RELEASE: 00-206

The first flare ever seen from a brown dwarf, or failed star, was
detected by NASA’s Chandra X-ray Observatory. The bright X-ray flare has
implications for understanding the explosive activity and origin of magnetic
fields of extremely low mass stars.

Chandra detected no X-rays at all from LP 944-20 for the first nine
hours of a twelve hour observation, then the source flared dramatically
before it faded away over the next two hours. “We were shocked,” said Dr.
Robert Rutledge of the California Institute of Technology in Pasadena, the
lead author on the discovery paper to appear in the July 20 issue of
Astrophysical Journal Letters. “We didn’t expect to see flaring from such a
lightweight object. This is really the mouse that roared.”

The energy emitted in the brown dwarf flare was comparable to a
small solar flare, and was a billion times greater than observed X-ray
flares from Jupiter. The flaring energy is believed to come from a twisted
magnetic field. “This is the strongest evidence yet that brown dwarfs and
possibly young giant planets have magnetic fields, and that a large amount
of energy can be released in a flare,” said Dr. Eduardo Martin, also of
Caltech and a member of the team.

Professor Gibor Basri of the University of California, Berkeley, the
principal investigator for this observation, speculated that “The flare
could have its origin in the turbulent magnetized hot material beneath the
surface of the brown dwarf. A sub-surface flare could heat the atmosphere,
allowing currents to flow and give rise to the X-ray flare — like a stroke
of lightning.”

LP 944-20 is about 500 million years old and has a mass that is
about 60 times that of Jupiter, or 6 percent of the sun’s mass. Its diameter
is about one-tenth that of the sun and it has a rotation period of less than
five hours. Located in the constellation Fornax in the southern skies, LP
944-20 is one of the best studied brown dwarfs because it is only 16 light
years from Earth.

The absence of X-rays from LP 944-20 during the non-flaring period
is in itself a significant result. It sets the lowest limit on steady X-ray
power produced by a brown dwarf, and shows that the million degree Celsius
upper atmospheres, or coronas, cease to exist as the surface temperature of
a brown dwarf cools below about 2500 degrees Celsius.

“This is an important confirmation of the trend that hot gas in the
atmospheres of lower mass stars is produced only in flares,” said Professor
Lars Bildsten of the University of California, Santa Barbara, also a member
of the team.

Brown dwarfs have too little mass to sustain significant nuclear
reactions in their cores. Their primary source of energy is the release of
gravitational energy as they slowly contract. They are very dim – less than
a tenth of a percent as luminous as the sun — and of great interest to
astronomers because they are poorly understood and probably a very common
class of objects that are intermediate between normal stars and giant

The 12-hour observation of LP 944-20 was made on December 15, 1999,
using the Advanced CCD Imaging Spectrometer (ACIS).

The ACIS instrument was built for NASA by the Massachusetts
Institute of Technology, Cambridge, and Pennsylvania State University,
University Park. NASA’s Marshall Space Flight Center in Huntsville, Ala.,
manages the Chandra program. 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.

Images associated with this release are available on the World Wide
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