A Neighbor of our Solar System Discovered
Institute For Astronomy news release, November 26, 2000
The nearest stars to us provide vantage points to study the different stellar populations in the Milky Way. They are brighter than similar stars located at larger distances, so they can be observed in fine detail. Considerable efforts are devoted to finding the stars that populate our own backyard. The most recent example is reported here. It is an object with a mass lower than a tenth of a solar mass, located at a distance of only 13 light years. An international team of astrophysicists, working in France, Spain and the United States of America, have discovered one of our closest neighbors using the infrared survey DENIS in the European Southern Observatory, and the Keck I telescope on Mauna Kea, on the Big Island of Hawaii. The discoverers have baptized this new celestial body with the prosaic name DENIS-P J104814.7-395606.1, according to the rules of the International Astronomical Union.
Xavier Delfosse, a French astronomer who was at the time working at the Institute of Astrophysics of the Canary Islands (Spain), and works now at the Observatoire de Grenoble (France), and Thierry Forveille, who works at the Canada-France-Hawaii Telescope Corporation, found this object last spring while they analyzed DENIS images. They identified a very red and relatively bright object. DENIS has found many red objects (including some of the very first brown dwarfs), but this one was unusually bright. It could be a very low-mass dwarf very close to us, or it could be a distant red giant. It was necessary to eliminate the possibility of a red giant. Thus, they contacted Eduardo Martin, a Spanish astronomer who now works at the Institute for Astronomy of the University of Hawaii.
Delfosse, Forveille and Martin have collaborated over the past four years in studying the nature of brown dwarf candidates found in the DENIS survey. Together with other collaborators they have proposed and developed a new classification scheme for ultracool dwarfs (L-type). Back in 1993, Martin was one of the inventors of a spectroscopic test to distinguish between very low-mass stars and brown dwarfs. The so-called "Lithium test'' has been applied ever since to confirm or refute the credibility of many brown dwarf candidates. On the night of 30 May 2000, Martin pointed the world's largest optical telescope, the 10-m Keck I of the W. M. Keck Observatory, at the Mauna Kea Observatories, to the DENIS candidate. He obtained a high-resolution spectrogram, which shows the presence of a strong cesium absorption line, as well as titanium and vanadium oxyde bands. The signature of lithium, however, is not present in the spectrum of the object. The Keck data demonstrated that this object must be a very nearby dwarf (cesium is not detected in giants), with a temperature of about 2200 K and a mass between 90 and 60 Jupiters. The lack of lithium implies that the mass must be larger than 60 Jupiter masses, but does not rule out that it could be a massive brown dwarf (the frontier between stars and brown dwarfs is at 75 Jupiter masses).
A very nearby dwarf should have an apparent motion with respect to the background of more distant stars. Jean Guibert and Francoise Crifo, at the Observatory of Paris, searched for DENIS-P J104814.7-395606.1 in old photographic plates, which have been scanned by the MAMA machine. They found it in images obtained in the last 30 years. It has moved considerably between 1986 and 1999 (see above image). The large proper motion (1.5 arcsec per year) confirms that this is one of our nearest neighbors. The astronomers estimate a distance of about 13 light-years. This distance is still uncertain because it is based on comparisons with objects of the same spectral type. DENIS-P J104814.7-395606.1 ranks between our 12th and 40th nearest neighbor. How could a star so close to us remain undiscovered until today? Despite of its proximity it is faint because of its low-mass and cool temperature (for example, it is 10,000 times too faint to see with the naked eye). These faint stars and brown dwarfs have so far escaped from the attention of astronomers, particularly in the southern hemisphere, which has been observed less systematically than the northern hemisphere. DENIS allows to identify them easily and will soon provide a census of very low-mass stars and brown dwarfs in the solar neighborhood. DENIS-P J104814.7-395606.1 becomes the brightest example of its spectral class, and it is therefore a benchmark for future studies of very low-mass stars and brown dwarfs.