THE ASTROPHYSICAL JOURNAL, 466:415-426, 1996 July 20

     SPECTROSCOPIC ORBITS FOR THREE BINARIES WITH LOW-MASS COMPANIONS AND
        THE DISTRIBUTION OF SECONDARY MASSES NEAR THE SUBSTELLAR LIMIT

       TSEVI MAZEH,[1,2,3] DAVID W. LATHAM,[2] AND ROBERT P. STEFNIK[2]


                                   ABSTRACT

        We present orbital solutions for three low-amplitude spectroscopic
     binaries discovered in a sample of 20 solar-type IAU radial velocity
     standard stars observed with the Digital Speedometers at the Harvard-
     Smithsonian Center for Astrophysics.  We update the orbital solutions
     for HD 114762 and HD 140913, and present a preliminary new solution
     for HD 29587.  For all three orbits, the minimum mass for the
     secondary is less than 0.08 Msun, the borderline between stellar and
     substellar masses.
        We consider the probability that all three binaries have small
     enough inclination angles so that their companions are above the
     substellar limit.  To do so, we treat the 20 IAU standards as a sample
     drawn from a population of binaries with a mass-ratio distribution
     that does not allow any substellar companions.  We calculate the
     probability that such a sample could still have three binaries, with
     the low-amplitude orbits actually found within the IAU sample.  We
     show that this probability is small, depending on the specific
     mass-ratio distribution.  For example, a flat mass-ratio distribution
     that assumes there are no substellar companions can be excluded at a
     high confidence level, 99.7%.  We further show that our three
     detections may imply that the secondary-mass distribution rises near
     the substellar limit.  However, the observations do not yet allow us
     to distinguish whether the unseen companions of HD 114762, HD 140913,
     and HD 29587 have stellar or substellar masses.  In particular, recent
     attempts to estimate the mass of the companion of HD 114762 based on
     assumptions about the intrinsic rotation of the primary are
     inconclusive, and the companion could easily have a mass as low as
     0.02 Msun.
        We compare our three detections with the null results of four very
     precise radial velocity searches for substellar companions.  The
     difference is indeed puzzling but can be accounted for if just a small
     fraction of the solar-type stars, of the order of a few percent, have
     companions with masses near the substellar limit.

     Subject headings: binaries: spectroscopic -- stars: individual
                       (HD 29587, HD 114762, HD 140913) -- stars:
                       low-mass, brown dwarfs

[1]  School of Physics and Astronomy, Raymond and Beverly Sackler Faculty
     of Exact Sciences, Tel Aviv University, Tel Aviv 69978, Israel;
     mazeh@wise7.tau.ac.il.
[2]  Harvard-Smithsonian Center for Astrophysics, 60 Garden Street,
     Cambridge, MA 02138; dlatham@cfa.harvard.edu,
     rstefanik@cfa.harvard.edu.
[3]  Laboratory for Astrophysics, National Air and Space Museum,
     Smithsonian Institution, Washington, DC 20560.