archive: SETI [ASTRO] Astronomers Examine Brown Dwarf's Hazy Atmosphere

SETI [ASTRO] Astronomers Examine Brown Dwarf's Hazy Atmosphere

Larry Klaes ( )
Mon, 21 Dec 1998 13:58:30 -0500

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>Date: Mon, 21 Dec 1998 3:47:50 GMT
>From: Ron Baalke <>
>Subject: [ASTRO] Astronomers Examine Brown Dwarf's Hazy Atmosphere
>Reply-To: Ron Baalke <>
>New Mexico State University
>Las Cruces, New Mexico
>December 15, 1998
>Astronomers examine brown dwarf's hazy atmosphere
>By Karl Hill
>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
>"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.