We report discovery of a young 0 \farcs 32 L dwarf binary , SDSS J2249+0044AB , found as the result of a Keck laser guide star adaptive optics imaging survey of young field brown dwarfs .
Weak K i , Na i , and FeH features as well as strong VO absorption in the integrated-light J -band spectrum indicate a low surface gravity and hence young age for the system .
From spatially resolved K -band spectra we determine spectral types of L3 \pm 0.5 and L5 \pm 1 for components A and B , respectively .
SDSS J2249+0044A is spectrally very similar to G196-3B , an L3 companion to a young M2.5 field dwarf .
Thus , we adopt 100 Myr ( the age estimate of the G196-3 system ) as the age of SDSS J2249+0044AB , but ages of 12–790 Myr are possible .
By comparing our photometry to the absolute magnitudes of G196-3B , we estimate a distance to SDSS J2249+0044AB of 54 \pm 16 pc and infer a projected separation of 17 \pm 5 AU for the binary .
Comparison of the luminosities to evolutionary models at an age of 100 Myr yields masses of 0.029 \pm 0.006 and 0.022 ^ { +0.006 } _ { -0.009 } M _ { \odot } for SDSS J2249+0044A and B , respectively .
Over the possible ages of the system ( 12–790 Myr ) , the mass of SDSS J2249+0044A could range from 0.011 to 0.070 M _ { \odot } and the mass of SDSS J2249+0044B could range from 0.009 to 0.065 M _ { \odot } .
Evolutionary models predict that either component could be burning deuterium , which could result in a mass ratio as low as 0.4 , or alternatively , a reversal in the luminosities of the binary .
We find a likely proper motion companion , GSC 00568-01752 , which lies 48 \farcs 9 away ( a projected separation of 2600 AU ) and has Sloan Digital Sky Survey and Two Micron All Sky Survey colors consistent with an early M dwarf .
We calculate a photometric distance to GSC 00568-01752 of 53 \pm 15 pc , in good agreement with our distance estimate for SDSS J2249+0044AB .
The space motion of SDSS J2249+0044AB shows no obvious coincidence with known young moving groups , though radial velocity and parallax measurements are necessary to refine our analysis .
The unusually red near-IR colors , young age , and low masses of the binary make it an important template for studying planetary-mass objects found by direct imaging surveys .