July 12, 2000 -- NASA's latest observatory, designed to see the most violent and stunning cosmic phenomena, captured something unexpected. The Chandra X-ray Observatory, orbiting in space about one-third of the way to the moon, saw the first-ever flare from what's known as a brown dwarf, or failed star.
 

"We were shocked," said Dr. Robert Rutledge of the California Institute of Technology (Caltech) in Pasadena, CA, 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."

Right: The approximate size of a brown dwarf (center) compared to Jupiter (left) and the Sun (right). Although brown dwarfs are similar in size to Jupiter, they are much more dense and produce their own light whereas Jupiter shines with reflected light from the Sun. (Illustration: CXC/K.Kowal)

The study of the bright X-ray flare will increase understanding of the explosive activity and origin of magnetic fields of extremely low-mass stars.

Chandra detected no X-rays at all from the object called LP 944-20 for the first nine hours of a twelve-hour observation, and then the source flared dramatically before it faded away over the next two hours.

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.

 

Above: For the first 9 hr 36 min of Chandra's observation, no X-rays were detected from the brown dwarf (left panel). Then the brown dwarf turned on with a bright X-ray flare (right panel) that gradually diminished over the last few hours of the observation. The grainy appearance of the image on the right is due to a shorter exposure time. The bright dots in the background are other X-ray sources, 7 of which have been identified as stars.

 

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 about 60 times that of Jupiter, or 6 percent of that of the Sun. 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.
 

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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 intermediate between normal stars and giant planets.

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, AL, manages the Chandra program. TRW, Inc., Redondo Beach, CA, is the prime contractor for the spacecraft. The Smithsonian's Chandra X-ray Center controls science and flight operations from Cambridge, MA.Web Links

 

X-ray Astronomy Field Guide: Brown Dwarfs

Chandra Home Page - from the Harvard Smithsonian Center for Astrophysics

Chandra Newsroom - from the NASA Marshall Space Flight Center