Bright X-rays, Dim Dwarfs
by Kristin Leutwyler

In Depth / July 17, 2000

Celestial brown dwarfs are not unlike Dopey of Snow White's set. In other words, they're far from the brightest lights in the sky. Though bigger than the biggest planets, they are smaller than the smallest stars, which themselves have just enough mass to sustain thermonuclear reactions in their cores. Because of their size, brown dwarfs, also called failed stars, can only shine about one tenth of a percent as brightly as our own sun. Indeed, these objects are so dim that, although astronomers predicted their existence in the 1960s, no one actually found any until 1995.

Image: NASA
FLARE of X-rays, seen in the bottom frame, shot unexpectedly from the brown dwarf LP 944-20 on December 15, 1999.

 

So imagine the surprise scientists working with NASA's Chandra X-ray Observatory felt when, nine hours into an otherwise uneventful, 12-hour look at the brown dwarf LP 944-20, it suddenly unleashed a bright x-ray flare. "We were shocked," says Robert Rutledge, a professor at the California Institute of Technology and the lead author of a paper published in the July 20 issue of Astrophysical Journal Letters. "This is really the mouse that roared. We didn't expect to see flaring from such a lightweight object."

Image: Pat Rawlings

BROWN DWARFS, contrary to their name, actually appear red and are sometimes companions to stars, as shown in this illustration.

What they had expected was to detect a few photons every hour, adds Lars Bildsten, a member of the team from the University of California at Santa Barbara. Rather, brown dwarfs, lacking the mass to support powerful fusion reactions, get most of their energy from the gradual release of gravitational energy as they contract a few inches a year. "It was as if we were searching for a dim bulb and instead found a bright flash of light." In fact, the energy emitted in the flare was comparable to that of a small solar flare and was one billion times greater than x-ray flares coming from Jupiter.

This surprisingly dynamic flare offers new insight into objects ranging in size from extremely low-mass stars down to gas-giant planets. "This is the strongest evidence yet that brown dwarfs and possibly even young giant planets have magnetic fields," noted another team member from Caltech, Eduardo Martin. (In 1992 Martin and his colleagues at the Astrophysics Institute of the Canary Islands were first to propose the lithium test, a clever method for detecting brown dwarfs.)


Principal investigator Gibor Basri guesses that the flare may have resulted from turbulent magnetized hot material below LP 944-20's surface that produced a twisted magnetic field. "A subsurface flare could heat the atmosphere, allowing currents to flow and give rise to the x-ray flare--like a stroke of lightning," he remarked. And the nine-hour absence of flaring, the team reports, shows that coronas, or million degree Celsius upper atmospheres, cease to exist when a brown dwarf's surface cools below 2,500 degrees C. It also sets the lowest limit for steady x-ray production from a brown dwarf.

Although this x-ray flare is the first ever witnessed from a brown dwarf, it most likely won't be the last. Brown dwarfs appear to be plentiful. "New sky surveys show that the objects may be as common as stars," Basri confirms. LP 944-20, a 500-million-year-old object about 60 times larger than Jupiter and 16 light-years from Earth, is just one of the closest. But other brown dwarfs are probably also sending up flares, waiting to be seen.