Using NASA’s Chandra X-ray Observatory, scientists have detected X-rays from
a low mass brown dwarf in a multiple star system, which is as young as 12
million years old. This discovery is an important piece in an increasingly
complex picture of how brown dwarfs – and perhaps the very massive planets
around other stars – evolve.
Chandra’s observations of the brown dwarf, known as TWA 5B, clearly resolve
it from a pair of Sun-like stars known as TWA 5A. The system is about 180
light years from the Sun and a member of a group of about a dozen young
stars in the southern constellation Hydra. The brown dwarf orbits the
binary stars at a distance about 2.75 times that of Pluto’s orbit around the
Sun. This is first time that a brown dwarf this close to its parent star(s)
has been resolved in X-rays.
“Our Chandra data show that the X-rays originate from the brown dwarf’s
coronal plasma which is some 3 million degrees Celsius,” said Yohko Tsuboi
of Chuo University in Tokyo and lead author of the April 10th issue of
Astrophysical Journal Letters paper describing these results. “The brown
dwarf is sufficiently far from the primary stars that the reflection of
X-rays is unimportant, so the X-rays must come the brown dwarf itself.
TWA 5B is estimated to be only between 15 and 40 times the mass of Jupiter,
making it one of the least massive brown dwarfs known. Its mass is rather
near the currently accepted boundary (about 12 Jupiter masses) between
planets and brown dwarfs. Therefore, these results may also have
implications for very massive planets, including those that have been
discovered as extrasolar planets in recent years.
“This brown dwarf is as bright as the Sun today in X-ray light, while it is
fifty times less massive than the Sun,” said Tsuboi. “This observation,
thus, raises the possibility that even massive planets might emit X-rays by
themselves during their youth!”
This research on TWA 5B also provides a link between an active X-ray state
in young brown dwarfs (about 1 million years old) and a later, quieter
period of brown dwarfs when they reach ages of 500 million to a billion
years.
Brown dwarfs are often referred to as “failed stars,” as they are believed
to be under the mass limit (about 80 Jupiter masses) needed to spark the
nuclear fusion of hydrogen to helium, which characterizes traditional stars.
Scientists hope to better understand the evolution of magnetic activity in
brown dwarfs through the X-ray behavior.
Chandra observed TWA 5B for about three hours on April 15, 2001, with its
Advanced CCD Imaging Spectrometer (ACIS). Along with Chandra’s mirrors,
ACIS can achieve the angular resolution of a half arc second.
“This brown dwarf is about 200 times dimmer than the primary and located
just two arcseconds away,” said Gordon Garmire of Penn State University who
led the ACIS team. “It’s quite an achievement that Chandra was able to
resolve it.”
Other members of the research team included Yoshitomo Maeda (Institute of
Space and Astronautical Science, Kanagawa, Japan), Eric Feigelson, Gordon
Garmire, George Chartas, and Koji Mori (Penn State University), and Steve
Prado (Jet Propulsion Laboratory).
NASA’s Marshall Space Flight Center in Huntsville, Ala., manages the Chandra
program, and TRW, Inc., Redondo Beach, Calif., is the prime contractor for
the spacecraft. The Smithsonian’s Chandra X-ray Center controls science and
flight operations from Cambridge, Mass., for the Office of Space Science at
NASA Headquarters, Washington.
Images and additional information about this result are available at:
and
