PP1 15: The first brown dwarf spectroscopic binary

PPI 15 is the first object to have been confirmed as a brown dwarf by the l ithium test (in 1995), though its inferred mass was very close to the subst ellar limit. It is a member of the Pleiades open cluster. Its position in a cluster color-magnitude diagram suggested that it might be binary, and pre liminary indications that it is a double-lined spectroscopic binary were re ported by us in 1997. Here we report on the results of a consecutive week o f Keck HIRES observations of this system, which yield its orbit. It has a p eriod of about 5.8 days, and an eccentricity of 0.4 +/- 0.05. The rotation of the stars is slow for this class of objects. Because the system luminosi ty is divided between two objects with a mass ratio of 0.85, each of them i s rendered an incontrovertible brown dwarf, with masses between 60 and 70 M -J. We show that component B is a little redder than A by studying their wa velength-dependent line ratios and that this variation is compatible with t he mass ratio. We confirm that the system has lithium but cannot support th e original conclusion that it is depleted (which would be surprising, given the new masses). This is a system of very close objects, which, if they ha d combined, would have produced a low-mass star. We discuss the implication s of this discovery for the theories of binary formation and formation of v ery low mass objects.