Abstract
During a survey for stars with disks in the Taurus star-forming region using the Spitzer Space Telescope, we have discovered a pair of young brown dwarfs, FU Tau A and B, in the Barnard 215 dark cloud. They have a projected angular separation of 57, corresponding to 800 AU at the distance of Taurus. To assess the nature of these two objects, we have obtained spectra of them and constructed spectral energy distributions. Both sources are young (∼1 Myr) according to their Hα emission, gravity-sensitive spectral features, and mid-infrared excess emission. The proper motion of FU Tau A provides additional evidence of its membership in Taurus. We measure spectral types of M7.25 and M9.25 for FU Tau A and B, respectively, which correspond to masses of ∼0.05 and ∼0.015 M☉ according to the evolutionary models of Chabrier and Baraffe. FU Tau A is significantly overluminous relative to an isochrone passing through FU Tau B and relative to other members of Taurus near its spectral type, which may indicate that it is an unresolved binary. FU Tau A and B are likely to be components of a binary system based on the low probability (∼3 × 10−4) that Taurus would produce two unrelated brown dwarfs with a projected separation of a ⩽ 6''. Barnard 215 contains only one other young star and is in a remote area of Taurus, making FU Tau A and B the first spectroscopically confirmed brown dwarfs discovered forming in isolation rather than in a stellar cluster or aggregate. Because they were born in isolation and comprise a weakly bound binary, dynamical interactions with stars could not have played a role in their formation, and thus are not essential for the birth of brown dwarfs.
Highlights
Several theoretical studies have suggested that dynamical interactions among young stars could be important—perhaps even essential—for the formation of brown dwarfs (Reipurth & Clarke 2001; Boss 2001; Bate et al 2002; Delgado-Donate et al 2003; Umbreit et al 2005; Goodwin & Whitworth 2007; Stamatellos et al 2007)
Because the ejection models require the presence of a stellar cluster, brown dwarfs should not form in isolation, either as singles or binaries, if dynamical interactions are necessary for their formation
We have shown that they have similar proper motions, but the accuracy of these measurements is insufficient to distinguish between a binary system and a pair of unrelated members of Taurus that are seen in projection near each other
Summary
Several theoretical studies have suggested that dynamical interactions among young stars could be important—perhaps even essential—for the formation of brown dwarfs (Reipurth & Clarke 2001; Boss 2001; Bate et al 2002; Delgado-Donate et al 2003; Umbreit et al 2005; Goodwin & Whitworth 2007; Stamatellos et al 2007). According to early versions of the ejection models, binary brown dwarfs should have separations that are no larger than ∼10 AU (Bate et al 2002) This prediction was consistent with the results from initial multiplicity surveys of brown dwarfs (Burgasser et al 2007, and references therein), but more recent observations have uncovered a small number of wide low-mass binaries in both the field and young clusters. Because of their fragile nature, it would seem difficult for a dynamical model to explain the existence of the widest and least massive systems.
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