Research Note

Binarity at the L/T brown dwarf transition^*

Adaptive optics search for companions

¹ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany e-mail: goldman@mpia.de
² Instituto de Astrofísica de Canarias, C/ vía Lactéa S/N, 38205 La Laguna, Tenerife, Spain
³ Astronomy Department, 601 Campbell Hall, UC Berkeley, Berkeley CA-94720-3411, USA

Received: 15 July 2008
Accepted: 17 August 2008

Abstract

Context. Current atmospheric models cannot reproduce some of the characteristics of the transition between the L dwarfs with cloudy atmospheres and the T dwarfs with dust-depleted photospheres. It has been proposed that a majority of the L/T transition brown dwarfs could actually be a combinaison of a cloudy L dwarf and a clear T dwarf. Indeed binarity seems to occur more frequently among L/T transition brown dwarfs.

Aims. We aim to refine the statistical significance of the seemingly higher frequency of binaries. Co-eval binaries would also be interesting test-beds for evolutionary models.

Methods. We obtained high-resolution imaging for six mid-L to late-T dwarfs, with photometric distances between 8 and 33 pc, using the adaptive optics systems NACO at the VLT, and the Lick system, both with the Laser Guide Star.

Results. We resolve none of our targets. Combining our data with published results, we obtain a frequency of resolved L/T transition brown dwarfs of %, compared to % and % for mid-L and T dwarfs (90% of confidence level). These fractions do not significantly support, nor contradict, the hypothesis of a larger binary fraction in the L/T transition. None of our targets has companions with effective temperatures as low as 360–1000 K at separations larger than 05.