WASHINGTON, D.C.--Thanks to newly sharpened vision, the world's largest telescope has detected a surprising sight: a tight pair of "failed stars" with strikingly different colors. It's the first time astronomers have seen such a mismatched couple, according to a report here 10 January at a meeting of the American Astronomical Society. Researchers speculate that the differences between the objects--known as brown dwarfs--may be due to the ebb and flow of iron-rich clouds in their respective atmospheres.
Brown dwarfs reside in a substellar limbo. They're bigger than Jupiter but much smaller than our sun, and they lack the internal pressures needed to ignite nuclear fusion. They glow a deep red and then cool off like cinders for millions of years, gradually fading from view. Astronomers think many brown dwarfs come in pairs, but they are so small that telescopes strain to tell them apart.
That has changed with a new adaptive optics system operating at the 10-meter Keck II Telescope at Mauna Kea, Hawaii, for the last year. A laser creates an artificial star 90 kilometers above the telescope. Computers analyze how Earth's turbulent atmosphere blurs the laser light, then flex a thin mirror to correct the distortion. The result is steady vision rivaling that of the Hubble Space Telescope.
Astronomer Michael Liu of the University of Hawaii at Manoa and his colleagues used the laser system to survey nearby brown dwarfs. Several are newfound sets of twins, but one pair stands out from the rest. One dwarf is reddish while the other is bluish, even though they probably formed at the same time in the same nursery of gas. Liu thinks faster cooling in the slightly smaller dwarf may explain the difference. If the red dwarf's atmosphere glows at about 1200°C, iron particles will drift within fairly dense clouds. But if the bluer dwarf is 200°C cooler or so, the clouds should disperse and leave a clearer atmosphere. "This pair straddles a special range in temperature," says Liu. Theoretical models predict such changing weather patterns, but no set of objects has displayed them so starkly, he notes.
That sequence is feasible, says theorist Adam Burrows of the University of Arizona in Tucson. But he offers another explanation: Despite their mutual birth, the two dwarfs may contain slightly altered mixes of gases, leading to distinct clouds even if the temperatures are similar. Brown-dwarf meteorology will become an even hotter field as telescopes with adaptive optics expose more pairs of dwarfs at this crucial phase in their evolution, Burrows believes.