Summer 2000 -- Vol. 17, No. 1

Brown Dwarf 'Missing Link' Discovered

A team of astronomers, including Donald Schneider and Larry Ramsey of Penn State, has identified three objects known as "brown dwarfs," intermediate between stars and planets, of a type never before observed, thus filling in what has until now been an elusive "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 are intriguing objects, often picturesquely described as "failed stars."  They are more massive than Jupiter, the largest planet in our solar system, but they fall short of the minimum mass a true star needs--8% of the Sun's mass.  Stars can shine constantly for billions of years because they generate nuclear energy from the fusion of hydrogen into helium.  Brown dwarfs, however, cannot sustain nuclear power production; after a modest initial flash, they cool and become progressively fainter.

Hundreds of young brown dwarfs are now known to exist in the Sun's neighborhood.  They have surface temperatures that range from about 3,500 K (3,200 degrees C) down to 1,500 K (1,200 degrees C).  Over most of this range their appearances are similar to cool stars of the same temperature.  However, as the surface of a brown dwarf cools below 1,500 K, a dramatic chemical change takes place: large amounts of methane form in the outer parts of the star, considerably altering the appearance of the brown dwarf.

The three newly discovered objects bridge the gap between the young, warmer group and the cooler methane 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 Johns Hopkins University.

A spectrum in visible light of the brightest of the three objects was obtained in March 2000 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.

"The infrared and visible spectra clearly revealed that the properties of these three brown dwarfs fall between the warmer and cooler groups previously known," Schneider says.  "Both methane and carbon monoxide show up weakly.  Methane is absent in the warmer group and strong in the cooler group, while carbon monoxide is the other way around--strong in the warmer group and not seen in the cooler group."  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.

Penn State
 

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