The cool atmospheres of the binary brown dwarf eps Indi B

Abstract

We have imaged $\hat{I}\mu$ Indi B, the closest brown dwarf binary known, with VISIR at the VLT in three narrow-band mid-infrared bandpasses located around 8.6$\hat{I}¼$m, 10.5$\hat{I}¼$m and 11.3$\hat{I}¼$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 % our photometry of $\hat{I}\mu$ 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 $\hat{I}\mu$ 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 $\hat{I}\mu$ 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 $\hat{I}\mu$ IndiBa.

4 pages, 2 figures