Indi Ba, Bb: a detailed study of the nearest known brown dwarfs ^*,**

¹ School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL, UK e-mail: rob@astro.ex.ac.uk
² Research & Scientific Support Department, ESA ESTEC, Keplerlaan 1, 2200 AG Noordwijk, The Netherlands
³ Institut für Astrophysik, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
⁴ Centre de Recherche Astrophysique de Lyon, UMR 5574: CNRS, Université de Lyon, École Normale Supérieure de Lyon, 46 allée d'Italie, 69364 Lyon Cedex 07, France
⁵ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
⁶ Instituto de Astrofísica de Canarias, vía Láctea s/n, 38200 La Laguna, Tenerife, Spain

Received: 24 July 2009
Accepted: 6 November 2009

Abstract

The discovery of ε Indi A, Bb, a binary brown dwarf system very close to the Sun, makes possible a concerted campaign to characterise the physical parameters of two T dwarfs. Recent observations suggest substellar atmospheric and evolutionary models may be inconsistent with observations, but there have been few conclusive tests to date. We therefore aim to characterise these benchmark brown dwarfs to place constraints on such models. We have obtained high angular resolution optical, near-infrared, and thermal-infrared imaging and medium-resolution (up to R ~ 5000) spectroscopy of ε Indi A, Bb with the ESO VLT and present VRIzJHKL'M' broad-band photometry and 0.63-5.1  spectroscopy of the individual components. The photometry and spectroscopy of the two partially blended sources were extracted with a custom algorithm. Furthermore, we use deep AO-imaging to place upper limits on the (model-dependent) mass of any further system members. We derive luminosities of log  = -4.699 ± 0.017 and -5.232 ± 0.020 for ε Indi A, Bb, respectively, and using the dynamical system mass and COND03 evolutionary models predict a system age of 3.7-4.3 Gyr, in excess of previous estimates and recent predictions from observations of these brown dwarfs. Moreover, the effective temperatures of 1352-1385 K and 976-1011 K predicted from the COND03 evolutionary models, for ε Indi A and Bb respectively, are in disagreement with those derived from the comparison of our data with the BT-Settl atmospheric models where we find effective temperatures of 1300-1340 K and 880-940 K, for ε Indi A and Bb respectively, with surface gravities of log g = 5.25 and 5.50. Finally, we show that spectroscopically determined effective temperatures and surface gravities for ultra-cool dwarfs can lead to underestimated masses even where precise luminosity constraints are available.