High-resolution spectroscopy during eclipse of the young substellar eclipsing binary 2MASS 0535-0546. I. primary spectrum: Cool spots versus opacity uncertainties

Abstract

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-∈ 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-∈ alone imply Teff = 2500 ± 50 K; (2) at this Teff, fits to K I imply logg = 3.0, 0.5 dex lower than the true value; and (3) at the true logg, K I fits yield Teff = 2650 ± 50K, ∼150K higher than from TiO-∈ 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 eff reversal 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 logg = 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-∈ 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). © 2010. The American Astronomical Society. All rights reserved. Printed in the U.S.A.