Keck Imaging of Binary L Dwarfs

We present Keck near-infrared imaging of three binary L dwarf systems, all ofwhich are likely to be sub-stellar. Two are lithium dwarfs, and a thirdexhibits an L7 spectral type, making it the coolest binary known to date. Allhave component flux ratios near 1 and projected physical separations between 5and 10 AU, assuming distances of 18 to 26 pc from recent measurements oftrigonometric parallax. These surprisingly similar binaries represent the soledetections of companions in ten L dwarf systems which were analyzed in thepreliminary phase of a much larger dual-epoch imaging survey. The detectionrate prompts us to speculate that binary companions to L dwarfs are common,that similar-mass systems predominate, and that their distribution peaks atradial distances in accord both with M dwarf binaries and with the radiallocation of Jovian planets in our own solar system. To fully establish theseconjectures against doubts raised by biases inherent in this small preliminarysurvey, however, will require quantitative analysis of a larger volume-limitedsample which has been observed with high resolution and dynamic range.