New Mexico State University, Las Cruces, New Mexico
Astronomers examine brown dwarf's hazy atmosphere
by Karl Hill, December 15, 1998
Brown dwarfs, sometimes known as failed stars, have a reputation for being the dim bulbs of the heavens. That's one reason the first real specimen, a brown dwarf named Gliese 229B, was discovered only three years ago. But Gliese 229B presented a puzzle to New Mexico State University astronomer Mark Marley and his colleagues as they studied the strange new object, because it seemed even darker than expected. "Brown dwarfs are supposed to be dim, but it was turning out to be much, much darker than we would have thought in the optical part of the spectrum," Marley said.
In a classic example of the high-tech detective work today's astronomers use to analyze distant objects, Marley and two colleagues have determined that the brown dwarf suffers from a malady similar to one Los Angeles is famous for - a hazy atmosphere. "The compounds are different, but it's like the red haze you see when you fly into Los Angeles," he said. While L.A.'s smog is caused by sunlight reacting with auto emissions and other particles in the air over the city, the brown dwarf's red haze is thought to be caused by a different sort of chemical reaction. It appears that gases in the brown dwarf's atmosphere, primarily methane, react with light from a nearby star that Gl229B orbits, causing them to form more complicated molecules that clump together to form extremely small drops, Marley said -- drops about one-hundredth the size of those that form clouds in the Earth's atmosphere. The drops tend to block the visible light from the brown dwarf but are transparent in other parts of the spectrum, he said.
Results of the analysis of Gl229B's atmosphere were published in the Dec. 11 issue of Science, the weekly journal of the American Association for the Advancement of Science. The article was written by Caitlin Griffith of Northern Arizona University's Department of Physics and Astronomy, Roger Yelle of Boston University's Center for Space Physics, and Marley, a planetary scientist in NMSU's Department of Astronomy. Solving this particular mystery about this particular brown dwarf, Marley said, adds to scientists' understanding of the universe around us. Since Gliese 229B was discovered, by a team of Cal Tech and Johns Hopkins scientists, "there are now dozens of brown dwarfs that have been discovered, and it's important to understand what their spectra can tell us about them," Marley said. "If these guys turn out to be a common part of the universe, we have to get a basic understanding of what's going on in their atmospheres, how hot they are, what they're made of."
Brown dwarfs are too small and cool to be stars and too massive to be planets. Scientists believe they form the same way stars do, but never accumulate enough mass to sustain nuclear fusion at their cores. They seem to share some characteristics with giant planets like Jupiter. Drawing on his research on Jupiter and other planets, Marley has developed computer models that help astronomers examine newly discovered objects such as brown dwarfs and planets orbiting stars beyond our solar system. His collaborators on the latest brown dwarf project have models that complement his. Using data obtained by the Keck Telescope in Hawaii, they found that Gliese 229B fit their models in most respects, but not in the optical part of the spectrum.
The optical part of the spectrum includes light waves that are visible to the human eye, plus a section of the spectrum between visible light and the infrared region that is not visible to the human eye. The brown dwarf's darkness in this part of the spectrum could not be caused by clouds, the scientists concluded. Its atmosphere is too warm to contain ice clouds like those on Jupiter and too cool to contain silicate clouds like those on low mass stars. Also, in parts of the spectrum where the brown dwarf is brighter, such as the near-infrared part of the spectrum, "the data look like there are no clouds at all -- a perfectly clear atmosphere," Marley said. "So there was this puzzle," he said. "It seemed to be cutting off light in one region (of the spectrum) but in other areas it looked just fine."
While the astronomers are confident they have solved the puzzle of Gliese 229B's atmosphere, by analysis of its spectrum and an understanding of how different particles scatter light, the results don't necessarily apply to other brown dwarfs, Marley said. "Most of the other brown dwarfs that have been discovered are isolated," he said. Gliese 229B is orbiting a nearby star, and ultraviolet light from the star is a factor in the chemical reaction in the brown dwarf's atmosphere. "And this one is still the coldest one so far," he added - another factor in the atmospheric makeup.