Title: The Young L Dwarf 2MASS J11193254-1137466 is a Planetary-Mass Binary,
Abstract: We have discovered that the extremely red, low-gravity L7 dwarf 2MASS
J11193254-1137466 is a 0.14" (3.6 AU) binary using Keck laser guide star
adaptive optics imaging. 2MASS J11193254-1137466 has previously been identified
as a likely member of the TW Hydrae Association (TWA). Using our updated
photometric distance and proper motion, a kinematic analysis based on the
BANYAN II model gives an 82% probability of TWA membership. At TWA's 10$\pm$3
Myr age and using hot-start evolutionary models, 2MASS J11193254-1137466AB is a
pair of $3.7^{+1.2}_{-0.9}$ $M_{\rm Jup}$ brown dwarfs, making it the
lowest-mass binary discovered to date. We estimate an orbital period of
$90^{+80}_{-50}$ years. One component is marginally brighter in $K$ band but
fainter in $J$ band, making this a probable flux-reversal binary, the first
discovered with such a young age. We also imaged the spectrally similar TWA L7
dwarf WISEA J114724.10-204021.3 with Keck and found no sign of binarity. Our
evolutionary model-derived $T_{\rm eff}$ estimate for WISEA J114724.10-204021.3
is $\approx$230 K higher than for 2MASS J11193254-1137466AB, at odds with their
spectral similarity. This discrepancy suggests that WISEA J114724.10-204021.3
may actually be a tight binary with masses and temperatures very similar to
2MASS J11193254-1137466AB, or further supporting the idea that near-infrared
spectra of young ultracool dwarfs are shaped by factors other than temperature
and gravity. 2MASS J11193254-1137466AB will be an essential benchmark for
testing evolutionary and atmospheric models in the young planetary-mass regime.