9 May 2000

Brown dwarf 'missing link' discovered

Brown dwarfs are intriguing objects, intermediate between stars and planets. 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. But brown dwarfs cannot sustain nuclear power production. After a modest initial flush, they cool off and become progressively fainter.

Young brown dwarfs are now known to exist in the hundreds in the Sun's neighbourhood. They have surface temperatures that range down from about 3,500 K (3,200 degrees C) 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, considerably altering the appearance of the brown dwarf.

The first methane-dominated brown dwarf to be discovered was found orbiting a nearby star by astronomers at Caltech in 1995. More have been found by astronomers at Caltech and Johns Hopkins University since early 1999, largely through two ongoing surveys of the night sky -- the Sloan Digital Sky Survey operating a single dedicated telescope in New Mexico, and the 2 Micron All-sky Survey (2MASS), which operates one telescope in Arizona and one in Chile. The methane brown dwarfs have turned out to be almost identical to each other. Their spectra are very similar to those of the giant Jupiter-like planets, even though they are considerably warmer.

The three newly discovered brown dwarfs bridge the gap between the young, warmer group and the cooler methane group. They are not identical, but form a sequence linking the warmer more star-like and the cooler more planet-like types.

Teams of astronomers have been searching intensively for such transition objects over the last year. In February 2000, following the discovery of several new brown dwarf candidates by the Sloan Survey, infrared measurements by Dr Sandy Leggett at UKIRT indicated that three of them might be this sought-after type. 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 and Alexander McDaniel, a Princeton University undergraduate student, working with Xiaohui Fan (Princeton graduate student) and Dr David Golimowski and Dr Todd Henry at the Johns Hopkins University.

The spectra clearly revealed that the properties of these three brown dwarfs fall between the warmer and cooler groups previously known. 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.

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

NOTES

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

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).

2. 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 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.

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