The First Discovery of a Wide Binary Brown Dwarf

I present observations of a new faint double, 2MASS J11011926-7732383AB, toward the Chamaeleon I star-forming region. From optical and near-infrared images of the pair, I measure a separation of 1.44" and extract RIJHK_s photometry of the components (I_A=17.21, dI=1.07, K_s A=11.97, dK_s=0.84). I use resolved optical spectroscopy to derive spectral types of M7.25 and M8.25 for the A and B components, respectively. Based on the strengths of gravity-sensitive features in these data, such as the Na I and K I absorption lines, I conclude that these objects are young members of Chamaeleon I rather than field stars. The probability that this pair is composed of unrelated late-type members of Chamaeleon I is low enough (~5x10^-5) to definitively establish it as a binary system. After estimating extinctions, effective temperatures, and bolometric luminosities for the binary components, I place them on the H-R diagram and infer their masses with the evolutionary models of Chabrier and Baraffe, arriving at substellar values of 0.05 and 0.025 M_sun. The projected angular separation of this system corresponds to 240 AU at the distance of Chamaeleon I, making it the first known binary brown dwarf with a separation greater than 20 AU. This demonstration that brown dwarfs can form in fragile, easily disrupted configurations is direct evidence that the formation of brown dwarfs does not require ejection from multiple systems or other dynamical effects. It remains possible that ejection plays a role in the formation of some brown dwarfs, but it is not an essential component according to these observations