The cool atmospheres of the binary brown dwarf ε Indi B

We have imaged e Indi B, the closest brown dwarf binary known, with VISIR at the VLT in three narrow-band mid-infrared bandpasses located around 8.6 μ m, 10.5 μ m and 11.3 μ m. We are able to spatially resolve both components, and determine accurate mid-infrared photometry for both components independently. In particular, our VISIR observations probe the NH 3 feature in the atmospheres of the cooler and warmer brown dwarfs. For the first time, we can disentangle the contributions of the two components, and find that e IndiBb is in good agreement with recent “cloud-free” atmosphere models having an effective temperature of $T_\mathrm{eff} =800$ K. With an assumed age of 1 Gyr for the e Indi system, component Ba agrees more with $T_\mathrm{eff} \approx 1100$ K rather than with $T_\mathrm{eff}=1200$ K, as suggested by SPITZER spectroscopic observations of the combined e Indi B system (Roellig et al. 2004). Even higher effective temperatures appear inconsistent with our absolute photometry, as they would imply an unphysical small size of the brown dwarf e IndiBa.