PP[CLC]l[/CLC] 15: The First Brown Dwarf Spectroscopic Binary

PPl 15 is the first object to have been confirmed as a brown dwarf by thelithium test (in 1995), though its inferred mass was very close to thesubstellar limit. It is a member of the Pleiades open cluster. Its position ina cluster color-magnitude diagram suggested that it might be binary, andpreliminary indications that it is a double-lined spectroscopic binary werereported by us in 1997. Here we report on the results of a consecutive week ofKeck HIRES observations of this system, which yield its orbit. It has a periodof about 5.8 days, and an eccentricity of 0.4+/-0.05. The rotation of the starsis slow for this class of objects. Because the system luminosity is dividedbetween 2 objects with a mass ratio of 0.85, this renders each of them anincontrovertible brown dwarf, with masses between 60-70 jupiters. We show thatcomponent B is a little redder than A by studying their wavelength-dependentline ratios, and that this variation is compatible with the mass ratio. Weconfirm that the system has lithium, but cannot support the original conclusionthat it is depleted (which would be surprising, given the new masses). This isa system of very close objects which, if they had combined, would have produceda low mass star. We discuss the implications of this discovery for the theoriesof binary formation and formation of very low mass objects.Comment: Latex, 18 pages, 4 figures, submitted to Astron.