Brown dwarf star takes astronomers by surprise
X-ray flare is first evidence of magnetic activity on these failed stars

By Robert Sanders, Public Affairs

16 AUGUST 00 | 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 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 Martin of the California Institute of Technology in Pasadena.

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, only 16 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.

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.



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