronomical Union Symposium on Brown
Dwarfs on the Big Island of Hawaii during this week,
namely, Dr. David Barrado y Navascues (Laboratory for
Fundamental Astrophysics in Madrid, Spain), Mr. Jose
Antonio Caballero (Instituto de Astrofisica de Canarias),
Prof. Eduardo Martin (University of Hawaii Institute for
Astronomy) and Dr. Maria Rosa Zapatero Osorio (Laboratory
for Fundamental Astrophysics in Madrid, Spain). Other
team members include Dr. Victor Bejar (Instituto de
Astrofisica de Canarias), Dr. Joachim Eisloeffel
(Thueringer Landessternwarte Tautenburg, Germany),
Dr. Reinhold Mundt (Max Plank Institut fur Astronomie,
Heidelberg, Germany), and Dr. Rafael Rebolo (Instituto
de Astrofisica de Canarias).

Members of the press and news media are invited to attend
all sessions of the Symposium at no cost. They are asked
to check in at the Conference Registration Desk at the
Outrigger Waikoloa Beach Hotel in order to obtain
Symposium materials and other information of interest.

More information about the Brown Dwarfs Symposium is
available at http://anansi.ifa.hawaii.edu/iau211/.


An image of the Sigma Orionis region. The multiple star
Sigma Orionis, which is visible with the naked eye, is
at the center. A box indicates the position of the
planet candidate, which is only 8.7 arcminutes from the
star. The image was taken from the Digital Sky Survey
and has a size of 23 x 22 square arcminutes. The inset
shows the infrared image obtained at the William Herschel
Telescope by Dr. Victor Bejar and Prof. Eduardo Martin.

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