A search for L dwarf binary systems

We present analysis of Hubble Space Telescope (HST) planetary camera images of 20 L dwarfs identified in the course of the Two Micron All Sky Survey. Four of the targets, 2MASSW J0746425+200032, 2MASSs J0850359+105716, 2MASSW J0920122+351742, and 2MASSW J1146345+223053, have faint red companions at separations between 0." 07 and 0." 29 (1.6-7.6 AU). Ground-based infrared i maging confirms the last as a common proper motion companion. The surface d ensity of background sources with comparable colors is extremely low, and w e identify all four as physical binaries. In three cases, the bolometric ma gnitudes of the components differ by less than 0.3 mag. Since the cooling r ate for brown dwarfs is a strong function of mass, similarity in luminositi es implies comparable masses. The faint component in the 2M0850 system, how ever, is over 1.3 mag fainter than the primary in the I band and similar to 0.8 mag fainter in M-bol. Indeed, 2M0850B is similar to0.8 mag fainter in I than the lowest luminosity L dwarf currently known, while the absolute mag nitude we deduce at J is almost identical with M-J for Gl 229B. We discuss the implications of these results for the temperature scale in the L/T tran sition region. 2M0850 is known to exhibit 6708 Angstrom Li I absorption, in dicating that the primary has a mass less than 0.06 M.. Theoretical models predict that the magnitude difference implies a mass ratio of approximate t o0.75. The apparent binary fraction of the current sample, 20%, is comparab le to the results of previous surveys of late-type M dwarfs in the field an d in the Hyades. However, the mean separation of the L dwarf binaries in th e current sample is smaller than the M dwarf value by a factor of 2, and on ly one system would be detected at the distance of the Hyades. We discuss t he likely binary frequency among L dwarfs in light of these new data.