Exploring the brown dwarf desert with Hipparcos

Abstract

The orbital elements of 11 spectroscopic binaries with brown dwarf candidates (M₂ sin i between 0.01 and 0.08 M_sun) are combined with the Hipparcos observations in order to derive astrometric orbits. Estimations of the masses of the secondary components are thus calculated. It appears that 5 secondary masses are more than 2 sigma_M2 above the limit of 0.08 M_sun, and are therefore not brown dwarfs. 2 other stars are still discarded at the 1 sigma_M2 level, 1 brown dwarf is accepted with a low confidence, and we are finally left with 3 viable candidates which must be studied by other means. A statistical approach is developed, based on the relation between the semi-major axes of the photocentric orbit, a_0, their errors, sigma _{a_0}, and the frequency distribution of the mass ratios, q. It is investigated whether the set of values of a_0 and sigma _{a_0} obtained for the sample is compatible with different frequency distributions of q. It is concluded that a minimum actually exists for M₂ between about 0.01 and 0.1 M_sun for companions of solar-type stars. This feature could correspond to the transition between giant planets and stellar companions. Due to the relatively large frequency of single brown dwarfs found recently in open clusters, it is concluded that the distribution of the masses of the secondary components in binary systems does not correspond to the IMF, at least for masses below the hydrogen-ignition limit. Based on photoelectric radial-velocity measurements collected at Haute-Provence observatory and on observations made with the ESA Hipparcos astrometry satellite.