Berkeley - The unexpected observation of a bright flare on a nearby brown dwarf has shown astronomers that failed and fading stars like this still have some life left in them.

The flare surprised astronomers who expected to see little or no activity on the brown dwarf during a planned 12-hour observation by the Chandra X-Ray Observatory.

Instead, after nine hours of seeing nothing, a flare flashed brightly, then faded out over the next two hours. The flare, very similar to the flares on our Sun, is the first ever observed on a brown dwarf.

"We really expected to see nothing - I hoped to see nothing, to prove that there was no hot corona surrounding the brown dwarf," said principal investigator Gibor Basri, a professor of astronomy at the University of California, Berkeley.

Basri and his collaborators originally planned the long observation by Chandra to eliminate the possibility that older brown dwarfs like this have hot coronae. A stellar corona - the upper atmosphere that stretches far into space and can reach a couple of million degrees Celsius - should emit copious X-rays. Work by Basri and his graduate student, Subu Mohanty, has suggested that brown dwarfs lose their hot coronae as they cool below about 2,200 degrees Celsius.

The fact that the X-ray satellite detected nothing for most of its 12-hour observation of the brown dwarf proves this hypothesis.

"The flare was a bonus," Basri said. "We've shown that older brown dwarfs don't have coronae, but the flare tells us they still have magnetic fields and also that subsurface flares occasionally punch through into the atmosphere."

"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 team member Eduardo L. Martin of the California Institute of Technology in Pasadena.

A paper describing the observations has been accepted by Astrophysical Journal Letters and will be published July 21. The paper is available on-line at http://xxx.lanl.gov/abs/astro-ph/0005559.

Brown dwarfs are failed stars somewhere in mass between a large planet and a small star. This makes them large enough to collapse and heat up, but not big enough to ignite the steady nuclear fires necessary to keep stars burning for billions of years. Instead, after a brief hot flash, brown dwarfs cool off until they become dead cinders.

They are extremely dim and went undetected until the recent construction of larger, more sensitive telescopes. Basri confirmed the first lithium brown dwarf in 1995 using Hawaii's Keck I Telescope, and since then several dozen have been found in nearby clusters or floating freely in the solar neighborhood.

The brown dwarf known as LP 944-20 is one of the nearest, only16 light years from Earth. In fact, it was first detected more than 25 years ago but was thought to be a very dim red star called a red dwarf. The recent observation of lithium in its atmosphere marks it as a brown dwarf.

LP 944-20, located in the constellation Fornax in the southern skies, is about 500 million years old and has a mass that is at most 60 times that of Jupiter, or six 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.

The 12-hour observation of LP 944-50 by the Advanced CCD Imaging Spectrometer of the Chandra X-Ray Observatory, a satellite operated by the National Aeronautics and Space Administration, took place on December 15, 1999.

Basri, who studies the evolution of brown dwarfs, teamed up several years ago with colleague Lars Bildsten, a theoretical astrophysicist formerly at UC Berkeley but now at the Institute for Theoretical Physics at UC Santa Barbara. Their coauthors on the paper reporting the flare are two former UC Berkeley post-doctoral researchers now working at Caltech: Robert Rutledge, who used to work with Bildsten; and Martin, who used to work with Basri.

Very young brown dwarfs are observed to have coronae, Basri said. They are hot enough to have sufficiently ionized atmospheres that can tangle with their magnetic fields. As they get entangled, the magnetic fields twist and occasionally cross, arcing like an electrical short and creating a flare. These flares are thought to inject high energy particles into the upper atmosphere or corona, producing temperatures up to several million degrees Celsius.

As the dwarfs cool, however, the atmosphere should cool and the gases de-ionize into a neutral gas. Magnetic fields do not interact with a neutral atmosphere, and thus atmospheric activity dies down and the corona disappears.

The observations with the Chandra satellite were designed to test this hypothesis on a relatively old brown dwarf, and put an upper limit on the amount of X-ray emission from the corona. The observations show no quiescent X-ray emission at all. The sensitivity of NASA's latest "Great Observatory" allows this limit to be pushed to new lows, Basri said.

"This is an important confirmation of the trend that hot gas in the atmospheres of lower mass stars is produced only in flares," said Bildsten, and not by any quiescent emission from the corona.

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.

"The flare could have its origin in the turbulent magnetized hot material beneath the surface of the brown dwarf," Basri said. "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."

The team plans future X-ray observations of other brown dwarfs to further explore flare activity.

The research was funded by NASA and the National Science Foundation.

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Print-quality images available for download
Images associated with this release, including high-resolution digital versions of the X-ray image, are available on the World Wide Web at http://chandra.harvard.edu & http://chandra.nasa.gov.