SETI [ASTRO] Sky Survey Scientists Discover New Celestial Dwarfs


Larry Klaes (lklaes@bbn.com)
Wed, 02 Jun 1999 15:24:20 -0400


>X-Authentication-Warning: brickbat12.mindspring.com: majordom set sender to owner-astro using -f >Date: Wed, 2 Jun 1999 14:50:20 GMT >From: Ron Baalke <BAALKE@kelvin.jpl.nasa.gov> >To: astro@lists.mindspring.com >Subject: [ASTRO] Sky Survey Scientists Discover New Celestial Dwarfs >Sender: owner-astro@brickbat12.mindspring.com >Reply-To: Ron Baalke <BAALKE@kelvin.jpl.nasa.gov> > >Sloan Digital Sky Survey > >Media Contacts: > >Steve Koppes >The University of Chicago, 773-702-8366 > >Judy Jackson >Fermilab, 630-840-3351 > >Georgia Whidden >The Institute for Advanced Study, 609-734-8239=20 > >Satoru Ikeuchi >Japanese Participation Group, 81-52-789-2427 > >Gary Dorsey >The John Hopkins University, 410-516-7906 > >Hans-Walter Rix >Max-Planck-Institut f=FCr Astronomie, 49-6221-528-210 > >Steven Dick >U.S. Naval Observatory, 202-762-1438 > >Mary Caffrey >Princeton University, 609-258-5748 > >Bruce Margon >University of Washington, 206-543-0089 > >Bruce Gillespie >Apache Point Observatory, 505-437-6822 > > >Embargoed for release after 12:30 p.m. Central Daylight Time Monday, >May 31, 1999 > >SDSS 98-4 > >Happy, Sleepy, Grumpy ... METHANE? > >Sky Survey Scientists Discover New Celestial Dwarfs > >Chicago, Ill. -- Scientists of the Sloan Digital Sky Survey announced today >(May 31) that early data from the Survey have revealed a new type of >astronomical object, smaller than a star but larger than a planet. Until >now, only one such object had ever been detected in the universe. > >Early this spring, while searching Sky Survey data for unusual objects such >as the universe's most distant quasars, graduate student Xiaohui Fan and >astronomer Michael Strauss, of Princeton University, found a faint but >extremely red dot of light in the night sky. Subsequent spectroscopic >observations revealed that the object was not a distant quasar but instead >an equally fascinating find -- a nearby cool, brown dwarf with properties >between those of a planet and a star. > >Until their discovery, only one of this type of "cool substellar object," >known as Gliese 229B, discovered in 1995, had ever been observed. However, >unlike Gliese 229B, which is a close companion to a star, the new object >was not orbiting a star, but occurred as a free-floating object about 30 >light years away in the constellation Ophiuchus. > >Then, last week, Sky Survey astronomers Zlatan Tsvetanov and Wei Zheng >of The Johns Hopkins University, along with fellow JHU astronomer David >Golimowski, confirmed the observation of another ultra-red, free-floating >object of comparable mass in the constellation Virgo, with a spectrum so >similar that Golimowski, a co-discoverer of Gliese 229B, described it as a >clone. > >"The JHU result shows that the Princeton discovery was not simply beginner's >luck but that these objects may be quite common," said astronomer Tom >Geballe of the Gemini observatory in Hawaii. "They are still so new to >astronomy that they require a new vocabulary. The name 'methane dwarf' has >emerged, because of the dramatic presence of bands of methane in their >spectra. Methane is characteristic of giant planets, like Jupiter, but it >never appears in normal stars -- they are much too hot -- or even in most >brown dwarfs." > >Objects whose mass falls between that of stars and planets are >extraordinarily elusive. Because they lack sufficient mass to generate >the nuclear reactions that make stars shine, they cool down from their >formation temperature and become too red and too dim to show up in most >searches of the sky. And, unlike planets, which are found in association >with stars, these objects may occur as isolated objects in interstellar >space. Thus, they are very hard to detect. > >Astronomers have previously observed objects with masses lower than the >minimum required to sustain nuclear reactions, the criterion that makes an >object a star. These lower-mass objects are called "brown dwarfs," a name >that reflects their temperature: red dwarfs are cooler than white ones, and >brown dwarfs are cooler still. The term "methane dwarf" distinguishes the >three known very cool brown dwarfs, which all show methane in their spectra, >from the population of hotter brown dwarfs that lack this signature. > >Astronomers also know that low-mass objects -- those with masses smaller >than the sun -- may be important contributors to the total mass of the Milky >Way. But to understand just how much they contribute, astronomers must >determine their masses -- and how many of them exist. > >"Just because we haven't seen these free-floaters before doesn't mean they >are rare," said University of Chicago astronomy graduate student Constance >Rockosi. "But to find them, you need to cover a lot of sky area and at the >same time be able to see very faint objects. The Sky Survey covers so much >celestial territory that at last we will begin to get a grip on how many >there really are." > >For now, the mass of the methane dwarfs remains unknown. They must be >smaller than stars, because they are so cool. However, their dimness >depends not only on their mass but on their age: brown dwarfs cool with >age, like embers drawn from a fire. So, although the methane dwarfs are >fainter than other brown dwarfs, they may not be less massive -- just much >older. Astronomers estimate their masses at 10 to 70 times the mass of >Jupiter. > >Both the Princeton and Johns Hopkins groups discovered the new methane >dwarfs while looking for objects so red that they were not even visible >with filters sensitive to other wavelengths. Princeton's Strauss described >the detective work that led to the confirmation of the object's true >identity. > >"The object we observed was the reddest object we have found thus far in >400 square degrees of observing," Strauss said. "We obtained its optical >spectrum with the 3.5 meter telescope at Apache Point Observatory in New >Mexico. The spectrum showed strong absorption by water, in the form of steam. >We immediately realized it might be an object similar to Gliese 229 B. At >the suggestion of Princeton astonomer Gillian Knapp, we called our colleagues >Tom Geballe, of the Gemini Observatory, and Sandy Leggett of the Joint >Astronomy Centre. By coincidence, they were observing at UKIRT, the United >Kingdom Infrared Telescope in Hawaii, which has one of the best near-infrared >spectrographs in the world. They obtained infrared spectra and showed that >our object was an almost exact twin to Gliese 229 B, with strong bands of >water and methane." > >A few weeks later, Geballe and Leggett received a similar call from the >Johns Hopkins group, and, on May 22 obtained the spectrum for the second >candidate object. > >"As anticipated," Golimowski said, "the spectrum revealed absorption >features from water vapor and methane. The third known methane dwarf was >now confirmed! Among many new questions, we need to understand why the >spectra of these three objects are so amazingly similar. The two new free- >floaters are ideal targets for searching for even lower-mass companions or >planets." > >Fan said Sky Survey collaborators are pleased at the objects' detection so >early in the life of their project. "Even the engineering test data from >the Sky Survey are proving very productive for science," he said. > >The wide-field capability of the Sky Survey telescope and camera are only >part of the technology necessary to advance the discovery rate of new >classes of astronomical objects. > >"The capability for precision five-color photometry is just as important," >said Knapp. "The work of Princeton software experts such as Robert Lupton >and Zeljko Ivezic was crucial." > >When the Sky Survey begins routine operations, on each clear moonless night >a powerful digital camera will generate 200 gigabytes of data containing >millions of objects. Most of the objects are more or less ordinary stars; >the interesting, one-in-(literally)-a-million objects must be carefully >filtered out from the vast quantities of Sky Survey data. Thus, another >crucial contribution to finding such rare objects as methane dwarfs so >early in the Survey is the extraordinary capability of the Sky Survey's >image-processing software, which enables scientists to zoom in on unusual >classes of objects with high reliability. > >The Sloan Digital Sky Survey (SDSS) is a joint project of The University of >Chicago, Fermilab, the Institute for Advanced Study, the Japan Participation >Group, The Johns Hopkins University, the Max-Planck-Institute for Astronomy, >Princeton University, the United States Naval Observatory, and the University >of Washington. Apache Point Observatory, site of the SDSS, is operated by the >Astrophysical Research Consortium. Funding for the project has been provided >by the Alfred P. Sloan Foundation, the SDSS member institutions, the National >Aeronautics and Space Administration, the National Science Foundation, the >U.S. Department of Energy and the Ministry of Education of Japan. The SDSS >Website is http://www.sdss.org . > >Images available: > >* First methane dwarf, > http://www.sdss.org/news/releases/19990531.dwarf.img1.html >* Second methane dwarf, > http://www.sdss.org/news/releases/19990531.dwarf.img2.html >* Spectrum of methane dwarf, > http://www.sdss.org/news/releases/19990531.dwarf.img3.html > >***** > >Two-Micron All-Sky Survey > >FOR RELEASE: 9:00 a.m. CDT, May 31, 1999 > >Discovery of Methane (T-type) Dwarfs Using 2MASS Data > >An example of one of the methane (T-type) dwarf discoveries. At left is a >5x5 arcminute piece of sky as seen in the near-infrared by the Two-Micron >All-Sky Survey (2MASS). In the center of the frame, marked with an arrow, >is the object known as 2MASSW J1217-03. On the right is the same piece o >sky as seen in visible light. (In this second view, 2MASSW J1217-03 is not >seen because it is at least 4000 times fainter here than in the >near-infrared.) In the 2MASS view, the object appears blue because of >absorption by methane. The discovery of methane in this object proves that >it is a brown dwarf -- an object which will forever continue to fade >because, unlike our Sun, it will never achieve thermonuclear fusion in its >core. In addition to this object, three similar methane dwarfs have also >been discovered by 2MASS. > >Because the spectrum, or chemical fingerprint, is very similar to that of >Jupiter, it means that this object is extremely cold. Before this year, >only one such object was known outside our own solar system: a companion to >the nearby star Gliese 229, announced in 1995. Discovery of the four 2MASS >methane dwarfs along with two others being announced today by the Sloan >Digital Sky Survey (SDSS) shows that these Jupiter-like objects can form by >themselves in the absence of a parent star. In fact, preliminary results >suggest that at least 90 of these isolated methane dwarfs may be discovered >after 2MASS has surveyed the entire sky, this despite the fact that 2MASS >can detect such objects out to only about 13 parsecs (about 40 light years). > >Because of methane in their spectra, a new spectral type "T" has been >proposed for these new discoveries. Spectral type "T" would be used for >objects just cooler than "L" dwarfs, which show no methane. Spectral type >"L" was announced just last year and follows "OBAFGKM" in the menagerie of >letters that astronomers use to denote various classes of stars. > >These results were announced to the press at the American Astronomical >Society meeting in Chicago, IL, on May 31, 1999. > >PHOTO CREDIT: >[http://spider.ipac.caltech.edu/staff/davy/2mass/science/tdwarf.jpg] > >(Left): 2MASS image courtesy of the University of Massachusetts and the >Infrared Processing and Analysis Center. > >(Right): Digitized Sky Survey image produced at the Space Telescope Science >Institute under U.S. Government grant NAG W-2166. The image is based on >photographic data obtained using the Oschin Schmidt Telescope located on >Palomar Mountain and operated by the California Institute of Technology >and Palomar Observatory. > >



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