Binarity at the L/T brown dwarf transition

TL;DR

This study investigates the frequency of binarity among L/T transition brown dwarfs, finding no significant difference from other spectral types, and highlights the challenges in modeling their atmospheric characteristics.

Contribution

The paper provides new high-resolution imaging data and a statistical analysis of binary fractions in L/T transition brown dwarfs, refining previous estimates and testing the binarity hypothesis.

Findings

Binary fraction in L/T transition dwarfs is approximately 31%, with large uncertainty.

No companions with temperatures 360-1000K found at separations >0.5".

Results do not conclusively support a higher binary fraction in the L/T transition.

Abstract

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. 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. We obtained high-resolution imaging for six mid-L to late-T dwarfs, with photometric distances between 8 and 33pc, using the adaptive optics systems NACO at the VLT, and the Lick system, both with the laser guide star. We resolve none of our targets. Combining our data with published results, we obtain a frequency of resolved L/T transition brown dwarfs of (31+21-15)%, compared to (21+10-7)% and (14+14-7)% 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 360-1000K at separations larger than 0.5".