* Astronomy

Title: The Young L Dwarf 2MASS J11193254-1137466 is a Planetary-Mass Binary
Author: William M. J. Best, Michael C. Liu, Trent J. Dupuy, Eugene A. Magnier

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±3 Myr age and using hot-start evolutionary models, 2MASS J11193254-1137466AB is a pair of 3.7^{+1.2}_{-0.9} MJup 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_eff estimate for WISEA J114724.10-204021.3 is ~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.

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