Methane Dwarf

Sky Survey Scientists Discover New Celestial Dwarfs
Sloan Digital SkySurvey, May 31, 1999

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.