DISCOVERY OF A LOW-LUMINOSITY, TIGHT SUBSTELLAR BINARY AT THE T/Y TRANSITION

We have discovered that the brown dwarf WISE J014656.66+423410.0 is a close binary (0 ''.0875 +/- 0 ''.0021, 0.93(-0.16)(+0.12) AU) from Keck laser guide star adaptive optics imaging. Our photometry for this system reveals that both components are less luminous than those in any known substellar binary. Combining a new integrated-light spectrum (T9p) and resolved YJH-band photometry from Keck allows us to perform spectral decomposition and assign component types of T9 and Y0. Many of the unusual features in the spectrum might be explained by high surface gravity: Y-band peak broadened to the blue; J-band peak broadened to the red; H-band peak shifted slightly to the red; and red Y - J colors. Interestingly, the very low component luminosities imply that the T9 primary is unexpectedly cold (T-eff = 345 +/- 45 K assuming an age of 10 Gyr), making it approximate to 100 K cooler than any other late-T dwarf and comparable to Y dwarfs. One intriguing explanation for this apparent discrepancy is that the J- and H-band spectral features that trigger the transition from T to Y spectral types are highly gravity dependent. This can be tested directly in the very near future by orbit monitoring. We constrain the orbital period to be less than or similar to 10 yr by combining evolutionary-model-based mass estimates for the components (approximate to 12-21M(Jup), 1 sigma at 10 Gyr) with a statistical constraint on the semimajor axis (less than or similar to 1.3 AU). Such a period is shorter than any other known T/Y transition binary, meaning that WISE J0146+4234AB will likely yield a dynamical mass within the next few years.