dwarf
Source: EDUARDO MARTIN

Some recent astronomical detective work has turned up a new object, either a small star or a large brown dwarf, right in our own backyard. Because of its low mass and cool temperature, it is very faint--some 10,000 times too faint to see with the naked eye. And it is for this reason, perhaps, that no one ever noticed the object--christened DENIS-P J104814.7-395606.1--despite the fact that astronomers now estimate it to be only 13 light years away from Earth.

French astronomers Xavier Delfosse and Thierry Forveille first spotted the object last spring as they analyzed images from the infrared survey DENIS in the European Southern Observatory. It was very red and comparatively bright, making it impossible for them to determine whether the feature was a nearby brown dwarf or a distant red giant. To rule out the latter possibility, they called in Eduardo Martin, now at the University of Hawaii. Martin was one of the inventors of the lithium test used to identify brown dwarfs, which are bigger than planets but not massive enough to sustain the thermonuclear reactions inside stars.

Martin observed the dwarf candidate through the 10-meter optical Keck I telescope at the W.M.Keck Observatory on May 30, 2000. The high-resolution spectrogram he obtained showed the presence of cesium, an element not found in red giants. What he didn't find was lithium, indicating that the object's mass was more than 60 times greater than Jupiter's mass. His study concluded that DENIS-P J104814.7-395606.1 had a temperature of about 2200 degrees Kelvin and a mass between 60 and 90 Jupiters. Thus, it was clearly nearby, and in the size range between dwarfs and stars.

Additional confirmation of the object's proximity came from Jean Guibert and Francoise Crifo at the Observatory of Paris, who searched for glimpses of DENIS-P J104814.7-395606.1 in photographic plates from the past 30 years. They discovered that in the images, it moved considerably between 1986 and 1999 (see images). Only something relatively close would cover that much ground. DENIS-P J104814.7-395606.1 is the brightest example of its spectral class, and so will undoubtedly serve as a benchmark in future studies of brown dwarfs and low-mass stars.