Israeli, German astronomers find 'brown dwarf' objects

Research provides insight on how planets form.

October 24, 2006 23:48
3 minute read.
Israeli, German astronomers find 'brown dwarf' objects

brown dwarf planet 88. (photo credit: )

Israeli and German astronomers have detected and become the first to directly image a "brown dwarf" companion to a star that also has a planet. The feat of detecting a solar system's faintest-known companions - carried out by Profs. Markus Mugrauer and Ralph Neuh user of the Friedrich-Schiller University of Jena, Andreas Seifahrt of the European Space Observatory and Tsevi Mazeh of Tel Aviv University - yields important information on the conditions under which planets form. "Brown dwarfs" are sub-stellar objects with a mass below that necessary to maintain hydrogen-burning nuclear fusion reactions in their cores. "Such a system is an interesting example that might prove that planet and brown dwarf can form around the same star," said Mugrauer, lead author of the paper presenting the discovery. These results were first presented at the recent International Astronomy Union general assembly in Prague and will soon be published in the Monthly Notices of the Royal Astronomical Society. The star HD 3651 is slightly less massive than our sun, located 36 light-years away in the constellation Pisces. For several years, it has been known to harbor a planet less massive than Saturn located closer to its parent star than Mercury is from the Sun; the planet accomplishes a full circle every 62 days. The brown dwarf was named HD 3651B. Comparing its characteristics with theoretical models, the astronomers inferred that the object has a mass between 20 and 60 Jupiter masses and a temperature between 500 and 600 degrees Celsius. It is thus 10 times colder and 300,000 times less luminous than the Sun. These properties place it in the category of "cool T brown dwarfs." Due to their faintness even in the infrared range, cool T dwarfs are very difficult to find, said Mugrauer. "Only two other brown dwarfs with similar brightness are presently known." The study will provide important insights on the atmospheric properties of cool sub-stellar objects," said Mugrauer. He and his colleagues first spotted the faint companion in 2003 on images from the United Kingdom Infrared Telescope (UKIRT), which is 3.8 meters in diameter and located in Hawaii. Observations in 2004 and 2006 using ESO's 3.6-meter-wide New Technology Telescope (NTT) at La Silla provided the crucial confirmation that the speck of light is not a spurious background star, but indeed a true companion. HD 3651B is 16 times further away from HD 3651 than Neptune is from the Sun. As it is not detected on the photographic plates of the Palomar All Sky Survey, the companion must be even fainter in the visible spectral range than in the infrared, meaning it is a very cool low-mass sub-stellar object, he said. More than 170 stars outside of the solar system are known to have planets. In some cases, these stars were also found to have one or several stellar companions, showing that planet formation can also take place in a more complex environment compared a system like our own solar system where planet formation occurred around a single isolated star. In 2001, Mugrauer and colleagues started an observational program to find out whether these "exoplanet host stars" are "single," that is with no large companions other than planets, or "married," that is a system that has a large companion that is not a planet, such as a brown or white dwarf. In this program, known exoplanet host stars are systematically imaged is at two different periods, at least several months apart. True companions can be distinguished from coincidental background objects as only they move together with the stars over time. With this effective search strategy several new companions of exoplanet host stars have been detected. Most of the detected companions are low-mass stars in the same evolutionary state as the Sun. In two cases, however, the astronomers found the companions to be "white dwarfs," that is, stars at the end of their life. These intriguing systems bear evidence that planets can even survive the troubled last moments in the life of a nearby star.

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