2010ApJ...722.1138M

We present high-resolution Keck optical spectra of the very young substellar eclipsing binary 2MASS J05352184-0546085, obtained during eclipse of the lower-mass (secondary) brown dwarf. The observations yield the spectrum of the higher-mass (primary) brown dwarf alone, with negligible (∼1.6%) contamination by the secondary. We perform a simultaneous fine analysis of the TiO-ε band and the red lobe of the K I doublet, using state-of-the-art PHOENIX DUSTY and COND synthetic spectra. Comparing the effective temperature and surface gravity derived from these fits to the empirically determined surface gravity of the primary (log g = 3.5) then allows us to test the model spectra as well as probe the prevailing photospheric conditions. We find that: (1) fits to TiO-ε alone imply T_eff= 2500±50 K; (2) at this T_eff, fits to K I imply log g = 3.0, 0.5 dex lower than the true value; and (3) at the true log g, K I fits yield T_eff= 2650±50 K, ∼150 K higher than from TiO-ε alone. On the one hand, these are the trends expected in the presence of cool spots covering a large fraction of the primary's surface (as theorized previously to explain the observed T_effreversal between the primary and secondary). Specifically, our results can be reproduced by an unspotted stellar photosphere with T_eff= 2700 K and (empirical) log g = 3.5, coupled with axisymmetric cool spots that are 15% cooler (2300 K), have an effective log g = 3.0 (0.5 dex lower than photospheric), and cover 70% of the surface. On the other hand, the trends in our analysis can also be reproduced by model opacity errors: there are lacks in the synthetic TiO-ε opacities, at least for higher-gravity field dwarfs. Stringently discriminating between the two possibilities requires combining the present results with an equivalent analysis of the secondary (predicted to be relatively unspotted compared to the primary).