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Astronomers Use "Laser-Vision" To Find (Strange) New Brown Dwarf Twins
Astronomers are reporting today discoveries of several ultracool brown dwarf binary systems found using the new Keck laser guide star adaptive optics system, including a strange kind of binary never seen before. The results are being announced by a team of U.S. astronomers led by Dr. Michael C. Liu of the Institute for Astronomy at the University of Hawaii. The team is presenting its findings at the 207th American Astronomical Society meeting in Washington, D.C.
The results shed light on the puzzling formation mechanism and atmospheres of brown dwarfs, and would not have been possible without the ultra-sharp images produced by the laser guide star system on the Keck Telescope. Among the discoveries, the team has determined that a brown dwarf previously considered a single object is actually a closely spaced pair of brown dwarfs with very unusual properties.
"Keck's new 'laser vision' is a big step forward in our quest to understand the origins and atmospheres of these intriguing objects, which are partly like stars and partly like planets," said Dr. Liu.
Discovered only a decade ago, brown dwarfs represent a class of low-mass, ultracool objects -- occupying the once theoretical gap between ordinary stars and giant planets. Hundreds of brown dwarfs are now known, but their origins and physical properties remain mysteries that have yet to be fully understood by astronomers. Part of the challenge lies in the fact that nearby brown dwarfs have a widely varied and unknown mix of ages, masses, and compositions.
To circumvent this problem, Dr. Liu and his collaborators have been conducting a survey for nearby brown dwarf binaries, namely pairs of brown dwarfs that are orbiting each other. Since binaries are born at the same time and made out of the same composition, comparing the two components removes much ambiguity in the interpretation.
"Biologists study human twins to understand whether differences among people can arise even if they have the same age and genetic makeup. In a similar fashion, we are finding and studying brown dwarf twins in order to understand what causes differences among these objects, and thereby learn about their physical properties," said Dr. Liu.
In the course of their survey, the team found a very unusual type of brown dwarf binary, whose two components have very different atmospheres. The object is called SDSS J1534+1615 and was first identified as a brown dwarf candidate in 2004 by the Sloan Digital Sky Survey (SDSS) and confirmed through follow-up ground-based infrared observations at the United Kingdom Infrared Telescope (UKIRT) and the NASA Infrared Telescope Facility (IRTF), both on Mauna Kea.
Initially thought to be a single object, Dr. Liu's team found that SDSS J1534+1615 is in fact a very closely spaced binary, with an angular separation of only 0.11 arcseconds or about 1/20,000 of the diameter of the full moon. The
physical separation is about four Astronomical Units, or four times the distance between the Earth and the Sun. SDSS J1534+1615 is about 110 light years away from Earth, located in the constellation of Serpens (the Serpent).
What is quite surprising about SDSS J1534+1615, and makes it unique compared to the ~40 previously known brown dwarf binaries, is that the two "twin" components have very different appearances, despite their identical ages and compositions. All previously known brown dwarf binaries have similar atmospheric colors, and one component is always brighter than the other regardless of which color filter is used for imaging. But in the case of SDSS J1534+1615, the two components have very different atmospheric colors in infrared light --- one is much redder than the other. And even more unusual, the brighter component depends on the observing filter.
"We've never seen a pair of brown dwarfs like this. They will tell us quite a bit about what is happening in the atmospheres of these planet-like objects," said Dr. Sandy K. Leggett of the United Kingdom Infrared Telescope and co-author on the study.
The very different atmospheres of the two components probably arise from differences in their cloud content. Just like planets in our own solar system, many brown dwarfs are thought to possess clouds in their atmospheres. However, brown dwarf clouds are composed of iron particles, unlike the water clouds on the Earth. These iron clouds cause the very red appearance of one component of SDSS J1534+1615. The other component likely has fewer and/or more fragmented clouds, leading to a very different appearance.
"The very different atmospheres of these otherwise very similar brown dwarfs are quite unusual. It's as if your house was completely clouded over, but your next-door neighbor sees sunny skies overhead," remarked co-author Dr. David A. Golimowski of Johns Hopkins University.
The discoveries are the result of an ongoing survey of nearby brown dwarfs being carried out by Dr. Liu and his collaborators at the 10-meter (400-inch) Keck II Telescope on Mauna Kea, Hawaii. Keck II is the first large (8-10 meter) telescope equipped with a laser guide star adaptive optics (AO) system.
Adaptive optics corrects astronomical images for the blurring caused by the Earth's turbulent atmosphere. While astronomers have been using this technology for nearly two decades, brown dwarfs have always been far too faint for traditional AO systems, which only work with bright stars. The Keck laser system creates an "artificial star" in the sky, which can then be pointed at the brown dwarfs to produce unprecedentedly sharp infrared images.
The resulting images are the sharpest ever obtained of any brown dwarf binary in infrared light, with angular resolution as good as 1/20 of an arcsecond, about 1/400,000 the diameter of the full moon. If a person's vision were as sharp as the Keck adaptive optics system, he would be able to read a magazine that was about two and a half miles away.
"This new capability provides us with infrared images three to four times sharper than those produced by the Hubble Space Telescope. This is essential to separate these very closely spaced brown dwarf binaries and to study their atmospheres. Without it, these objects would falsely appear to be a single object," Dr. Liu added.
Brown dwarfs are very faint, ultra-cool objects with masses less than seven percent of the Sun's mass (or about 70 times the mass of Jupiter). They are commonly referred to as "failed stars," since they lack sufficient mass to generate their own energy via nuclear fusion like stars do. As a result, they have very low surface temperatures, less than one-fourth of the Sun's surface temperature, and are very dim. In many respects, brown dwarfs are much closer to the gas giant planets in our solar system, given that both planets and brown dwarfs have much lower masses and temperatures than stars."
In fact, we suspect that some other nearby brown dwarfs that are thought to be single objects may in fact be thus-far undetected binaries. And some of them may belong to this strange new class of binaries established by SDSS J1534+1615. We're eager to look at more nearby brown dwarfs with Keck's ultra-sharp vision in the coming months," Dr. Liu exclaimed.
Co-authors on the results presented today are Sandy Leggett of the United Kingdom Infrared Telescope; David Golimowski and Kuenley Chiu of Johns Hopkins University; Xiaohui Fan of the University of Arizona; Tom Geballe of Gemini Observatory; Donald Schneider of the Pennsylvania State University; and Jon Brinkmann of Apache Point Observatory.
The results are described in an upcoming paper in The Astrophysical Journal. This research has been supported by the National Science Foundation and the Alfred P. Sloan Foundation.
The Institute for Astronomy at the University of Hawaii conducts research into galaxies, cosmology, stars, planets, and the sun. Its faculty and staff are also involved in astronomy education, deep space missions, and in the development and management of the observatories on Haleakala and Mauna Kea.
Established in 1907 and fully accredited by the Western Association of Schools and Colleges, the University of Hawaii is the state's sole public system of higher education. The UH System provides an array of undergraduate, graduate, and professional degrees and community programs on 10 campuses and through educational, training, and research centers across the state. UH enrolls more than 50,000 students from Hawaii, the U.S. mainland, and around the world.
Funding for the SDSS has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, the U.S. Department of Energy, the National Aeronautics and Space Administration, the Japanese Monbukagakusho, the Max Planck Society, and the Higher Education Funding Council for England. The SDSS Web Site is http://www.sdss.org/. The SDSS is managed by the Astrophysical Research Consortium for the Participating Institutions. The Participating Institutions are the American Museum of Natural History, Astrophysical Institute Potsdam, University of Basel, Cambridge University, Case Western Reserve University, University of Chicago, Drexel University, Fermilab, the Institute for Advanced Study, the Japan Participation Group, Johns Hopkins University, the Joint Institute for Nuclear Astrophysics, the Kavli Institute for Particle Astrophysics and Cosmology, the Korean Scientist Group, the Chinese Academy of Sciences (LAMOST), Los Alamos National Laboratory, the Max-Planck-Institute for Astronomy (MPA), the Max-Planck-Institute for Astrophysics (MPIA), New Mexico State University, Ohio State University, University of Pittsburgh, University of Portsmouth, Princeton University, the United States Naval Observatory, and the University of Washington.
UKIRT is operated by the Joint Astronomy Centre on behalf of the U.K. Particle Physics and Astronomy Research Council.
The IRTF is operated by the University of Hawaii under Cooperative Agreement no. NCC 5-538 with the National Aeronautics and Space Administration, Office of Space Science, Planetary Astronomy Program.