Letter to the Editor

The cool atmospheres of the binary brown dwarf ε Indi B

¹ European Southern Observatory, Casilla 19001, Santiago 19, Chile e-mail: msterzik@eso.org
² DSM/DAPNIA/Service d'Astrophysique, CEA/Saclay, 91191 Gif-sur-Yvette, France
³ European Southern Observatory, 85748 Garching b. München, Germany

Received: 15 March 2005
Accepted: 30 April 2005

Abstract

We have imaged ε Indi B, the closest brown dwarf binary known, with VISIR at the VLT in three narrow-band mid-infrared bandpasses located around 8.6 μm, 10.5 μm and 11.3 μm. 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 NH₃ 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 ε IndiBb is in good agreement with recent “cloud-free” atmosphere models having an effective temperature of  K. With an assumed age of 1 Gyr for the ε Indi system, component Ba agrees more with  K rather than with  K, as suggested by SPITZER spectroscopic observations of the combined ε 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 ε IndiBa.