Brown Dwarf "Missing Link" Discovered__

May 16, 2000

University Park, Pa. --- A team of astronomers, including Drs. Donald Schneider and Lawrence Ramsey, Penn State professors of astronomy and astrophysics, have identified three objects intermediate between stars and planets, called "brown dwarfs," of a type never before observed. The three objects fill in what has, until now, been a "missing link" in the range of properties of known brown dwarfs.

The discovery resulted from a collaboration between astronomers using the United Kingdom Infrared Telescope (UKIRT) in Hawaii, the Hobby-Eberly Telescope (HET) in Texas, and scientists associated with the Sloan Digital Sky Survey (SDSS).

Brown dwarfs, unlike true stars, cannot sustain nuclear power production. After a modest initial flash, they cool and become progressively fainter.

The three newly discovered objects bridge the gap between the young, warmer group and the cooler fainter group. "They are not identical, but form a sequence linking the star-like and planet-like types," Schneider says.

Astronomers have been searching intensively for such transition objects over the last year. "In February 2000, following the discovery by the Sloan Survey of several new candidate brown dwarfs, infrared measurements by Dr. Sandy Leggett, at UKIRT, indicated that three of them might be in this 'missing link' range," Schneider says.

These infrared spectra were taken at UKIRT by the observing team of Leggett; Dr. Thomas Geballe of the Gemini Observatory in Hawaii; Professor Gillian Knapp of Princeton University; Alexander McDaniel, a Princeton University undergraduate student working with Xiaohui Fan, a Princeton University graduate student; and Dr. David Golimowski and Dr. Todd Henry at the Johns Hopkins University.

A spectrum in visible light of the brightest of the three objects was obtained in March with the HET by Schneider, Ramsey and Gary Hill from the University of Texas.

"Brown dwarfs emit but a small fraction of their luminosity in the visible band, so it was a real tour de force for the HET to obtain the spectrum and provide this important insight," explains Schneider. Detailed analysis of the spectra is under way to deduce more about the nature of these objects, which may resemble Jupiter and Saturn shortly after they formed about 5 billion years ago.

In the past year, Schneider and Ramsey have been using the HET to observe candidate brown dwarfs from the Sloan Survey; to date they have found nearly a dozen. "These challenging observations are the type of projects that the HET was designed for," remarked Ramsey, the telescope's project scientist and co-designer of the HET, with former Penn State professor Daniel Weedman.

A paper reporting these findings will be published in the Astrophysical Journal. Reports also are being presented at a meeting from May 28 to June 1 in Jackson Hole, Wyoming, and at the 196th meeting of the American Astronomical Society in Rochester, N.Y., from June 4 to 8.

A copy of the full paper can be obtained at http://xxx.lanl.gov/abs/astro-ph/0004408

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http://www.science.psu.edu/alert/Schneider5-2000.htm