Summer 2004 -- Vol. 21

X-Rays Found from a Lightweight Brown Dwarf

	The approximate size of brown dwarf TWA 5B (center) compared to Jupiter (right) and the Sun (left). 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. TWA 5B is estimated to be between 15 and 40 times the mass of Jupiter, making it one of the least massive brown dwarfs known. Credit: CXC/M. Weiss

Using NASA’s Chandra X-ray Observatory, scientists have detected X-rays from a low mass brown dwarf in a multiple star system, which is as young as 12 million years old. This discovery is an important piece in an increasingly complex picture of how brown dwarfs—and perhaps the very massive planets around other stars—evolve.

Chandra’s observations of the brown dwarf, known as TWA 5B, clearly resolve it from a pair of Sun-like stars known as TWA 5A. The system is about 180 light years from the Sun and a member of a group of about a dozen young stars in the southern constellation Hydra. The brown dwarf orbits the binary stars at a distance about 2.75 times that of Pluto’s orbit around the Sun. This is first time that a brown dwarf this close to its parent star(s) has been resolved in X-rays.

“Our Chandra data show that the X-rays originate from the brown dwarf’s coronal plasma, which is some 3 million degrees Celsius,” said Yohko Tsuboi, the lead author of the Astrophysical Journal Letters paper describing these results, who was a Japan Society for the Promotion of Science Postdoctoral Fellow at Penn State until she recently accepted a position at Chuo University in Tokyo. “The brown dwarf is sufficiently far from the primary stars that the reflection of X-rays is unimportant, so the X-rays must come the brown dwarf itself.”

TWA 5B is estimated to be only between 15 and 40 times the mass of Jupiter, making it one of the least massive brown dwarfs known. Its mass is rather near the currently accepted boundary (about 12 Jupiter masses) between planets and brown dwarfs. Therefore, these results may also have implications for very massive planets, including those that have been discovered as extrasolar planets in recent years.

“This brown dwarf is as bright as the Sun today in X-ray light, while it is fifty times less massive than the Sun,” said Tsuboi. “This observation, thus, raises the possibility that even massive planets might emit X-rays by themselves during their youth.”

This research on TWA 5B also provides a link between an active X-ray state in young brown dwarfs (about 1 million years old) and a later, quieter period of brown dwarfs when they reach ages of 500 million to a billion years.

Brown dwarfs are often referred to as “failed stars,” as they are believed to be under the mass limit (about 80 Jupiter masses) needed to spark the nuclear fusion of hydrogen to helium, which characterizes traditional stars. Scientists hope to better understand the evolution of magnetic activity in brown dwarfs through the
X-ray behavior.

Chandra observed TWA 5B for about three hours on April 15, 2001, with its Advanced CCD Imaging Spectrometer (ACIS). Along with Chandra’s mirrors, ACIS can achieve the angular resolution of a half arc second.

“This brown dwarf is about 200 times dimmer than the primary stars and located just two arcseconds away,” said Gordon Garmire of Penn State University who led the ACIS team. “It’s quite an achievement that Chandra was able to resolve it.”

Other members of the research team included Yoshitomo Maeda (Institute of Space and Astronautical Science, Kanagawa, Japan), Eric Feigelson, Gordon Garmire, George Chartas, and Koji Mori (Penn State University), and Steve Pravdo (Jet Propulsion Laboratory).

Megan Watzke, Chandra X-Ray Center

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