THE MASS-LUMINOSITY RELATION IN THE L/T TRANSITION: INDIVIDUAL DYNAMICAL MASSES FOR THE NEW J-BAND FLUX REVERSAL BINARY SDSS J105213.51+442255.7AB

We have discovered that SDSS J105213.51+442255.7 (T0.5 +/- 1.0) is a binary in Keck laser guide star adaptive optics imaging, displaying a large J- to K-band flux reversal (Delta J = -0.45 +/- 0.09 mag, Delta K= 0.52 +/- 0.05 mag). We determine a total dynamical mass from Keck orbital monitoring (88 +/- 5 M-Jup) and a mass ratio by measuring the photocenter orbit from CFHT/WIRCam absolute astrometry (M-B/M-A= 0.78 +/- 0.07). Combining these provides the first individual dynamical masses for any field L or T dwarfs, 49 +/- 3 M-Jup for the L6.5 +/- 1.5 primary and 39 +/- 3 M-Jup for the T1.5 +/- 1.0 secondary. Such a low mass ratio for a nearly equal luminosity binary implies a shallow mass-luminosity relation over the L/T transition (Delta log L-bol/Delta log M = 0.6(-0.8)(+0.6)). This provides the first observational support that cloud dispersal plays a significant role in the luminosity evolution of substellar objects. Fully cloudy models fail our coevality test for this binary, giving ages for the two components that disagree by 0.2 dex (2.0 sigma). In contrast, our observed masses and luminosities can be reproduced at a single age by hybrid evolutionary tracks where a smooth change from a cloudy to cloudless photosphere around 1300 K causes slowing of luminosity evolution. Remarkably, such models also match our observed JHK flux ratios and colors well. Overall, it seems that the distinguishing features SDSS J1052+ 4422AB, like a J-band flux reversal and high-amplitude variability, are normal for a field L/T binary caught during the process of cloud dispersal, given that the age (1.11(-0.20)(+0.17) Gyr) and surface gravity (log g = 5.0-5.2) of SDSS J1052+ 4422AB are typical for field ultracool dwarfs.