A Substellar Companion to van Maanen 2

Abstract

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: a₀=27 mas, P=1.57 yr, i=89^deg, ω=95^deg, Ω=92^deg, and T₀=1991.26. Assuming a mass of 0.83 M_solar for the primary white dwarf, the mass of the secondary is 0.08 M_solar. 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_solar.