SPACE TELESCOPE SCIENCE INSTITUTE

It takes two.
Astronomers have taken precise measurements of two brown dwarfs.

Brown Dwarfs Weigh In

Like 19th century archaeologists suddenly gaining the secret of hieroglyphics, astronomers for the first time have precisely measured the size, mass, and temperature of not one but two elusive brown dwarfs--hot, luminous objects that are not quite stars but are too large to be planets. Their observations neatly matched several theoretical predictions, a sign that models of stellar evolution are on the right track.

Astronomers detected the pair locked in a tight orbit around each other almost directly edge-on to Earth-based telescopes. The orientation allowed the team to compute the pair's masses by applying Newton's equations for orbits to the periods at which they eclipse each other's light. "It's Physics 101," explains team leader Keivan Stassun, an astronomer at Vanderbilt University in Nashville, Tennessee. "This binary pair is a Rosetta stone that will help to unlock many of the mysteries regarding brown dwarfs."

Until now, the small size and dim light of brown dwarfs prevented accurate measurements of their masses--the critical element in stellar evolution and destinies. In this case, the dwarfs fell right in the anticipated range: between 13 and 80 times as massive as Jupiter, too small to ignite the nuclear fusion reactions that power stars. The data, combined with measurements of their brightness, provide critical points on a mass-luminosity relationship curve, which plots the characteristics of many celestial objects. Because similar stars tend to have similar masses, the new brown-dwarf readings should help astronomers pinpoint the masses of other brown dwarfs.

Stassun and colleagues Robert Mathieu of the University of Wisconsin, Madison, and Jeff Valenti of the Space Telescope Science Institute in Baltimore, Maryland, collected and analyzed about 1500 images accumulated from 300 nights of observations over 12 years. In the 16 March issue of Nature, they report that the two dwarfs are about 1500 light-years away in the Orion Nebula's stellar nursery, meaning they must be very young--perhaps only a million years old. The data also show they are remarkably large--nearly sun-sized. That combination of youth and largeness supports another hypothesis: that brown dwarfs are nearly as big as stars at birth but gradually contract over very long lifetimes.

One surprise is that the heavier of the two is slightly cooler--about 2650 kelvin, versus 2790 K for its twin. "One possible explanation is that the two objects have different origins and ages," Stassun says. If so, he adds, it could validate recent computer models that predict many brown dwarfs are born separately but close together.

The new data fill in details about the most uncertain portion of brown dwarf evolution, says Subhanjoy Mohanty of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. "As such, their study is critical for a better understanding of the evolution and properties of brown dwarfs in general."

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