Brown Dwarf "Missing Link" Discovered

A team of astronomers, including Penn State Professors Donald Schneider and Lawrence Ramsey, have 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 the 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 the 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.

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

ADDITIONAL INFORMATION:

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

CLASSIFICATION OF THE SPECTRA OF BROWN DWARFS:

A system of classification has been devised for brown dwarfs, which builds on the established method of classifying stars (types O, B, A, F, G, K, M in order of decreasing temperature) and is similarly based on spectral features. New classes introduced following the discovery of brown dwarfs are L and T, only loosely defined at present.

Cool dwarf stars and the younger, warmer, brown dwarfs have similar appearances and share portions of the M and L classifications. The M-type objects, with surface temperatures ranging down to 2,100 K (1,800 degrees C), have water and strong oxide features in their spectra. They may be stars or brown dwarfs, depending on their mass.

The next cooler group, with temperatures of roughly 1,500 to 2,100 K (1,200 to 1,800 degrees C) are the L dwarfs (L0 to L8), have spectra characterized by hydride features, and even deeper water bands. The coolest, least massive, stars fall into the warmer half of this temperature range. Their temperatures cannot be lower than 1,800 K (1,500 degrees C). Objects with temperatures between 1,500 and 1,800 K (1,200 and 1,500 degrees C) must be L-type brown dwarfs.

The 'missing link' objects found in the study reported here are believed to have surface temperatures in the range 1,000 to 1,500 K (700 to 1,200 degrees C).

The methane (T-type) brown dwarfs found to date are the coolest objects so far detected. Their surface temperatures range from about 1000 K down to 800 K (700 to 500 degrees C). Their spectra show strong absorption by methane and water.

TELESCOPE/PROJECT OPERATION:

The United Kingdom Infrared Telescope is operated by the Joint Astronomy Centre in Hilo, Hawaii, on behalf of the UK Particle Physics and Astronomy Research Council.

The Hobby-Eberly Telescope is a joint project of the University of Texas, the Pennsylvania State University (Penn State), Stanford University, Ludwig-Maximilians-Universität München, and Georg-August-Unversität Göttingen.

The Sloan Digital Sky Survey is a joint project of the University of Chicago, Fermilab, Institute for Advanced Study, Japan Participation Group, Johns Hopkins University, Max-Planck Institute for Astronomy, Princeton University, the United States Naval Observatory, and the University of Washington.

CONTACTS:

Lawrence Ramsey, 814-865-0418, lwr@astro.psu.edu
Donald Schneider, 814-863-9554, dps@astro.psu.edu
Barbara Kennedy (PIO), 814-863-4682, science@psu.edu