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We present high-resolution Keck optical spectra of the very young substellareclipsing binary 2MASS J05352184-0546085, obtained during eclipse of thelower-mass (secondary) brown dwarf. The observations yield the spectrum of thehigher-mass (primary) brown dwarf alone, with negligible (~1.6%) contaminationby the secondary. We perform a simultaneous fine-analysis of the TiO-epsilonband and the red lobe of the KI doublet, using state-of-the-art PHOENIX Dustyand Cond synthetic spectra. Comparing the effective temperature and surfacegravity derived from these fits to the {\it empirically} determined surfacegravity of the primary (logg=3.5) then allows us to test the model spectra aswell as probe the prevailing photospheric conditions. We find that: (1) fits toTiO-epsilon alone imply Teff=2500 \pm 50K; (2) at this Teff, fits to KI implylogg=3.0, 0.5 dex lower than the true value; and (3) at the true logg, KI fitsyield Teff=2650 \pm 50K, ~150K higher than from TiO-epsilon alone. On the onehand, these are the trends expected in the presence of cool spots covering alarge fraction of the primary's surface (as theorized previously to explain theobserved Teff reversal between the primary and secondary). Specifically, ourresults can be reproduced by an unspotted stellar photosphere with Teff=2700Kand (empirical) logg=3.5, coupled with axisymmetric cool spots that are 15%cooler (2300K), have an effective logg=3.0 (0.5 dex lower than photospheric),and cover 70% of the surface. On the other hand, the trends in our analysis canalso be reproduced by model opacity errors: there are lacks in the syntheticTiO-epsilon opacities, at least for higher-gravity field dwarfs. Stringentlydiscriminating between the two possibilities requires combining the presentresults with an equivalent analysis of the secondary (predicted to berelatively unspotted compared to the primary).

astrophysical journal/the astrophysical journal

High Resolution Spectroscopy during Eclipse of the Young Substellar  Eclipsing Binary 2MASS 0535-0546. I. Primary Spectrum: Cool Spots versus  Opacity Uncertainties