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V. Makarov (Michelson Science Center, Caltech)
Most of the presently known low-mass companions and planets in extrasolar systems have been discovered via a periodic variation of radial velocity of the primary star. The astrometric method, although currently less accurate, is an alternative and independent way to look for brown dwarf and planetary companions. It is based on the reflex stellar motion caused by the orbital motion in the system. The astrometric method may provide important information on the physical size and inclination of the orbit, and, subsequently, a dynamical estimation of the total and secandary masses. It works fine for objects that may be difficult for the radial velocity search, e.g., very hot primary stars or almost face-on orbits. Using the Hipparcos Intermediate Astrometry Data, we are conducting a systematic screening of nearby (distances less than 20-30 pc) stars, paying special attention to astrometric binaries with considerable accelerations or discrepant long-term and short-term proper motions. Several new systems have been discovered with probable brown dwarf or giant planet companions, including van Maanen 2 (GJ 35), HD 219571, GJ 914A (85 Peg A), GJ 533, GJ 9616 and GJ 9387. Preliminary astrometric solutions are obtained, statistical confidence levels are computed, and secondary masses are estimated. The companion to van Maanen 2 has a mass of about 0.08 Msun, and may be the nearest boundary object between the classes of brown dwarfs and super-giant planets. It orbits the nearest cool white dwarf 3.67 Byr of age, at a distance of just 4.4 pc from the Sun. The estimated period is 1.57 yr, and the predicted semi-amplitude of radial velocity is 0.48 km/s. The maximum separation between the primary white dwarf and the secondary substellar object is approximately 0.3 arcsec, which leaves the possibility of direct imaging of the latter with existing facilities.
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Bulletin of the American Astronomical Society, 35#5
© 2003. The American Astronomical Soceity.