A substellar companion to van Maanen 2

摘要

The Hipparcos data for the nearest white dwarf van Maanen 2 (=HIP 3829; distance of 4.4 pc) indicate astrometric binarity, in that the proper motion has a large first derivative not accounted for as a perspective acceleration due to changing distance. The presence of a companion was suspected, but never demonstrated, from the pre-Hipparcos astrometric measurements of photographic plates. Inspection of a digital position probability chart based on the Hipparcos Intermediate Astrometry Data reveals a pendulum-like motion of the photocenter mostly in the east-west direction with a period of about 1.5 yr. Having this period as an initial constraint, an orbital fit was obtained with the following parameters: a(0) = 27 mas, P = 1.57 yr, i = 89degrees, omega = 95degrees, Omega = 92degrees, and T-0 = 1991.26. Assuming a mass of 0.83 M. for the primary white dwarf, the mass of the secondary is 0.08 M,. Covariance analysis shows, however, that the uncertainty of the semimajor axis estimate and, subsequently, of the mass estimate is great for this nearly edge-on orbit. A more robust estimate is obtained for the semiminor axis, providing a lower bound mass of the companion of. 0.06 +/- 0.02 M.. [References: 9]